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RENAL HEMODYNAMICS AND CARDIORENAL INTEGRATION
1- and 2-adrenoceptors on renal sensory nerve fibers1Departments of Internal Medicine and Pharmacology, Department of Veterans Affairs Medical Center and University of Iowa Carver College of Medicine, Iowa City, Iowa; and 2Department of Neuroscience, Karolinska Institutet, Stockholm, Sweden
Submitted 5 July 2007 ; accepted in final form 15 August 2007
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ABSTRACT |
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-adrenoceptors on the renal sensory nerves, we examined the effects of renal pelvic administration of the 1- and 2-adrenoceptor antagonists prazosin and rauwolscine on the ARNA responses to reflex increases in ERSNA (placing the rat's tail in 49°C water) and renal pelvic perfusion with norepinephrine in anesthetized rats. Hot tail increased ERSNA and ARNA, 6,930 ± 900 and 4,870 ± 670%·s (area under the curve ARNA vs. time). Renal pelvic perfusion with norepinephrine increased ARNA 1,870 ± 210%·s. Immunohistochemical studies showed that the sympathetic and sensory nerves were closely related in the pelvic wall. Renal pelvic perfusion with prazosin blocked and rauwolscine enhanced the ARNA responses to reflex increases in ERSNA and norepinephrine. Studies in a denervated renal pelvic wall preparation showed that norepinephrine increased substance P release, from 8 ± 1 to 16 ± 1 pg/min, and PGE2 release, from 77 ± 11 to 161 ± 23 pg/min, suggesting a role for PGE2 in the norepinephrine-induced activation of renal sensory nerves. Prazosin and indomethacin reduced and rauwolscine enhanced the norepinephrine-induced increases in substance P and PGE2. PGE2 enhanced the norepinephrine-induced activation of renal sensory nerves by stimulation of EP4 receptors. Interaction between ERSNA and ARNA is modulated by norepinephrine, which increases and decreases the activation of the renal sensory nerves by stimulating 1- and 2-adrenoceptors, respectively, on the renal pelvic sensory nerve fibers. Norepinephrine-induced activation of the sensory nerves is dependent on renal pelvic synthesis/release of PGE2. substance P; EP4 receptor; pelvis; prazosin; rauwolscine
3 mmHg in renal pelvic pressure (21), and its abolition resulting in salt-sensitive hypertension (20). Among the mechanisms involved in the activation of the renal mechanosensory nerves produced by stretching the renal pelvic wall are induction of cyclooxygenase-2 (COX-2) leading to increased renal pelvic synthesis of PGE2 (23, 24, 27). PGE2 increases the release of substance P via activation of the cAMP-PKA signal transduction pathway. Substance P activates the afferent renal nerves by stimulating neurokinin-1 receptors in the renal pelvic area (29).
One possible mechanism involved in the interaction between ERSNA and ARNA is the sympathetic neurotransmitter norepinephrine. Similar to the interaction between ERSNA and ARNA, there is evidence for a relationship between efferent and afferent nerve activity in the sinoaortic arterial baroreflex and the carotid chemoreceptor reflex (9). Norepinephrine has been shown to increase baroreceptor afferent nerve activity by a direct action on the baroreceptors (31). Early studies by Niijima (39) suggested that norepinephrine activates renal sensory nerves. Although the renal vasoconstriction produced by electrical renal nerve stimulation and intrarenal administration of norepinephrine may confound the interpretation of the data, the results showed an increase in ARNA in response to these two interventions. Also, there is considerable evidence for norepinephrine enhancing the activation of peripheral nociceptors (15).
We therefore examined the role of norepinephrine in the ARNA response to reflex increases in ERSNA and the subtype of adrenoceptors involved. ERSNA was increased by thermal cutaneous simulation produced by placing the tail of the anesthetized rat in 49°C water. These studies were complemented by examination of the effects of renal pelvic administration of norepinephrine on ARNA in vivo and substance P release in vitro to minimize the influence of central and systemic mechanisms activated by thermal cutaneous stimulation. In this way, we could also study whether the effects produced by reflex ERSNA involved activation of pre- or postsynaptic adrenoceptors. PGE2 plays an important role in the activation of renal mechanosensory receptors (23, 24, 26, 27) and sensitizes the sensory receptors to numerous stimuli (e.g., 11, 16, 37, 40). Therefore, we examined whether PGE2 was also involved in the norepinephrine-induced activation of renal pelvic sensory nerves. Because the majority of the renal sensory nerves are unmyelinated and located in the renal pelvic wall (25, 26, 32), we performed immunohistochemical studies to examine whether a similar close relationship between unmyelinated efferent and afferent nerve fibers could be demonstrated in the renal pelvic wall, as had been demonstrated between unmyelinated sympathetic nerves and myelinated afferent nerves in the renal cortical/juxtamedullary border (3).
