Abstract
Phosphoinositide (3,5)-bisphosphate [PI(3,5)P(2)] is a newly identified phosphoinositide that modulates intracellular Ca(2+) by activating ryanodine receptors (RyRs). Since the contractile state of arterial smooth muscle depends on the concentration of intracellular Ca(2+), we hypothesized that by mobilizing sarcoplasmic reticulum (SR) Ca(2+) stores PI(3,5)P(2) would increase intracellular Ca(2+) 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 Ca(2+), and it also contracted aortic rings. Both the rise in intracellular Ca(2+) 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 Ca(2+) stores with thapsigargin or caffeine and/or ryanodine blunted the Ca(2+) response and greatly attenuated the contraction elicited by PI(3,5)P(2). The removal of extracellular Ca(2+) or addition of verapamil to inhibit voltage-dependent Ca(2+) channels reduced but did not eliminate the Ca(2+) 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 LaCl(3) inhibited those aspects of the PI(3,5)P(2) response attributable to extracellular Ca(2+). Thus, full and sustained aortic contractions to PI(3,5)P(2) required the release of SR Ca(2+), probably via the activation of RyR, and also extracellular Ca(2+) entry via voltage-dependent Ca(2+) channels.