The role of superficially bound calcium in the regulation of myocardial contractility and the positive inotropic response to ouabain

Abstract

Positive inotropic effects induced in hearts by paired pacing and ouabain treatment were found to be dependent on the content of calcium in a superficial pool, Ca₁. Two possible mechanisms for the expansion of this pool were investigated in isolated cardiac myocytes in which the cell surface structures were preserved from proteolytic digestion. Guinea pig ventricles were disrupted mechanically to obtain intact myocardial cells. The cell yield from this preparation was enriched by incubation with Cytodex^® beads and the characteristics of calcium binding to the cell–bead suspension were determined by continuous flow equilibrium dialysis. Scatchard plots revealed two components for calcium binding; a high affinity (K_A ~ 65 mM⁻¹) and a low affinity (K_A ~ 140 M⁻¹) pool. Ouabain (10⁻⁹ to 10⁻⁶ M) increased, in a dose related manner, the K_A for calcium binding to the low affinity sites without affecting the total number of binding sites. The calculated amount of calcium bound to the low affinity pool at [Ca]_e = 1.8 mM was closely correlated with the positive inotropic effect (PIE) measured in guinea pig hearts perfused with the same calcium concentration. At [Ca]_e = 0.45, 0.9, or 3.6 mM, the response to ouabain (1 × 10⁻⁷ and 5 × 10⁻⁷ M) was greater than expected on the basis of the increase in calcium binding to the low affinity sites. The results suggest that at least one event in the inotropic response to ouabain involves increased affinity for calcium binding to low affinity sites on the cell surface, but that the increase in calcium bound at any [Ca]_e is insufficient to produce the PIE directly.