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 respo nse and Ca2+ binding at low affinity sites induced by K+ (60 mM), with smaller effects on the. phasic response and the high affinity Ca2+ si tes, in Taenia coli. However, when the muscle was kept in Zn2+-contain ing medium following the first stimulation with the K+, the phasic res ponse and the high affinity Ca2+ sites were more severely inhibited du ring the second stimulation with K+. This probably indicates that Zn2 reduced the tonic tension response to K+ mainly by inhibiting Ca2+ in flux at the cell membranes of Taenia coli. However, when Zn2+ is conti nuously present, Ca2+ is not supplied at the storage sites and is not available for the phasic response to a second stimulation with K+.