Contribution of myocardial diffuse double-layer calcium to contractile function.
The role of diffuse double-layer calcium in cardiac excitation-contraction coupling was examined using rabbit interventricular septa, cultured neonatal rat myocardial cells, and gas-dissected sarcolemmal membranes. The diffuse double layer refers to the space directly adjacent to the sarcolemma where the ionic composition of the media is a direct function of the membrane surface potential. The divalent cation dimethonium was used as a specific probe for the diffuse double layer. According to Gouy-Chapman theory, replacement of sodium with sucrose should increase the amount of calcium located in this compartment. Dimethonium (10 mM) was found to decrease calcium uptake and contractility during low-sodium (33 mM) perfusion when the perfusate contained sucrose but not LiCl. Dimethonium did not decrease calcium uptake or contractility during control perfusion. The results suggest that calcium present in the myocardial diffuse double layer can be augmented or reduced in accordance with Gouy-Chapman theory. Changes in diffuse double-layer calcium are accompanied by small (7.8%) but significant changes in contractility.