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METHODS |
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The experimental protocols were approved by the Institutional Animal Care and Use Committee and performed according to the "Guide for the Care and Use of Laboratory Animals" from the National Institutes of Health. Anesthesia was induced with pentobarbital sodium (0.2 mmol/kg ip; Abbott Laboratories, Abbott Park, IL).
Immunohistochemistry
The immunohistochemical procedures for kidney tissue have been previously described in detail (25, 26). In brief, anesthetized male Sprague-Dawley rats were transcardially perfused with calcium-free Tyrode solution followed by phosphate-buffered (0.1 M, pH 7.4) fixative containing 4% wt/vol paraformaldehyde and 0.2% wt/vol picric acid. The kidneys were quickly dissected out, postfixed in fixative for 90 min and stored in 10% sucrose at 4°C. The tissue was frozen using CO2, 14-µM-thick sections, cut on a cryostat (Microm, Heidelberg, Germany) and thaw-mounted onto chromium potassium sulfate/gelatin-coated slides. The sections were rinsed in PBS and incubated for 24 h with the rabbit antibody against the norepinephrine transporter (NE-t; 1:1,000; HPA004063, www.proteinatlas.org). Immunoreactivity was visualized using the tyramide signal amplification system (TSA-Plus: PerkinElmer Life and Analytical Sciences, Waltham, MA). The following day, horseradish-peroxidase conjugated with swine antirabbit IgG (1:200; DAKO, Copenhagen, Denmark) was applied followed by biotinyl tyramide-fluorescein. After completion of the protocol for TSA for detection of NE-t, the tissue sections were further processed by the indirect immunofluorescence technique. The tissue sections were incubated overnight with primary antiserum for CGRP (mouse, 1:400; provided by Drs. J. H. Walsh and H. C. Wong, Department of Veterans Affairs/University of California, Los Angeles, CA). The tissue-bound antibodies were detected by cyanin (Cy3)-conjugated donkey anti-mouse antibody (1:100, Jackson ImmunoResearch, West Grove, PA).
The sections were examined using a Nikon Eclipse E600 fluorescence microscope (Tokyo, Japan) equipped with epifluorescence with the appropriate filter combinations. Photographs were taken with a Hamamatsu ORCA-ER C4762-80 digital camera (Hamamatsu City, Japan) using Hamamatsu photonics Wasabi 1.5 software. For confocal analysis, a Zeiss confocal laser-scanning microscope (Model 510, Zeiss, Jena, Germany) equipped with appropriate excitation and emission filters was used. Digital images from the microscope were optimized for image resolution, brightness, and contrast, and color images were merged using Adobe Photoshop 7.0 (Adobe Systems, San Jose, CA).
In Vivo Studies
After induction of anesthesia, an intravenous infusion of pentobarbital sodium (0.04 mmol·kg–1·h–1) at 50 µl/min into the femoral vein was started and maintained throughout the course of the experiment. Arterial pressure was recorded from a catheter in the femoral artery. The left renal pelvis was perfused with vehicle or various perfusates, described below, throughout the experiment at 20 µl/min via a PE-10 catheter placed inside a PE-60 catheter located in the ureter. ERSNA and ARNA were recorded from the central and peripheral portions, respectively, of the cut ends of two adjacent left renal nerve branches, which were placed on bipolar silver-wire electrodes. ERSNA and ARNA were integrated over 1-s intervals, the unit of measure being microvolts per second per 1 second. All data were collected at 500 Hz and averaged over 2 s. Post mortem renal nerve activity, assessed by crushing the renal nerve bundles central or peripheral to the recording electrode, was subtracted from all values of ERSNA and ARNA, respectively. Renal nerve activity was expressed in percentage of its baseline value during the control period (21–30).
Stimulation of renal sensory nerves. The studies were divided into two main groups. In the first group, renal sensory nerves were stimulated by reflex-mediated increases in ERSNA, which were produced by placing the rat's tail in 49°C water. In second group, we examined whether renal pelvic perfusion with norepinephrine at a concentration that did not alter systemic hemodynamics also increased ARNA, and if so, whether the mechanisms involved were similar to those activated by reflex induced increases in ERSNA. In all groups, a 10-min control and a 10-min recovery period bracketed the experimental period.
