BLOCKADE BY ZINC IONS OF K+-INDUCED CONTRACTION AND CALCIUM IN GUINEA-PIG TAENIA-COLI

Preincubation with 0.3 mM Zn2+ markedly inhibited both the tonic response and Ca2+ binding at low affinity sites induced by K+ (60 mM), with smaller effects on the. phasic response and the high affinity Ca2+ sites, in Taenia coli. However, when the muscle was kept in Zn2+-containing medium following the first stimulation with the K+, the phasic response and the high affinity Ca2+ sites were more severely inhibited during the second stimulation with K+. This probably indicates that Zn2+ reduced the tonic tension response to K+ mainly by inhibiting Ca2+ influx at the cell membranes of Taenia coli. However, when Zn2+ is continuously present, Ca2+ is not supplied at the storage sites and is not available for the phasic response to a second stimulation with K+.