title: Ryanodine and the calcium content of cardiac SR assessed by caffeine and rapid cooling contractures.
abstract: 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 less than 5 degrees 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 eliminated. 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.(ABSTRACT TRUNCATED AT 250 WORDS)
mesh_terms: D000470:Alkaloids / Q000494:pharmacology*; D000818:Animals; D002110:Caffeine; D002118:Calcium / Q000378:metabolism*; D003080:Cold Temperature; D009119:Muscle Contraction; D009206:Myocardium / Q000378:metabolism* / Q000648:ultrastructure; D010210:Papillary Muscles / Q000378:metabolism / Q000648:ultrastructure; D011817:Rabbits; D012433:Ryanodine / Q000494:pharmacology*; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
chemical_list: D000470:Alkaloids; D012433:Ryanodine; D002110:Caffeine; D002118:Calcium