Increased At1 Receptors and Giα Expression Contribute to Greater Angiotensin Ii-Mediated Stimulation of Na,H-Exchanger in Proximal Tubules of Hypertensive Obese Rats
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/HCO3 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. [125I]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.