Prolonged potentials in gastrointestinal muscles induced by calcium chelation

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Abstract
When stomach muscles of skate [Raja clavata], toad [Bufo marinus] or frog [Rana pipiens] or intestinal muscle of cat were treated with Ca-free physiological solutions containing 2-5 mM EGTA or EDTA, spontaneous spikes and slow waves disappeared reversibly. With continued treatment, depolarization of 25-30 mV from resting potentials of -65 mV occurred and rhythmic prolonged potentials of several seconds duration appeared. They showed rapid depolarization to near zero and rapid repolarization, and they could continue for several hours. The prolonged potentials disappeared when Na was replaced by Li, Tris or choline. They are insensitive to TTX [tetradotoxin]. The EGTA-induced waves were abolished by Mn, Co, La, verapamil and D 600. After 10-15 min in 5 mM EGTA, voltage-current curves of circular muscle showed increased leakage current and abolition of anomalous rectification. When bound Ca is removed by a chelator, nonspecific reduction in resistance occurs and Na+ may enter rhythmically through channels normally used by Ca.