EFFECTS OF NIFEDIPINE RYANODINE AND CYCLOPIAZONIC ACID ON TENSION DEVELOPMENT OF ILEAL LONGITUDINAL MUSCLE BY MANGANESE IONS IN CA2-FREE HIGH-K+ MEDIUM()

1. Mn2+ (5 mM) could evoke the contraction in a Ca2+-free, high-K+ (60 mM) medium in ileal longitudinal muscle. Nifedipine, L-type Ca2+ chan nel blocker, inhibited dose-dependently both the contraction and manga nese uptake in the presence of 5 mM Mn2+, in the Ca2+-free, high-K+ me dium. 2. However, both ryanodine, Ca2+ release blocker from SR and cyc lopiazonic acid, specific SR Ca2+-ATPase blocker, had no effects on th e contractions evoked by Mn2+ and the manganese uptake in the Ca2+-fre e, high-K+ medium. 3. These results suggest that Mn2+ did not liberate Ca2+ at the storage sites during the Mn2+-induced contraction in Ca2-free, high-K+ medium. Mn2+ is entering via voltage-dependent Ca2+ cha nnels in the ileal cell membranes and it directly activates the contra ctile elements.