In Vitro Studies
Substance P and PGE2 release from an isolated renal pelvic wall preparation. To further examine whether norepinephrine modulates the responsiveness of the renal sensory nerves by a mechanism(s) independent of systemic and/or central effects, we examined whether norepinephrine added to the incubation bath containing isolated renal pelvises induced a release of substance P. Because the isolated renal pelvic preparation by design is sympathetically denervated, these studies will further show whether norepinephrine modulates the activation of the renal sensory nerves by mechanisms involving postsynaptic receptors. Our previous studies have shown an important role for PGE2 in the release of substance P and activation of the renal sensory nerves. Therefore, we also examined whether the norepinephrine-induced release of substance P was associated with renal pelvic release of PGE2.
The procedures for measuring the release of substance P from an isolated rat renal pelvic wall preparation have been previously described in detail (18, 21, 23, 25–27). In brief, following anesthesia, renal pelvises dissected from the kidneys were placed in wells containing 400 µl HEPES buffer (25 mM HEPES, 135 mM NaCl, 3.5 mM KCl, 2.5 mM CaCl2, 1 mM MgCl2, 3.3 mM D-glucose, 0.1 mM ascorbic acid, 0.1% BSA, 10 µM dl-thiorphan, 1 mM Phe-Ala, 50 µM p-chloromercuriphenylsulfonic acid, pH 7.4) maintained at 37°C. Each well contained the pelvic wall from one kidney.
The renal pelvic walls were allowed to equilibrate for 130 min. The incubation medium was replaced with fresh HEPES every 10 min for the first 120 min and every 5 min thereafter. The incubation medium was collected in siliconized vials and stored at –80°C for later analysis of substance P. The experimental protocol consisted of four 5-min control periods, one 5-min experimental period and four 5-min recovery periods. Norepinephrine was added to the incubation bath to both the ipsilateral and contralateral pelvises during the experimental periods.
Experimental Protocols
Group I (in vivo): effects of an 1- or 2-adrenoceptor antagonist on the ARNA responses to reflex increases in ERSNA.
The experiment was divided into two parts. During each part, the rat's tail was placed in 49°C water during two 3-min experimental periods. Following each experimental period, the rat's tail was immediately placed in room temperature water to quickly terminate the heat stimulation. Twenty minutes after the end of the first part, the renal pelvic perfusate was switched from vehicle to the 1-adrenoceptor antagonist prazosin, 0.5 µM (n = 9) or the 2-adrenoceptor antagonist rauwolscine, 0.1 µM (n = 11). Ten minutes later, the control, experimental and recovery periods were repeated. Time control experiments (n = 7) were performed similarly, except the renal pelvis was perfused with vehicle throughout the experiment.
Group II (in vivo/in vitro): Effects of an 1- or 2-adrenoceptor antagonist on the norepinephrine-induced increases in ARNA, substance P, and PGE2.
IN VIVO.
The experiment was divided into three parts. During each part, norepinephrine, 10 pM, was added to the renal pelvic perfusate during the 5-min experimental period. At the end of the first part, the renal pelvic perfusate was switched from vehicle to prazosin, 0.5 µM (n = 9) or rauwolscine, 0.1 µM (n = 13). Five minutes later, the control, experimental, and recovery periods were repeated. At the end of the second part, the renal pelvic perfusate was switched from prazosin or rauwolscine to vehicle. Ten minutes later, the control, experimental, and recovery periods were repeated once more.
IN VITRO.
Initial experiments were performed to determine the concentration of norepinephrine required to increase the release of substance P. Ipsilateral and contralateral renal pelvises were incubated in HEPES buffer as described above. During the experimental period, the ipsilateral and contralateral pelvises were exposed to norepinephrine at 10 and 50 pM (n = 6), respectively, or 50 and 250 pM (n = 7), respectively. Only the renal pelvic release of substance P was measured in these experiments. In subsequent experiments examining the effects of prazosin and rauwolscine, ipsilateral and contralateral pelvises were exposed to norepinephrine, 250 pM and 50 pM, respectively, during the experimental period. In these experiments, the ipsilateral pelvis was incubated in HEPES buffer and the contralateral renal pelvis was incubated in HEPES buffer containing prazosin 5 µM (n = 13) or rauwolscine, 0.1 µM (n = 18) throughout the control, experimental, and recovery periods. Renal pelvic release of substance P and PGE2 into the incubation bath was measured throughout the experiment.
