Reversible effects of a high-molecular weight sulfhydryl reagent on the contractile activity of myocardial cells isolated from adult rat heart.

The Ca2+ ion dependency of electrically stimulated beating activity of isolated ventricular myocytes of adult rat was examined by using video recording and high-speed movie photography of the cellular contraction. The myocyte exhibited a transient positive inotropy on treatment with dextran-pCMB having a minimum molecular weight of 200,000 and the maximum degree and initial velocity of contraction as measured from the movie photographs increased 1.8 and 2.2 times, respectively, as compared with those before dextran-pCMB treatment. Then the myocyte showed fibrillatory movement which was not a response to electric stimulation. These effects were similar to those caused by ouabain, and they were reversed by dithiothreitol. The sarcomere length of the myocyte in the resting state was reversibly shortened by treatment with dextran-pCMB only in the presence of added Ca2+. K+-PNPPase activity of the myocyte was inhibited by dextran-pCMB. It is suggested that dextran-pCMB binds to a protein factor(s) in the cytoplasmic membrane from the exterior of the myocyte to give rise to an increase of intracellular concentration of Ca2+.