Ryanodine and the calcium content of cardiac SR assessed by caffeine and rapid cooling contractures

The ability of rabbit ventricular muscle sarcoplasmic reticulum (SR) to accumulate and retain Ca during Na-free perfusion was assessed using caffeine contractures and rapid cooling contractures (RCC). Muscles were exposed to a Na-free medium for 15 min, during which time a transient contracture developed and relaxed back to the resting tension level. Muscles were then either exposed to 20 mM caffeine or rapidly cooled to <5°C, both of which produced large contractures. The magnitudes of those contractures are believed to be a relative index of SR Ca content. Reduction of extracellular [Ca] from 2 to 0.2 mM did not significantly alter the magnitude of either Na-free caffeine contractures or RCC. These are not the maximum contractures that can be obtained. The possibility that low extracellular [Ca] ([Ca](o)) may increase passive Ca permeability is suggested in explanation of this effect. After equilibration with 100 nM ryanodine, both Na-free caffeine contractures and RCC are virtually eliminted. This suggests that even if the SR could accumulate Ca during the initial Na-free exposure in the presence of ryanodine, it could not retain that Ca in Na-free medium. It is proposed that the sarcolemmal Ca pump can extrude Ca from the cells at a rate sufficient to deplete the ryanodine-treated SR. When removal of Na was accompanied by increase of [K](o) (to 12, 20, or 30 mM), the Na-free RCC were enhanced. Increasing [K](o) during Na-free superfusion in the presence of ryanodine resulted in demonstrable RCC that were graded with [K] and completely suppressed by nifedipine. It appears that the SR can still be Ca loaded in the presence of ryanodine. However, this SR Ca loading would appear to be more dependent on transsarcolemmal Ca flux imbalance than cytoplasmic [Ca] (at least as indicated by the lack of resting force).