Difference in water intake but not in renal sympathetic nerve activity in response to central salt-loading or angiotensin II in awake Dahl salt-sensitive and -resistant rats

Aquaporin-2 transcript is differentially regulated by dietary salt in Sprague-Dawley and Dahl SS/Jr rats

Aquaporin-2 (AQP-2) is a vasopressin-regulated water channel in the kidney collecting duct. AQP-2 transcript has been identified by transcriptional profiling of rat kidneys as being regulated by dietary salt. We compared renal AQP-2 transcript expression in Sprague-Dawley and Dahl salt-sensitive (SS/Jr) rats using real-time RT-PCR. Expression of AQP-2 transcript is 5-fold less (P<0.01) in the Sprague-Dawley and 3-fold greater in Dahl SS/Jr rats (P<0.01) on high versus basal NaCl diets. The AQP-2 coded sequence was identical in Sprague-Dawley and Dahl SS/Jr rats. The present results provide evidence that: (1)AQP-2 plays a role in salt adaptation and (2) regulation of aquaporin transcript expression by salt is altered in the Dahl SS/Jr rat.

Is osmolality a long-term regulator of renal sympathetic nerve activity in conscious water-deprived rats?

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/kgH2O, P < 0.01). When plasma osmolality was subsequently lowered to normal (-17 +/- 1 mosmol/kgH2O) 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.

Participation of arterial baroreceptors input and peripheral vasopressin in the suppression of renal sympathetic nerve activity induced by central salt loading in conscious rats

We examined whether renal sympathetic nerve activity (RSNA) is suppressed in response to intracerebroventricular (i.c.v.) administration of hypertonic saline (HS) in conscious rats. RSNA was suppressed by i.c.v. administration of HS (0.3 M, 0.67 M, and 1.0 M, 1 microl/min for 20 min) in a concentration-dependent manner, which was attenuated under pentobarbital anesthesia. To elucidate mechanisms responsible for central HS-induced decrease in RSNA, possible involvement of arterial baroreceptors and peripheral arginine vasopressin (AVP) secreted from the posterior pituitary gland was examined using sinoaortic denervated (SAD) rats and non-peptide vasopressin receptor antagonists. The maximum suppression of RSNA (-81.5 +/- 5.5%) in control rats was significantly attenuated to -32.5 +/- 6.7% in SAD rats and to -55.8 +/- 5.7% in rats pretreated with intravenous vasopressin V1 receptor antagonist, OPC-21268 (5 mg/kg, i.v.). However, in SAD rats, pretreatment with vasopressin V1 receptor antagonist did not further affect the RSNA inhibition induced by central salt loading. The results suggest that the suppression of RSNA during central salt loading is mainly dependent on the arterial baroreceptors input and the 'additive' role of peripheral vasopressin.