Effect of ouabain on potassium exchange in hypothermic mammalian heart

Potassium exchange was studied in the intact working hypothermic guinea pig heart in vitro with K42. As at 37 C, buildup and washout experiments demonstrated two compartments of K exchange, but these behaved differently with reductions in temperature to 20 C. The rate of K exchange of the "fast" compartment decreased with lowered temperatures, whereas the rate of "slow" compartment exchange either remained unaffected or increased slightly. Ouabain had no apparent effect on the fast compartment K exchange. Toxic levels of ouabain, which inhibited entrance of K into the slowly exchanging phase at 37 C, showed a minimal effect on this compartment at 20 C. The decreased ouabain inhibition at 20 C was paralleled by a concomitant decrease in toxicity (contracture and ventricular fibrillation). It was postulated that intracellular cardiac K exchange involved two separate processes which responded differently to low temperatures. Ouabain action was indicated to be specifically on that process which was insensitive to cold temperatures.