DIRECT EVIDENCE FOR THE INTIMATE RELATIONSHIP BETWEEN THE UPTAKE OF DIGITOXIN AND AN ACTIVITY OF GUINEA PIG HEART

Influence of extracellular potassium concentration on myocardial uptake and inotropic effect of tritiated digoxin

Because of conflicting reports on the influence of extracellular potassium concentration ([K] 0 ) on the inotropic action of cardiac glycosides, we compared the effects of varying [K] 0 (1.5, 4.5, and 7.5m M ) on the inotropic action and myocardial concentration of tritiated digoxin (5 x 10 -7 M ) using the isolated cat papillary muscle. The time course of the inotropic response to digoxin was significantly altered by [K] 0 in an inverse manner; the rate of change of the active tension response to digoxin (g/mm 2 · min -1 ) was 0.048±0.004 with low [K] 0 , 0.025±0.003 with normal [K] 0 and 0.014±0.002 with high [K] 0 . There were no differences, however, in the peak inotropic responses achieved at each [K] 0 . Muscles were analyzed for 3 H-digoxin content after 45 minutes, and at the time of the peak inotropic response. The relationships found between [K] 0 and 3 H-digoxin content were similar to those found between [K] 0 and active tension. Moreover, a significant positive correlation between digoxin content and the increment in active tension was observed. Thus, an inverse relationship exists between [K] 0 and the rate of development of the inotropic response to digoxin, changes that may be due to a [K] 0 induced alteration in myocardial digoxin content.