Group IIIa (in vitro): effects of a COX inhibitor on the norepinephrine-induced release of substance P and PGE2. The ipsilateral pelvis was incubated in HEPES buffer, and the contralateral renal pelvis was incubated in HEPES buffer containing the COX inhibitor indomethacin, 0.14 mM (n = 10) throughout the control, experimental, and recovery periods. During the experimental period, the ipsilateral and contralateral pelvises were exposed to norepinephrine at 250 pM.
Group IIIb (in vivo): effects of a COX inhibitor or an EP4 receptor antagonist on the norepinephrine-induced increase in ARNA. Because the in vitro studies suggested an important role for PGE2 in the norepinephrine-induced release of substance P, we examined the role of PGE2 in the ARNA response to norepinephrine by studying the effects of renal pelvic perfusion with indomethacin, 0.14 mM, the EP4 receptor antagonist, L-161,982, 5 µM (26, 34), or L-161,983, 5 µM, the inactive enantiomer of L-161,983, on the increases in ARNA produced by norepinephrine, 10 pM, using a similar protocol as in group II (in vivo).
Group IIIc (in vitro): effects of PGE2 on norepinephrine-induced release of substance P. The important role of PGE2 in the activation of renal sensory nerves was further studied by comparing the release of substance P produced by norepinephrine at 10 pM in the absence and presence of PGE2, 0.03 µM, a subthreshold concentration for substance P release (21) (n = 6). Ipsilateral and contralateral pelvises were incubated in HEPES buffer containing indomethacin, 0.14 mM, throughout the experiment to minimize the influence of endogenous PG synthesis. In addition, the contralateral pelvis was exposed to PGE2 throughout the experiment, starting from 10 min before the experimental period.
In additional experiments (n = 8), ipsilateral and contralateral pelvises were exposed to PGE2, 0.03 µM, as described above. In these experiments, the contralateral pelvis was also exposed to L-161,982, 5 µM, throughout the experiment. During the experimental period, the ipsilateral and contralateral pelvises were exposed to norepinephrine, 10 pM.
Group IV (in vitro): effects of 1- and 2-adrenoceptor antagonists on norepinephrine-induced release of substance P in the presence of PGE2.
Ipsilateral and contralateral pelvises were incubated in HEPES/indomethacin buffer containing PGE2, 0.03 µM, as detailed above. In addition, the contralateral pelvises were exposed to prazosin at 0.5 µM (n = 10) or 5 µM (n = 8) or rauwolscine, 0.1 µM (n = 8). During the experimental period, the ipsilateral and contralateral pelvises were exposed to norepinephrine, 2 pM, and 10 pM, in the experiments examining the effects of rauwolscine and prazosin, respectively.
Drugs
L-161,982 and L-161,983 were gifts from Merck Frosst Canada, Center for Therapeutic Research (Kirkland Quebec, Canada). Substance P antibody (IHC 7451) was acquired from Peninsula Laboratories (San Carlos, CA), and PGE2 was obtained from Cayman Chemicals (Ann Arbor, MI). All other reagents/chemicals were from Sigma Chemicals (St. Louis, MO), unless otherwise stated. Norepinephrine was dissolved in 0.1% ascorbic acid in incubation buffer or 0.15 M NaCl. Indomethacin was dissolved together with Na2CO3 (2:1 weight ratio) in incubation buffer or 0.15 M NaCl. All other agents were dissolved in incubation buffer (in vitro studies) or 0.15 M NaCl (in vivo studies).
Analytical Procedures
Substance P and PGE2 in the incubation medium were measured by ELISA, as previously described in detail (18, 21, 23–27).
Statistical analysis.
IN VIVO.
Increases in ERSNA, ARNA, mean arterial pressure (MAP) and heart rate (HR) produced by placing the tail in 49°C water were evaluated by calculating the area under the curve of each parameter vs. time, with baseline being the average value of each control period. Likewise, the ARNA responses to norepinephrine were calculated as the area under the curve of ARNA vs. time. Friedman two-way ANOVA and shortcut ANOVA were used to determine the effects of heat and norepinephrine on the various parameters within each rat.
IN VITRO.
The release of substance P and PGE2 during the experimental period was compared with the substance P and PGE2 release during the control and recovery periods using Friedman two-way ANOVA and shortcut ANOVA. The Wilcoxon matched-pairs signed-rank test was used to compare the norepinephrine-induced increases in renal pelvic release of substance P and PGE2 between the ipsilateral and contralateral pelvises. A significance level of 5% was chosen. Data in text and figures are expressed as means ± SE (48, 54).
