Influence of ANG II on cytoplasmic sodium in cultured rabbit nonpigmented ciliary epithelium

Angiotensin (ANG) II receptors have been reported in the nonpigmented ciliary epithelium (NPE) of the eye. In cultured NPE, we found ANG II caused a dose-dependent rise of cytoplasmic sodium. The sodium increase was inhibited by the AT₁-AT₂ receptor antagonist saralasin (IC₅₀ = 3.7 nM) and the AT₁ antagonist losartan (IC₅₀ = 0.6 nM) but not by the AT₂ antagonist PD-123319. ANG II also caused a dose-dependent increase in the rate of ouabain-sensitive ⁸⁶Rb uptake. The ANG II-induced cell sodium increase and ⁸⁶Rb uptake increase were reduced by dimethylamiloride (DMA; 10 μM). On the basis of this finding, we propose that Na⁺/H⁺ exchange is stimulated by ANG II. Simultaneously, ANG II appears to inhibit H⁺-ATPase-mediated proton export. Thus Ang II (10 nM) did not alter the baseline cytoplasmic pH (pH_i) but reduced pH_i in cells that were also exposed to 10 μM DMA. Consistent with the notion of H⁺-ATPase inhibition in ANG II-treated NPE, bafilomycin A₁ (100 nM) (BAF) and ANG II were both observed to suppress the pH_i increase that occurs upon exposure to a mixture of epinephrine (1 μM) and acetylcholine (10 μM) and the pH_i increase elicited by depolarization. In ATP hydrolysis measurements, H⁺-ATPase activity (bafilomycin A₁-sensitive ATP hydrolysis) was reduced significantly in cells that had been pretreated 10 min with 10 nM ANG II. In summary, these studies suggest that ANG II causes H⁺-ATPase inhibition and an increase of cell sodium due to activation of Na⁺/H⁺ exchange.