IP3 Constricts Cerebral Arteries via IP3 Receptor–Mediated TRPC3 Channel Activation and Independently of Sarcoplasmic Reticulum Ca Release

Vasoconstrictors that bind to phospholipase C–coupled receptors elevate inositol-1,4,5-trisphosphate (IP3). IP3 is generally considered to elevate intracellular Ca concentration ([Ca ]i) in arterial myocytes and induce vasoconstriction via a single mechanism: by activating sarcoplasmic reticulum (SR)-localized IP3 receptors, leading to intracellular Ca release. We show that IP3 also stimulates vasoconstriction via a SR Ca 2 release–independent mechanism. In isolated cerebral artery myocytes and arteries in which SR Ca was depleted to abolish Ca release (measured using D1ER, a fluorescence resonance energy transfer–based SR Ca indicator), IP3 activated 15 pS sarcolemmal cation channels, generated a whole-cell cation current (ICat) caused by Na influx, induced membrane depolarization, elevated [Ca ]i, and stimulated vasoconstriction. The IP3-induced ICat and [Ca 2 ]i elevation were attenuated by cation channel (Gd , 2-APB) and IP3 receptor (xestospongin C, heparin, 2-APB) blockers. TRPC3 (canonical transient receptor potential 3) channel knockdown with short hairpin RNA and diltiazem and nimodipine, voltage-dependent Ca channel blockers, reduced the SR Ca release–independent, IP3-induced [Ca 2 ]i elevation and vasoconstriction. In pressurized arteries, SR Ca depletion did not alter IP3-induced constriction at 20 mm Hg but reduced IP3-induced constriction by 39% at 60 mm Hg. [Ca 2 ]i elevations and constrictions induced by endothelin-1, a phospholipase C–coupled receptor agonist, were both attenuated by TRPC3 knockdown and xestospongin C in SR Ca -depleted arteries. In summary, we describe a novel mechanism of IP3-induced vasoconstriction that does not occur as a result of SR Ca release but because of IP3 receptor–dependent ICat activation that requires TRPC3 channels. The resulting membrane depolarization activates voltage-dependent Ca channels, leading to a myocyte [Ca ]i elevation, and vasoconstriction. (Circ Res. 2008;102:1118-1126.)