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RESULTS |
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Localization of NE-t and CGRP in renal pelvic tissue. NE-t-immunoreactive (ir) fibers were mainly localized in the pelvic muscle layer (Fig. 1A). In contrast, there were numerous CGRP-ir fibers both in the pelvic muscle layer and in the subepithelial connective tissue (Fig. 1B). Double-labeling experiments showed that the CGRP-ir and NE-t-ir fibers often overlapped and were in close contact in the pelvic muscle layer (Fig. 1C). The close anatomical relationship between the NE-t-ir and CGRP-ir fibers was further evaluated by confocal microscopy, which showed that these two fiber populations were adjacent but separate from each other and in many cases were intertwined (Fig. 1D).
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1- or 2-adrenoceptor antagonist on the ARNA responses to reflex increases in ERSNA.
Placing the rat's tail in 49°C water (hot tail) resulted in reversible increases in MAP, ERSNA, ARNA (Fig. 2) and HR (not shown). Renal pelvic perfusion with prazosin reduced the ARNA response without altering the ERSNA response to hot tail (Fig. 3A). The duration of the ARNA and ERSNA responses, before vs. after prazosin, were ARNA: 197 ± 25 vs. 38 ± 20 s (P < 0.01) and ERSNA: 250 ± 40 vs. 211 ± 40 s. Conversely, renal pelvic perfusion with rauwolscine enhanced the ARNA response and slightly reduced the ERSNA response to hot tail (Fig. 3B). The duration of the ARNA and the ERSNA responses, before vs. after rauwolscine, were ARNA: 195 ± 19 vs. 281 ± 27 s (P < 0.01) and ERSNA: 225 ± 32 vs. 219 ± 39 s. Before renal pelvic administration of prazosin and rauwolscine, hot tail increased MAP, 3,670 ± 690 and 7,030 ± 870 (AUC, mmHg/s), respectively, and HR 21,670 ± 3,450 and 37,870 ± 5,740 (AUC, beats/min·s), respectively. These MAP and HR responses to hot tail were unaltered by the two agents.
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1- or 2-adrenoceptor antagonist on the norepinephrine-induced increases in ARNA, substance P, and PGE2.
On the basis of the results in group I, we hypothesized that the increased ERSNA produced by hot tail increased ARNA by a release of norepinephrine, which modulated ARNA by activating 1- and/or 2-adrenoceptors. Therefore, we examined whether renal pelvic administration of norepinephrine increased ARNA, and if so, if the norepinephrine-induced ARNA responses were similarly modulated by prazosin and rauwolscine as the ERSNA-induced increase in ARNA. To examine whether pre- or postsynaptic mechanisms were involved in the increased ARNA, we examined the effects of renal pelvic administration of norepinephrine on the activation of renal sensory nerves in vitro.
IN VIVO.
Renal pelvic administration of norepinephrine, 10 pM, resulted in a reversible increase in ARNA in the absence of changes in MAP and HR. The increase in ARNA produced by norepinephrine was reversibly reduced by prazosin and enhanced by rauwolscine (Fig. 4, A and B). Basal MAP, 117 ± 2 and 113 ± 4 mmHg, and HR, 312 ± 18 and 314 ± 21 beats/min, remained unaltered throughout the experiments in the two groups.
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1- and 2-adrenoceptor antagonists on norepinephrine-induced release of substance P in the presence of PGE2.
In view of the results in group IIIc, we hypothesized that PGE2 plays an important facilitatory role in the activation of renal sensory nerves in response to norepinephrine-induced activation of -adrenoceptors. We tested this theory by examining the effects of prazosin and rauwolscine on the norepinephrine-induced release of substance P in the presence of a concentration of PGE2 which is subthreshold for substance P release. Similar to the results in group IIIc, norepinephrine, 10 pM, resulted in a reversible increase in substance P release in the presence of PGE2, 0.03 µM, in the incubation bath (Table 4). The norepinephrine-induced substance P release in the presence of PGE2, 0.03 µM, was reduced by 0.5 µM prazosin and blocked by 5 µM prazosin in the incubation bath. Further studies showed that rauwolscine enhanced the release of substance P produced by norepinephrine, 2 pM, in the presence of PGE2 in the bath (Table 5).
