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1 Department of Physiology and Pharmacology, Oregon Health & Science University, Portland, Oregon 97201; and 2 Department of Pharmacology and Experimental Therapeutics, Loyola University Chicago, Stritch School of Medicine, Maywood, Illinois 60153
Acute increases in osmolality suppress
renal sympathetic nerve activity (RSNA). However, it is not known
whether prolonged physiological increases in plasma osmolality
chronically inhibit RSNA. To address this hypothesis, mean arterial
blood pressure (MAP), heart rate (HR), and RSNA were measured during
acute normalization of plasma osmolality in conscious rats made
hyperosmotic by 48 h of water deprivation. Water deprivation
significantly elevated MAP (120 ± 1 vs. 114 ± 3 mmHg,
P < 0.05) and plasma osmolality (306 ± 1 vs. 293 ± 1 mosmol/kgH20, P < 0.01). When plasma
osmolality was subsequently lowered to normal (
17 ± 1 mosmol/kgH20) with a 2-h (0.12 ml/min) infusion of 5%
dextrose in water (5DW), MAP decreased (11 ± 1 mmHg), and RSNA
increased (25 ± 10% baseline). To assess the role of circulating
vasopressin in these changes, rats were pretreated with a
V1-vasopressin receptor antagonist before infusion of 5DW.
The antagonist lowered MAP (4 ± 1 mmHg) and raised RSNA
(31 ± 3% baseline) and HR (25 ± 5 beats/min) in water-deprived rats (all changes P < 0.05). However,
V1-vasopressin receptor blockade did not increase RSNA or
HR independently of baroreflex responses to decreases in arterial
pressure. After V1 blockade, infusion of 5DW lowered blood
pressure (8 ± 1 mmHg) but did not further affect HR or RSNA. An
isotonic saline infusion that produced the same volume expansion as 5DW
lowered MAP (5 ± 2 mmHg) and HR (68 ± 2 beats/min) but
had no effect on osmolality or RSNA in water-deprived rats. Finally,
5DW infusion had negligible effects in water-replete animals. In
conclusion, these results fail to support the hypothesis that sustained
increases in plasma osmolality, either directly or via increased
vasopressin, tonically suppress RSNA.
water deprivation; vasopressin; arterial pressure; heart rate; RSNA
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