Involvement of a GTP-binding protein in stimulating action of angiotensin II on calcium channels in vascular smooth muscle cells

Abstract

The possible involvement of a GTP-binding protein in the regulation of Ca2+ channels by angiotensin II (Ang II) in vascular muscle cells was investigated by the whole-cell voltage-clamp method. Single cells were freshly isolated from guinea pig portal vein. The pipette solution contained high Cs+ to inhibit K+ currents and thereby isolate the Ca2+ channel current. Ba2+ (2 mM) was in the bath solution as a charge carrier for the Ca2+ channel. Application of Ang II (0.1-100 nM) produced an increase in peak amplitude of the Ba2+ current, with a shift of the current-voltage curve in the negative direction. These effects were inhibited by pretreatment with an antagonist of the Ang II receptor, [Sar1,Ile8]-Ang II. Presence of 0.1 mM gTP in the pipette solution stabilized the Ang II action, but 0.3-1.0 mM GDP-β-S and 1.0 mM GTP-γ-S inhibited it. GTP-γ-S alone produced a slowly progressing increase in the basal (unstimulated) current amplitude. Preincubation of muscle tissues with pertussis toxin (1 μg/ml, for up to 6 hours at 36°C) or intracellular application of preactivated pertussis toxin (1 μg/ml) plus NAD (1 mM) did not inhibit the Ang II action. Cholera toxin (10 μg/ml) also had no effect on the Ang II action. These results suggest that the Ang II stimulation of Ca2+ channels in smooth muscle of guinea pig portal vein may be mediated by a G protein that is insensitive to both pertussis toxin and cholera toxin.