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DISCUSSION |
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1-adrenoceptor antagonist blocked the ARNA response without altering the increases in ERSNA, MAP, or HR produced by hot tail. Conversely, renal pelvic administration of an 2-adrenoceptor antagonist enhanced the ARNA response and slightly reduced the ERSNA response without altering the MAP and HR responses to hot tail. Likewise, renal pelvic administration of norepinephrine resulted in increased ARNA and increased renal pelvic release of PGE2 and substance P that were reduced and enhanced by renal pelvic administration of prazosin and rauwolscine, respectively. Renal pelvic administration of PGE2 resulted in an enhancement of the norepinephrine-induced substance P release that was blocked by prazosin and enhanced by rauwolscine. Indomethacin blocked the norepinephrine-induced increases in renal pelvic release of PGE2, substance P, and ARNA. Taken together, these data suggest that the interaction between ERSNA and ARNA is mediated by norepinephrine modulating the release of substance P by acting on postsynaptic 1- and 2-adrenoceptors on or close to the renal sensory nerve endings. The norepinephrine-induced increase in renal pelvic PGE2 synthesis/release contributes to the mechanisms involved in the interaction between ERSNA and ARNA by enhancing the activation of postsynaptic 1-adrenoceptors. CGRP-ir and substance P-ir nerve fibers are colocalized in the renal pelvic wall (25) demonstrating that an antibody for either neuropeptide may be used as a marker for sensory nerves in the pelvic wall. The NE-t- and CGRP-ir fibers were often running in the same nerve bundles in the muscle layer of the pelvis, and initial inspection suggested that the two markers may be present in the same nerves. However, using confocal microscopy with high resolution, it was evident that the NE-t- and CGRP-ir nerves were separate fibers, often intertwined. On the other hand in the subepithelial connective tissue, almost all nerve fibers were only CGRP-positive, with some fibers reaching into the uroepithelial layer (26). Although the current study was not designed to allow measurements of the distance between adjacent NE-t- and CGRP-ir fibers, an estimate of the average distance showed it being <0.01 µm based on confocal images. Throughout the kidney, many NE-t-ir fibers were found in the smooth muscle layer of the renal vasculature and close to the basolateral side of various nephron segments (not shown), a similar distribution as previously shown for norepinephrine-containing nerve fibers (2).
Reflex increases in ERSNA increase ARNA.
In agreement with our previous studies (30), hot tail produced reflex increases in ERSNA, MAP, and HR. Reflex increases in ERSNA also increased ARNA. It is unlikely that the ERSNA-induced increases in ARNA were related to the changes in systemic hemodynamics because the increases in ARNA produced by hot tail were blocked by renal pelvic administration of prazosin or prior renal denervation (30), neither intervention altering the increases in MAP or HR. Also, hot tail does not change renal hemodynamics (30), further supporting our hypothesis that increases in ARNA are directly related to the increases in ERSNA activating 1-adrenoceptors. Further studies showed that renal pelvic administration with rauwolscine enhanced the ARNA response to hot tail. Taken together, these findings suggest that reflex-induced stimulation of ERSNA releases norepinephrine with a resultant increase in ARNA, which is dependent on the activation of stimulatory 1- and inhibitory 2-adrenoceptors.
Mechanisms involved in the interaction between ERSNA and ARNA.
Similarly to reflex renal nerve stimulation, renal pelvic administration of norepinephrine resulted in increases in ARNA that were blocked by prazosin and enhanced by rauwolscine. Because the norepinephrine-induced increases in ARNA were not associated with any changes in systemic hemodynamics, these data support our hypothesis that the reflex ERSNA-induced changes in ARNA are due to ERSNA-induced release of norepinephrine activating 1- and 2-adrenoceptors located on the peripheral renal nerve fibers. Regarding the concentration of norepinephrine used in the current study, 10 pM, it is noteworthy that it is lower than urinary norepinephrine concentration in anesthetized and conscious rats, being in the range of 2–4 pmol/min (e.g., 17, 49). Although, there is little information on renal pelvic norepinephrine concentration, whole rat kidney norepinephrine concentration is less than 1 pmol/mg tissue weight (41). It may be argued that the norepinephrine-induced increase in ARNA may, at least in part, be due to renal pelvic muscle contractions. Although, it is possible that reflex-induced ERSNA may elicit renal pelvic muscle contractions (6, 45), which could contribute to activation of mechanosensory nerves in the muscle layer, it is unlikely that the norepinephrine-induced increases in ARNA were related to pelvic muscle contractions, since norepinephrine was administered at a concentration that is much lower than that required for renal pelvic and/or ureteral muscle contractions (e.g., 13, 45).
