Acidosis-induced protein tyrosine phosphorylation depends on Ca ²⁺ influx via voltage-dependent Ca²⁺ channels in SHR aorta

The contractile response to acidosis in isolated aorta from spontaneously hypertensive rat (SHR) depends upon tyrosine phosphorylation of phosphatidylinositol 3 kinase (PI3-kinase) and Ca²⁺ influx via voltage-dependent Ca²⁺ channels (VDCC). In this study, verapamil, a VDCC inhibitor, was shown to markedly inhibit acidic pH-induced contraction, whereas the residual contraction in the presence of verapamil was unaffected by the PI3-kinase inhibitor, 2-(4-morpholinyl)-8-phenyl-4H-1-benzopyran-4-one hydrochloride (LY-294002). Interestingly, the LY-294002-insensitive component of contraction was further inhibited by verapamil in the presence of LY-294002. Western blotting revealed that acidosis stimulated tyrosine phosphorylation of p85, which was abolished when tissues were pretreated with tyrphostin 23, a tyrosine kinase inhibitor, verapamil or EGTA. In fura-2-loaded aortic strips, acidosis induced a rise in intracellular Ca²⁺ ([Ca²⁺] _i) that was partially inhibited by LY-294002. The residual increase in [Ca²⁺]_i caused by acidosis in the presence of LY-294002 was abolished by verapamil. These findings suggest that acidosis-induced Ca²⁺ influx through VDCC is the upstream event leading to the tyrosine phosphorylation of PI3-kinase, which in turn contributes to the enhancement of Ca²⁺ entry to some extent in SHR aorta.