Characteristics of spontaneous contraction in the circular smooth muscles of cat ileum.

We investigated the characteristics of spontaneous contraction in feline ileal circular smooth. Smooth muscle contractions were recorded by an isometric force transducer. The neurotoxin tetrodotoxin did not alter the spontaneous contraction of the circular muscles. Atropine or guanethidine also did not affect on the contraction. In Ca2+-free Krebs, the spontaneous contraction completely disappeared in the muscles. An L-type Ca2+ channel blockade with nimodipine decreased the amplitude of spontaneous contraction. A selective inhibitor of the sarcoplasmic reticulum Ca2+-ATPase, thapsigargin, had not no effect on spontaneous contraction within incubation time for 10 min. However, phosphoinositide-specific phospholipase C inhibitor, U-73122, decreased the frequency of the contraction in circular smooth muscle These results suggest that the neuronal component, adrenergic or cholinergic innervation was not involved in spontaneous activity and that extracellular Ca2+ influx via an L-type Ca2+ channel may mediate the contractile amplitude in circular smooth muscles of cat ileum.