EFFECTS OF CARDIAC-GLYCOSIDES ON EXCITATION-CONTRACTION COUPLING IN FROG SKELETAL-MUSCLE FIBERS

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 fibre s of the frog. 2. Both cardiac glycosides increased both calcium trans ients 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 extracellu lar digoxin concentration of 5 nM and an estimated intracellular ouaba in concentration of 1-2 nM. Digoxin and ouabain thus exerted their eff ects at the same concentration on calcium release in skeletal muscle a s previously observed in isolated cardiac-type ryanodine receptor (RyR ) calcium release channels. 3. The peak of SR calcium release increase d at all voltages, with the largest potentiation at intermediate membr ane potentials. This increase in calcium release flux was attained des pite an unchanged SR calcium content. The attenuated release rate ther efore reflected an increased number of open RyR channels rather than i ncreased SR loading. 4. These effects could be attributed to an increa se in calcium release activation and not a decrease in the rate of ina ctivation. Rather, the rate of inactivation was enhanced at all voltag es as expected from the increased calcium concentration in the triadic junction. 5. In contrast, CMA (17 alpha-acetoxy-6-chloro-4,6-pregnadi ene-3,20-dione; 5 mu M), a Na+-K+-ATPase inhibitor with no positive in otropic effects on the heart, neither influenced SR calcium release no r antagonized the effects of ouabain. 6. Both digoxin and ouabain pres erved total intramembrane charge apart from a small negative shift in the mid-point voltage and increase in slope factor. 7. Both digoxin an d ouabain induced calcium release from heavy XR vesicles at rates comp arable to that induced by ryanodine or caffeine. 8. It is concluded th at at least part of the inactivating component of SR calcium release i nvolves distinct RyR calcium release channels that resemble the cardia c RyR isoform in its specific sensitivity to cardiac glycosides.