Angiotensin II negatively modulates L-type calcium channels through a pertussis toxin-sensitive G protein in adrenal glomerulosa cells

In bovine adrenal glomerulosa cells, angiotensin II and extracellular K+ st imulate aldosterone secretion in a calcium-dependent manner. In these cells , physiological concentrations of extracellular potassium activate both T-t ype (low threshold) and L-type (high threshold) voltage-operated calcium ch annels. Paradoxically, the cytosolic calcium response to 9 mM K+ is inhibit ed by angiotensin II. Because K+-induced calcium changes observed in the cy tosol are almost exclusively due to L-type channel activity, we therefore s tudied the mechanisms of L-type channel regulation by angiotensin II. Using the patch-clamp method in its perforated patch configuration, we observed a marked inhibition (by 63%) of L-type barium currents in response to angio tensin II. This effect of the hormone was completely prevented by losartan, a specific antagonist of the AT(1) receptor subtype, Moreover, this inhibi tion was strongly reduced when the cells were previously treated for I nigh t with pertussis toxin. An effect of pertussis toxin was also observed on t he modulation by angiotensin II of the K+ (9 mm)-induced cytosolic calcium response in fura-2-loaded cells, as well as on the angiotensin II-induced a ldosterone secretion, at both low (3 mM) and high (9 mM) K+ concentrations. Finally, the expression of both G(o) and G(i) proteins in bovine glomerulo sa cells was detected by immunoblotting, Altogether, these results strongly suggest that in bovine glomerulosa cells, a pertussis toxin-sensitive G pr otein is involved in the inhibition of L-type channel activity induced by a ngiotensin II.