Calcium pools mobilized during excitation-contraction coupling in flounder intestinal smooth muscle

Abstract

1. 1. The effects of Ca 2+ free solutions (0 mM Ca 2+ /5 mM EGTA) and low Ca 2+ media (no added Ca 2+) on the 100 mM K + and 10 −5M ACh contractions of the flounder intestine were examined. 2. 2. Ca 2+-free solutions abolished the K +-contractions and reduced the normal ACh response to a smaller transient event. Low Ca 2+ media blocked the prolonged tonic phase of the K +-contractions more readily than the initial phasic component. 3. 3. It was concluded that both ACh and K + stimulate Ca 2+ entry into the cell and that the phasic component of the K +-contraction relies on a more tightly bound extracellular Ca 2+-fraction than that utilized during the tonic phase. ACh can also mobilize an intracellular Ca 2+ -store. 4. 4. Ultrastructural studies suggested that this ACh-releasable intracellular Ca 2+-store may reside on the inner surface of the plasma membrane or within the peripheral S.R.