Effects of cardiac glycosides on excitation-contraction coupling in frog skeletal muscle fibres.

1. The effects of digoxin and ouabain on the calcium release flux from the sarcoplasmic reticulum (SR), isometric tension and intramembrane charge movement were studied in voltage clamped skeletal muscle fibres of the frog. 2. Both cardiac glycosides increased both calcium transients and simultaneously recorded tension at all membrane potentials, showing different effects on the peak and on the steady components of the calcium release flux. These effects were attained at an extracellular digoxin concentration of 5 nM and an estimated intracellular ouabain concentration of 1-2 nM. Digoxin and ouabain thus exerted their effects at the same concentration on calcium release in skeletal muscle as previously observed in isolated cardiac-type ryanodine receptor (RyR) calcium release channels. 3. The peak of SR calcium release increased at all voltages, with the largest potentiation at intermediate membrane potentials. This increase in calcium release flux was attained despite an unchanged SR calcium content. The attenuated release rate therefore reflected an increased number of open RyR channels rather than increased SR loading. 4. These effects could be attributed to an increase in calcium release activation and not a decrease in the rate of inactivation. Rather, the rate of inactivation was enhanced at all voltages as expected from the increased calcium concentration in the triadic junction. 5. In contrast, CMA (17 alpha-acetoxy-6-chloro-4, 6-pregnadiene-3,20-dione; 5 microM), a Na(+)-K(+)-ATPase inhibitor with no positive inotropic effects on the heart, neither influenced SR calcium release nor antagonized the effects of ouabain. 6. Both digoxin and ouabain preserved total intramembrane charge apart from a small negative shift in the mid-point voltage and increase in slope factor. 7. Both digoxin and ouabain induced calcium release from heavy SR vesicles at rates comparable to that induced by ryanodine or caffeine. 8. It is concluded that at least part of the inactivating component of SR calcium release involves distinct RyR calcium release channels that resemble the cardiac RyR isoform in its specific sensitivity to cardiac glycosides.