INFLUENCE OF K⁺ AND Ca⁺⁺ ON THE EFFECT OF OUABAIN ON Ca⁴⁵ ENTRY AND CONTRACTURE IN RABBIT ATRIA

¹ Department of Pharmacology, University of Mississippi School of Medicine, Jackson, Mississippi

Factors affecting the rate of onset and reversibility of ouabain-induced contractures have been investigated. Parallel studies on the changes in rate of Ca⁴⁵ entry have been included. In quiescent preparations treated with ouabain, contractures and increased Ca entry rates did not occur. Elevation of the K content of Ringer's solution prevented contractures, increased entry rates, relaxed contracted atria and restored rates of Ca uptake to normal levels. Reduction of the Ca concentration prevented an increase in resting tension induced by ouabain, but if the Ca concentration was restored to normal, contractures appeared and Ca entry rates were again increased. From such studies it was concluded that ouabain acts on initial steps in the excitation-contraction coupling mechanism. These steps are apparently Ca-dependent. The binding of ouabain was blocked by K, but unaffected by changes in Ca concentration or stimulus frequency.