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Departments of 1 Pediatrics, 2 Physiology and Biophysics, and 4 Medicine, Georgetown University Medical Center, Washington, District of Columbia 20007; and 3 Department of Pediatrics, University of Michigan Medical Center, Ann Arbor, Michigan 48109
To
determine if the defective interactions among D1-like
receptors, G proteins, and Na+/H+ exchanger 3 (NHE3) are consequences of hypertension, we studied these interactions
in rats, before (2-3 wk) and after (12 wk) the establishment of
hypertension. To eliminate the confounding influence of second
messenger action on D1 receptor-NHE3 interaction, studies
were performed in renal brush-border membranes (BBM) devoid of
cytoplasmic second messengers. NHE3 activity increased with age in
Wistar-Kyoto (WKY) rats (3 wk = 1.48 ± 0.39, n = 13; 12 wk = 2.83 ± 0.15, n = 16, P < 0.05) but not in
spontaneously hypertensive rats (SHRs; 3 wk = 2.52 ± 0.37, n = 11; 12 wk = 2.81 ± 0.20, n = 16). D1 receptor protein tended to
decrease, whereas NHE3 protein tended to increase with age in both WKY
and SHRs. However, the inhibitory effect of a D1-like
agonist, SKF-81297, on NHE3 activity increased with age in WKY rats (3 wk =
40.7 ± 5.3%, n = 10, 12 wk =
58.7 ± 4.6%, n = 12, P < 0.05) but not in SHRs (3 wk =
27.6 ± 5.9%,
n = 11, 12 wk =
25.1 ± 3.2%,
n = 11). The decreased inhibitory effect of another
D1-like agonist, fenoldopam, on NHE3 activity in SHRs was
not caused by increased activity and binding of G
to NHE3 as has
been reported in young WKY rats. Gs
mediates, in part,
the inhibitory effect of D1-like agonists on NHE3 activity. In WKY rats, fenoldopam increased Gs
/NHE3 binding to the
same extent in 2-wk-old (1.5-fold, n = 4) and adult
(1.5-fold, n = 4) rats. In contrast, in SHRs,
fenoldopam decreased the amount of Gs
bound to NHE3 in
2-wk-old SHRs and had no effect in 4-wk-old and adult SHRs. These
studies indicate that the decreased inhibitory effect of
D1-like agonists on NHE3 activity in SHRs (compared with
WKY rats) precedes the development of hypertension. This may be caused,
in part, by a decreased interaction between Gs
and NHE3
in BBM secondary to impaired D1-like receptor function.
brush-border membrane; G protein; fenoldopam; sodium/hydrogen exchanger 3
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