Role of intracellular calcium in the angiotensin II-mediated tyrosine phosphorylation and dephosphorylation of PLC-γl

Angiotensin II induces the rapid temporal tyrosine phosphorylation and activation of phospholipase C-γ1 (PLC-γl) and the elevation of intracellular calcium levels. To investigate the relationship of these intracellular signaling events, rat aortic smooth muscle cells were treated with the calcium chelator BAPTA-AM, the calcium channel blocker verapamil, the intracellular calcium antagonist TMB-8, and the calcium ionophore ionomycin. The effects of these agents on PLC-γl tyrosine phosphorylation were then measured. We found that treatment of these cells with the calcium inhibitors augmented the basal level of PLC-γl tyrosine phosphorylation, without changing the peak level of tyrosine phosphorylation induced by angiotensin II. The rapid dephosphorylation of PLC-γ1 that follows angiotensin II stimulation was prevented by these calcium antagonists. In contrast, angiotensin II-induced tyrosine phosphorylation of PLC-γl was inhibited by ionomycin. These results suggest that the angiotensin II-induced tyrosine phosphorylation of PLC-γ1 is calcium-independent, while the dephosphorylation is calcium-dependent.