Calcium binding to cardiac sarcolemmal vesicles: potential role as a modifier of contraction.

Passive Ca binding to cardiac sarcolemmal vesicles isolated from rabbit ventricles was measured under ionic conditions similar to intracellular and extracellular media. The first of two main goals was to evaluate whether certain agents induce changes in Ca binding at the external sarcolemmal surface that might contribute to the overall effect of these agents on cardiac muscle contraction. The agents studied were ouabain, verapamil, nifedipine, Bay K 8644, caffeine, ryanodine, and milrinone over a broad range of concentrations, including concentrations at which these agents exert strong effects on cardiac contractile performance. None of these agents produced significant alterations in Ca binding, such that it is unlikely that any part of their actions can be attributed to changes in Ca binding to the external sarcolemmal surface. In contrast, when [Na] is reduced from 140 mM, sarcolemmal Ca binding increases or decreases depending on what replacement is used to avoid changes of osmolarity. Thus the possible effect of Na reduction on surface Ca must be considered in physiological experiments where extracellular [Na] is changed. The second main goal was to evaluate the effects of membrane potential, Na and Mg on Ca bound to the inner surface of the sarcolemma under ionic conditions similar to those expected intracellularly (e.g., [Ca] = 0.3-5.0 microM). Ca binding was inhibited by physiological concentrations of Na and Mg and was sensitive to membrane potential such that depolarization of a normally polarized cell would cause Ca to be released from these sarcolemmal sites. From a quantitative standpoint, it is not clear whether the effect of depolarization would be to contribute sarcolemmal Ca to the activation of the myofilaments or merely to limit the ability of the inner sarcolemmal surface to buffer the rise in intracellular [Ca] associated with contraction.