Cardiac Sarcotubular Vesicles

Abstract

The transport of calcium through the membranes of the sarcoplasmic reticulum plays an important role in the relaxation-contraction cycle of the heart. To study and characterize this system, fragmented sarcotubular vesicles from dog hearts were prepared by ultracentrifugation. The fraction between 15,000 and 40,000 g was partially purified by gradient centrifugation. Calcium accumulation by the vesicles in the presence of ATP, Mg, ⁴⁵Ca, and oxalate was studied. The rate of calcium uptake increased with the ATP concentration and was not affected by changes in K or Na concentration nor by substitution of isosmotic amounts of sucrose for 75% of the Na ions. Splitting of ATP was increased upon substitution of sucrose. In aged preparations of vesicles, ouabain or acetylstrophanthidin decreased the rate of their calcium accumulation and partially reversed the inhibition of myofibrillar ATPase by the vesicles. It is suggested that the partial inhibition of calcium uptake increases the concentraton of free calcium, and thus makes more calcium available for contraction of the myofibril.