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

Calcium uptake by 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 (< 1μM). 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. These data indicate that 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.