Insulin and glucose 6-phosphate stimulation of Ca2+ uptake by skinned muscle fibers.

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Abstract
Ca uptake by [rabbit] skinned muscle fibers is stimulated by physiological concentrations of insulin. These fibers, which lack a functional plasma membrane, are permeable to macromolecules but retain extensive portions of their sarcolemma in the form of transverse tubules intercalated between the myofibrils. They have an active sarcoplasmic reticulum that removes 45Ca2+ from solution at concentrations below the threshold that initiates contraction (< 1 .mu.M). The Ca2+ uptake activity is stimulated by insulin, presumably in response to its binding to those receptors located in the transverse tubules. Addition of glucose 6-phosphate, whose intracellular concentration increases in response to insulin, also stimulates Ca2+ uptake, a unique property of this preparation. Insulin and glucose 6-phosphate act in concert to stimulate the sarcoplasmic reticulum. The resulting decrease in myoplasmic Ca2+ and the increase in glucose 6-phosphate would serve to mediate some of the anabolic effects of the hormone.