Low [Mg2+](o) induces contraction and [Ca2+](i) rises in cerebral arteries: roles of Ca2+, PKC, and PI3

Removal of extracellular Ca2+ concentration ([Ca2+](o)) and pretreatment of canine basilar arterial rings with either an antagonist of voltage-gated C a2+ channels (verapamil), a selective antagonist of the sarcoplasmic reticu lum Ca2+ pump [thapsigargin (TSG)], caffeine plus a specific antagonist of ryanodine-sensitive Ca2+ release (ryanodine), or a D-myo-inositol 1,4,5-tri sphosphate [Ins(1,4,5) P-3]-mediated Ca2+ release antagonist (heparin) mark edly attenuates low extracellular Mg2+ concentration ([Mg2+](o))-induced co ntractions. Low [Mg2+](o)-induced contractions are significantly inhibited by pretreatment of the vessels with Go-6976 [a protein kinase C-alpha (PKC- alpha)- and PKC-betaI-selective antagonist], bisindolylmaleimide I (Bis, a specific antagonist of PKC), and wortmannin or LY-294002 [selective antagon ists of phosphatidylinositol-3 kinases (PI3Ks)]. These antagonists were als o found to relax arterial contractions induced by low [Mg2+](o) in a concen tration-dependent manner. The absence of [Ca2+](o) and preincubation of the cells with verapamil, TSG, heparin, or caffeine plus ryanodine markedly at tenuates the transient and sustained elevations in the intracellular Ca2+ c oncentration ([Ca2+](i)) induced by low-[Mg2+](o) medium. Low [Mg2+](o)-pro duced increases in [Ca2+](i) are also suppressed markedly in the presence o f Go-6976, Bis, wortmannin, or LY-294002. The present study suggests that b oth Ca2+ influx through voltage-gated Ca2+ channels and Ca2+ release from i ntracellular stores [both Ins( 1,4,5) P3 sensitive and ryanodine sensitive] play important roles in low-[Mg2+](o) medium-induced contractions of isola ted canine basilar arteries. Such contractions are clearly associated with activation of PKC isoforms and PI3Ks.