Calcium Efflux from a Heavy Sarcotubular Fraction

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Abstract
Calcium efflux has been measured from two different fractions of rabbit skeletal muscle partially loaded with calcium oxalate, under conditions in which the inward calcium pump is inoperative. Efflux from the heavy sarcotubular fraction (2000–8000×g) is stimulated by ryanodine and caffeine, whereas calcium efflux from a light fraction (12000–35000×g) is much less sensitive to these drugs. Quinidine and chlorpromazine stimulate calcium efflux from both muscle fractions. These effects of ryanodine and caffeine are completely blocked, and those of quinidine and chlorpromazine partially blocked, by 5 mM magnesium, although magnesium does not affect calcium efflux in the absence of drugs. Zinc stimulates and uranyl ion depresses calcium efflux.

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