Angiotensin II (Ang II) via the activation of AT1 receptors and subsequent stimulation of the tubular sodium transporters increases sodium and water reabsorption in the proximal tubule. An enhanced tubular action of Ang II is implicated in obesity related hypertension; however, the mechanism of such a phenomenon is unknown. Present study was designed to determine the AT1 receptor numbers and function in the proximal tubule of obese and lean Zucker rats. Obese Zucker rats were hypertensive and hyperinsulinemic. The plasma renin activity was similar in the lean and obese rats. Angiotensin II stimulated the Na,H-exchanger (NHE) activity in the proximal tubule, but the stimulatory response was markedly greater in obese than in lean rats. Similarly, Ang II caused greater inhibition in cAMP accumulation in the proximal tubule of obese compared to lean rats. The (125I]sar-Ang II binding revealed a 100% increase in the AT1 receptor number in the brush border membrane (BBM) of obese compared to lean rats. The Western blot analysis revealed a 36-51% increase in the Gi(alpha)1 and Gi(alpha)3 in the BBM of obese compared to lean rats. We conclude that increases in the AT1 receptor number and abundance of the Gi(alpha) on BBM may be responsible for the enhanced signaling and subsequent greater stimulation of NHE by Ang II in proximal tubules of obese rats. The greater stimulation of NHE by Ang II may contribute to the increased tubular sodium reabsorption and to the hypertension in obese Zucker rats.

Dopamine via the activation of D1-like receptors inhibits Na,K-ATPase and Na,H-exchanger and subsequently increases sodium excretion. We have previously reported that dopamine failed to inhibit Na,K-ATPase in the proximal tubules (PTs) of obese Zucker rats. The present study was designed to determine the effect of dopamine on Na,H-exchanger in PTs of lean and obese Zucker rats, and examine D1-like receptor-coupled signal transduction pathway mediating the inhibition of Na,H-exchanger. We found that dopamine inhibited Na,H-exchanger in the PTs of lean rats but this response was absent in obese rats. In brush border membranes, [3H]SCH 23390 binding revealed a approximately 45% reduction in D1-like receptor binding sites in obese compared to lean rats. Dopamine stimulated cAMP accumulation in PTs of lean but not in obese rats. Forskolin-mediated stimulation of cAMP was similar in lean and obese rats. Dopamine as well as forskolin and dibutyryl cAMP-mediated stimulation of protein kinase A (PKA) was reduced in PTs of obese compared to lean rats. The data suggest that reduction in D1-like receptor binding sites, defective coupling with signaling pathway and inability of PKA activation may be responsible for the failure of dopamine to inhibit Na,H-exchanger in PTs of obese rats. This phenomenon may contribute to an increase in sodium reabsorption and development of hypertension in obese Zucker rats.

P184 Angiotensin II (ang II) is a powerful stimulator of sodium and water reabsorption in the proximal tubules (PTs) through the activation of AT1 receptors and subsequent stimulation of sodium transporters, Na,H-exchanger (NHE), Na,K-ATPase and Na/HCO 3 co-transporter. AT1 receptors are linked to adenylyl cyclase via Gi proteins. Ang II is implicated in the development of obesity related hypertension, however the mechanism is unknown. Present study was designed to investigate the effects of ang II on NHE activity, cAMP accumulation in PTs and to measure AT1 receptors and the expression of Giαin brush border membranes (BBM) of obese and lean Zucker rats. Obese rats exhibited higher blood pressure (165±2/114±3 mmHg) when compared with lean rats (133±5/99±2 mmHg), a modest increase (12%) in blood glucose and a 6-fold increase in plasma insulin. Plasma renin activity was similar in lean and obese rats. Ang II (0.01-10 pM) stimulated NHE activity in PTs of lean and obese rats, however the stimulatory response was markedly greater in obese rats. Ang II (0.1-100 pM) inhibited cAMP accumulation in PTs of lean and obese rats, but the inhibitory effect was greater in obese rats. There was no difference in basal and forskolin-mediated stimulation of cAMP between lean and obese rats. [ 125 I]sar-ang II binding revealed 100% increase in AT1 receptor binding sites without a change in dissociation constant (Kd) to the ligand in BBM of obese compared to lean rats. Western blot analysis revealed 2-3-fold increase in Giα1&3 in BBM of obese compared to lean rats. We conclude that increased AT1 receptor binding sites and expression of Giαmay be responsible for greater ang II-mediated inhibition of cAMP and subsequent increased stimulation of NHE in PTs of obese rats. This phenomenon of increased stimulation of NHE by ang II may contribute to increased tubular sodium retention and development of hypertension in obese Zucker rats.

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