Studies showing the expression of 1-adrenoceptor mRNA in dorsal root ganglia (DRG) (59) and phenylephrine-induced increases in intracellular calcium and depolarization of neurons in DRG (55) suggest the presence of 1-adrenoceptors on sensory nerve fibers. There is also evidence in the literature for the concept of 1-adrenoceptors being involved in the activation of peripheral sensory nerve nerves. It is well known that sympathetic nerve activity enhances pain and activation of 1-adrenoceptors contributes to neurogenic inflammation (15, 58, 59). Studies in humans showed that norepinephrine, released by subcutaneous administration of tyramine, produced an increase in capsaicin-induced pain that was reduced by a local administration of an -adrenoceptor antagonist (7). Likewise, studies in rats showed that local administration of an 1-adrenoceptor antagonist reduced the increases in central afferent nerve activity produced by intradermal injection of capsaicin (58).
In addition to the well-known location of 2-adrenoceptors on presynaptic sympathetic nerve terminals, whose activation results in inhibition of further release of norepinephrine (51, 52), there is also evidence for 2-adrenoceptors located on sensory neurons in DRG (5, 47). Functional evidence for 2-adrenoceptors on peripheral sensory nerve endings has been demonstrated in various organs. Studies of hypoxia-induced activation of the carotid sinus showed enhanced activation following sympathectomy, which was further enhanced by the addition of an 2-adrenoceptor antagonist (42). Also, electrically induced nonadrenergic noncholinergic iris sphincter contractions were modulated by 2-adrenoceptor mediated inhibition of sensory neurotransmitter release (10). Moreover, capsaicin-induced bronchoconstriction was reduced by 2-adrenoceptor agonists (33).
From our in vivo studies examining the effects of norepinephrine on ARNA in a sympathetically innervated kidney preparation, it is difficult to deduct whether the norepinephrine-induced changes in ARNA are related to pre- or postsynaptic mechanisms. Therefore, we turned to the isolated renal pelvic wall preparation to examine whether norepinephrine modulates substance P release by activating 1- and 2-adrenoceptors on the renal sensory nerves. The data show that norepinephrine induced a reversible release of substance P that was suppressed by prazosin and enhanced by rauwolscine in the bath. Because the isolated renal pelvic wall preparation is sympathetically denervated, these studies provide support for our hypothesis that norepinephrine modulates ARNA by activating 1- and 2-adrenoceptors on or close to the renal pelvic sensory nerve fibers.
In agreement with the current studies are the findings in an isolated bladder strip preparation, which showed that phenylephrine resulted in a release of substance P from the isolated bladder strip, which was blocked by an 1-adrenoceptor antagonist (55). Furthermore, studies in an isolated lung preparation (33) and the mesenteric vascular bed (53) showed that the release of CGRP produced by bradykinin and endotoxin, respectively, was reduced by an 2-adrenoceptor agonist. These studies taken together with the current studies support the notion that norepinephrine modulates ARNA by influencing the release of substance P via activation of 1- and 2- adrenoceptors on or close to the renal sensory nerve fibers.
Although there is considerable evidence for rauwolscine being a serotonin receptor antagonist (e.g., 14), in addition to an 2-adrenoceptor antagonist, it is unlikely that activation of serotonin receptors on the renal sensory nerve terminals contributed to the effects produced rauwolscine in the current studies. There is little evidence in the current literature for serotonergic nerve fibers and/or 5-hydroxytryptamine (HT) receptors in the renal pelvic wall. Furthermore, our studies showed that the norepinephrine-induced increases in substance P in vitro and ARNA in vivo produced by rauwolscine were achieved at norepinephrine concentrations far below those required for release of 5-HT, as demonstrated in in vitro studies in the pineal gland (1).
Role of PGE2 in the norepinephrine-induced modulation of renal sensory nerves. Because our previous studies, together with studies by others, have shown an important role for PGE2 in the direct activation and/or sensitization of sensory nerves (11, 16, 23, 24, 26, 27, 37, 40), we examined whether PGE2 contributed to the norepinephrine-induced activation of renal pelvic sensory nerves. Our initial studies showed that norepinephrine administered to the isolated renal pelvic wall preparation resulted in a reversible release of PGE2 into the incubation bath. Indomethacin lowered baseline PGE2 release and inhibited the norepinephrine-induced release of PGE2 and substance P. These in vitro findings were supported by our in vivo studies showing that renal pelvic perfusion with indomethacin blocked the ARNA response to norepinephrine, suggesting that norepinephrine increased PGE2 synthesis which, in turn, contributed to activation of renal sensory nerves. Comparing the concentrations of norepinephrine required to activate renal sensory nerves in vivo and in vitro showed that a 25-fold higher concentration was required in vitro vs. in vivo. 250 vs. 10 pM. We hypothesized that the increased responsiveness of the renal sensory nerves in vivo was related to greater renal PGE2 synthesis in the anesthetized surgically operated rat compared with the in vitro preparation (42). Our subsequent in vitro studies confirmed this view. Adding PGE2 to the incubation bath at a concentration that is subthreshold for substance P release lowered the concentration of norepinephrine required to elicit substance P release from 250 pM to 10 pM, that is, a similar concentration as that producing increases in ARNA in vivo.
