alpha-Adrenoceptor-mediated depletion of phosphatidylinositol 4, 5-bisphosphate inhibits activation of volume-regulated anion channels in mouse ventricular myocytes.

Volume-regulated anion channels (VRACs) play an important role in cell-volume regulation. alpha(1)-Adrenoceptor stimulation by phenylephrine (PE) suppressed the hypotonic activation of VRAC current in mouse ventricular cells and regulatory volume decrease (RVD) was also absent in PE-treated cells. We examined whether the effects of alpha(1)-adrenoceptor stimuli on VRAC current were modulated by phosphatidylinositol signalling. Whole-cell patch-clamp method was used to record the hypotonicity-induced VRAC current in mouse ventricular cells. RVD was analyzed by videomicroscopic measurement of cell images. The attenuation of VRAC current by PE was suppressed by alpha(1A)-adrenoceptor antagonists (prazosin and WB-4101), anti-G(q) protein antibody and a specific phosphoinositide-specific phospholipase C (PLC) inhibitor (U-73122), but not by antagonists for alpha(1B)-, alpha(1D)- or beta-adrenoceptor, or protein kinase C inhibitors. The inhibition of VRAC by PE was antagonized by intracellular excess phosphatidylinositol 4,5-bisphosphate (PIP(2)), while intracellular anti-PIP(2) antibody (PIP(2) Ab) inhibited the activation of VRAC currents. When cells were loaded with phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) with or without PIP(2) Ab, PE little affected the VRAC current. Extracellular m-3M3FBS (an activator of PLC) suppressed VRAC in the absence of PE, and this effect was reversed by intracellular excess PIP(2). Our results indicate that the stimulation of alpha(1A)-adrenoceptors by PE inhibited the activation of cardiac VRAC current via PIP(3) depletion brought about by PLC-dependent reduction of membrane PIP(2) level.