Functional coupling of human L-type Ca²⁺ channels and angiotensin AT(1A) receptors coexpressed in Xenopus laevis oocytes: Involvement of the carboxyl-terminal Ca²⁺ sensors

A human recombinant L-type Ca²⁺ channel (α(1C,77)) was coexpressed with the rat angiotensin AT(1A) receptor in Xenopus laevis oocytes. In oocytes expressing only α(1C,77) channels, application of human angiotensin II (1-10 μM) did not affect the amplitude or kinetics of Ba²⁺ currents (I(Ba)). In sharp contrast, in oocytes coexpressing α(1C,77) channels and AT(1A) receptors, application of 1 nM to 1 μM angiotensin gradually and reversibly inhibited I(Ba), without significantly changing its kinetics. The inhibitory effect of angiotensin on I(Ba) was abolished in oocytes that had been preincubated with losartan (an AT(1A) receptor antagonist) or thapsigargin or injected with 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'- tetraacetate, pertussis toxin, guanosine-5'-O-(2-thio)diphosphate, or heparin, suggesting that the recombinant α(1C) channels were regulated by angiotensin through G protein-coupled AT(1A) receptors via activation of the inositol trisphosphate-dependent intracellular Ca²⁺ release pathway. Consistent with this hypothesis, no cross-signaling occurred between the AT(1A) receptor and a splice variant of α(1C) lacking Ca²⁺ sensors (α(1C,86)). The data suggest that the regulation of recombinant L-type Ca²⁺ channels by angiotensin is mediated by inositol trisphosphate-induced intracellular Ca²⁺ release and occurs at the molecular motif responsible for the Ca²⁺induced inactivation of the channels.