PGE receptors have been classified into four general subtypes: EP1, EP2, EP3, and EP4 based on cloning and pharmacological interventions (4, 36). Our previous studies have shown that PGE2 contributes to the activation of renal mechanosensory nerves by activating EP4 receptors on or close to the renal sensory pelvic nerve fibers (26). PGE2-induced activation of EP4 receptors leads to activation of adenylyl cyclase and the cAMP/PKA pathway, eventually leading to an increase in intracellular calcium and substance P release (18, 23). The current studies showing that the PGE2-mediated enhancement of the norepinephrine-induced substance P release and increase in ARNA was blocked by an EP4 receptor antagonist further support the hypothesis that PGE2 enhances the responsiveness of the renal sensory nerves to norepinephrine. Regarding the possible mechanisms involved in the norepinephrine-induced PGE2 synthesis/release, our current studies showing that prazosin reduces and rauwolscine enhances the norepinephrine-induced PGE2 release suggest an important role for 1- and 2- adrenoceptors on or close to the renal sensory nerves in modulating PGE2 synthesis. The findings that prazosin blocked the norepinephrine-induced substance P release in the presence of PGE2 together with the data showing that rauwolscine in the presence of PGE2 lowered the concentration of norepinephrine required to increase substance P release suggest that PGE2 facilitates the norepinephrine-mediated activation of 1-adrenoceptors.
The mechanisms involved in the 1-adrenoceptor-mediated release of PGE2 and PGE2-mediated sensitization of -adrenoceptors are beyond the scope of the current studies. However, numerous studies in cultured DRGs have established that PGE2-induced activation of the cAMP signaling pathway results in PKA-mediated phosphorylation of various proteins leading to modulation of the activity of ion channels with a resultant increased excitability of sensory neurons and neuropeptide release (8, 12, 50, 56). We speculate that similar mechanisms are involved in the PGE2-induced sensitization of 1-adrenoceptors in the current studies because our previous studies have shown an important role for the cAMP/PKA pathway in the PGE2-induced release of substance P and activation of renal mechanosensory nerves (23).
The data showing that norepinephrine via increasing PGE2 synthesis/release leads to sensitization of 1-adrenoceptors suggest the interesting hypothesis that the sensory nerves adjust their responsiveness to alterations in sympathetic nervous system activity by synthesizing PGs. Support for this hypothesis is derived from our previous studies showing the presence of COX-2 in sensory nerve bundles in the renal pelvic wall (24) and EP4 receptors on numerous renal pelvic sensory nerve fibers (26). Also, studies in isolated DRG show that sensory neurons exposed to arachidonic acid synthesize PGE2-like products (57).
There is considerable evidence for norepinephrine increasing PGE2 release in both central and peripheral neural tissue (e.g., 35, 44). Although not examined in the current studies, it is possible that the increase in ARNA produced by reflex increases in ERSNA is also modulated by PGE2 limiting the release of norepinephrine via stimulation of presynaptic EP3 receptors.
In summary, the present study shows that the interaction between ERSNA and ARNA is associated with a close anatomical relationship between efferent sympathetic and sensory renal nerves in the renal pelvic wall. Our data suggest that reflex-induced increases in ERSNA result in increases in norepinephrine that activate 1- and 2-adrenoceptors located on or close to the renal pelvic sensory nerve fibers (Fig. 10). Activation of 1-adrenoceptors increases and activation of 2-adrenoceptors decreases renal pelvic PGE2 synthesis/release resulting in a net increase in PGE2 release during the current experimental conditions. Increased PGE2 synthesis/release increases and/or facilitates the activation of the renal sensory nerves via increased release of substance P. The increased ARNA exerts a negative feedback control of ERSNA via activation of the renorenal reflexes in the overall goal of maintaining a low level of ERSNA, so as to limit sodium retention.
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
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1, 
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P < 0.01 vs. the RNA response in the presence of vehicle.
P < 0.05, 
