Zinc depresses the contractile force of electrically driven rat atria logarithmically with time. The threshold concentration is about 5 X 10 -6 M zinc and the half-time for contractile depression at 10 -4 M is about 25 minutes. Zinc also depresses spontaneous activity of atria and alters the transmembrane potential parameters in a manner similar to quinidine. Unlike quinidine, zinc causes an elevation of the resting potential and an elevation of cellular potassium which varies with time in the same way as the resting potential. Exposure to 10 -4 M zinc for 60 minutes causes a statistically significant fall in atrial calcium content and an amount of radioactively labeled zinc is taken up which is quantitatively equal to the calcium lost. Zinc has no effect on rigor caused by iodoacetate but inhibits rigor caused by l-fluoro-2,4 dinitrobenzene. It is postulated that zinc depression of contractile force is not due to metabolic inhibition, probably not due to quinidine-like action on the cell membrane, but may be due to an interference in the handling of calcium by the cell.

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