Phosphatidylinositol 3,5-bisphosphate increases intracellular free Ca2+ in arterial smooth muscle cells and elicits vasocontraction

Silswal N, Parelkar NK, Wacker MJ, Brotto M, Andresen J. Phosphatidylinositol 3,5-bisphosphate increases intracellular free Ca2+ in arterial smooth muscle cells and elicits vasocontraction. Am J Physiol Heart Circ Physiol 300: H2016-H2026, 2011. First published March 18, 2011; doi: 10.1152/ajpheart.01011.2010.-Phosphoinositide (3,5)-bisphosphate [PI(3,5) P-2] is a newly identified phosphoinositide that modulates intracellular Ca2+ by activating ryanodine receptors (RyRs). Since the contractile state of arterial smooth muscle depends on the concentration of intracellular Ca2+, we hypothesized that by mobilizing sarcoplasmic reticulum (SR) Ca2+ stores PI(3,5) P-2 would increase intracellular Ca2+ in arterial smooth muscle cells and cause vasocontraction. Using immunohistochemistry, we found that PI(3,5) P-2 was present in the mouse aorta and that exogenously applied PI(3,5) P-2 readily entered aortic smooth muscle cells. In isolated aortic smooth muscle cells, exogenous PI(3,5) P-2 elevated intracellular Ca2+, and it also contracted aortic rings. Both the rise in intracellular Ca2+ and the contraction caused by PI(3,5) P-2 were prevented by antagonizing RyRs, while the majority of the PI(3,5) P-2 response was intact after blockade of inositol (1,4,5)-trisphosphate receptors. Depletion of SR Ca2+ stores with thapsigargin or caffeine and/or ryanodine blunted the Ca2+ response and greatly attenuated the contraction elicited by PI(3,5) P2. The removal of extracellular Ca2+ or addition of verapamil to inhibit voltage-dependent Ca2+ channels reduced but did not eliminate the Ca2+ or contractile responses to PI(3,5) P-2. We also found that PI(3,5) P-2 depolarized aortic smooth muscle cells and that LaCl3 inhibited those aspects of the PI(3,5) P-2 response attributable to extracellular Ca2+. Thus, full and sustained aortic contractions to PI(3,5) P-2 required the release of SR Ca2+, probably via the activation of RyR, and also extracellular Ca2+ entry via voltage-dependent Ca2+ channels.