Modulation of effect of extracellular calcium buffering in cardiac muscle

The buffering of extracellular calcium by citrate, with identical free calcium levels in the buffered and unbuffered medium, was previously found to markedly reduce tension in frog and guinea pig atria. We now report the following results. 1) In guinea pig, postest contractions are not reduced by citrate. 2) In frog the negative inotropic effect of citrate is greatly attenuated by partially replacing extracellular sodium concentration ([Na+]o) with lithium or sucrose. In contrast, in low [Na+]o, sodium salts of weak acids (which cause intracellular acidosis) still reduce tension. These findings strongly support the suggestion that citrate does not reduce tension directly, e.g., by causing intracellular acidosis or by reducing voltage-dependent calcium currents. 3) Higher stimulation rates also decrease the effect of citrate. 4) Treatment with neuraminidase or phospholipase D, both of which alter sarcolemmal calcium binding, dose not change the effect of citrate. 5) The positive inotropic effect of strophanthidin is reversibly lost in the presence of citrate. Our results provide a different, novel approach to support earlier suggestions that the extracellular matrix and/or the sarcolemma contain abundant calcium-binding sites that supply some of the calcium for contraction. Citrate presumably binds calcium more strongly, thereby "trapping" calcium released from the extracellular "stores," so that less calcium is made available for contraction. The inotropic effect of cardiac glycosides may also depend on the calcium that is bound to these stores.