Dependence of cycloplazonic-acid-induced muscle contractures on extracellular Ca2+

Inhibition of Ca2+ uptake by the sarcoplasmic reticulurn decreases cytosolic Ca2+ clearance and also triggers Ca2+ influx in response to Ca2+ store depletion. The role of extracellular Ca2+ in the contractures evoked by cyclopiazonic acid (CPA) and thapsigargin (TG), Ca2+ pump inhibitors, was assessed in mouse diaphragm. At 3-100 muM, CPA elicited a rapid-onset contracture followed by a large elevation of muscle tone, which corresponded temporally to the monophasic slow contracture evoked by TG (1-30 muM). Irrespective of the differences in profiles, contractures were prevented and inhibited by the removal of extracellular Ca2+, but not by nicardipine and SK&F96365, blockers of voltage-gated (L-type) and receptor-operated Ca2+ channels. Mn2+ and Ni2+ preferentially depressed the fast-phase contracture, whereas long-term pretreatment with LY294002, U73122, and 2-aminoethoxydiphenylborance, inhibitors of phosphatidylinositol kinase, phospholipase C, and inositol trisphosphate receptors, suppressed the slow-phase contracture. When contracture was inhibited, the twitch response remained augmented and prolonged by CPA and TG, indicating that the inhibition was not due to malfunction of the contractile apparatus. For preparations incubated in Ca2+-free medium containing CPA, a monophasic fast upstroke of muscle tone developed as extracellular Ca2+ was restored. The results suggest that the bimodal contracture induced by CPA is mediated by the recruitment of distinct Mn2+- and U73122-sensitive Ca2+ entries. The ongoing two-component Ca2+ entries might merge if the muscle preparation was preconditioned with CPA in Ca2+-free medium to deplete cellular Ca2+ stores.