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 7.5mM) on the inotropic action and myocardial concentration of tritiated digoxin (5 x 10-7M) 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/mm2 · 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 3H-digoxin content after 45 minutes, and at the time of the peak inotropic response. The relationships found between [K]0 and 3H-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.
- force-velocity relations
- isometric tension
- maximal velocity of shortening
- cardiac glycosides
- cat papillary muscle
- Received May 27, 1970.
- Accepted January 13, 1971.
- © 1971 American Heart Association, Inc.