Myocardial Contraction Frequency and Onset of Cardiac Glycoside Action

Left atrial strips and right ventricular papillary muscles of kittens and left atrial strips of guinea pigs, rabbits, dogs, and chickens were exposed to ouabain, acetylstrophanthidin, or digoxin at 37°C during rest or while contracting at frequencies from 0.06 to 60/min. Both the positive inotropic effect and increases in resting tension developed fully in all preparations exposed to the three drugs during complete quiescence. Contraction frequency had some influence on the rate of development of these effects, but this influence was not the result of the number of beats during exposure. In ventricular myocardium, exposure times required for the attainment of the maximum positive inotropic effect and for the induction of contracture at different contraction frequencies were proportional to the increase in tension development. It is concluded that the development of the myocardial effects of digitalis-like compounds does not require cardiac activity. The limited influence of contraction frequency on the rate of development of these effects may be partly due to frequency-dependent changes in the functional state of the myocardium which affect the magnitude of the positive inotropic action of cardiac glycosides.