B. Swynghedauw

Respective involvements of high- and low- affinity digitalis receptors in the inotropic response of isolated rat heart to ouabain

High- and low-affinity digitalis receptors coexist in rat cardiac sarcolemma. In this study, their relative involvement in the inotropic effect of ouabain was evaluated on an isolated Langendorff rat heart preparation working under isovolumic conditions at a low external calcium concentration (0.25 mM). This involvement was estimated according to both the development of the inotropic response to ouabain (10(-8)-10(-4)M) and the time course of the washing out of the biological effect. In each phenomenon considered, and whatever the index of inotropy chosen, the high-affinity digitalis receptor (EC50: 1-2 X 10(-8) M) contributed to 25-40% of the maximal inotropy (evoked by 10(-4) M ouabain). Low-affinity receptors (EC50: 1-2 X 10(-5) M) accounted for 60-75%. These apparent affinities were identical to those previously determined in sarcolemma isolated from rat heart perfused with 0.25 mM Ca2+. The biphasic effect of ouabain was related to both the inhibition of high- and low-sensitivity Na+, K+-ATPase forms and the corresponding number of ouabain-binding sites occupied. These results support the concept that the Na+, K+-ATPase highly sensitive to ouabain as revealed by lowering calcium is the in vivo manifestation of the high-sensitivity inotropic component.

Effects of calcium on the heterogeneity of the Na+, K+-ATPase forms in rat heart.

The sensitivity of the Na+, K+-ATPase to ouabain has been studied in sarcolemma vesicles isolated from normal rat heart. Two enzyme forms exhibiting high and low sensitivities to ouabain have been observed in Ca2+-free perfused heart. The half-maximal inhibitory effects occurred with 1-2 X 10(-8) M ouabain. The high sensitivity form undetectable in hearts maintained at a physiological Ca2+ level might represent altered low affinity sites or latent enzyme forms unmasked by low calcium concentrations. The heterogeneity of the Na+, K+-ATPase forms was found to be also modulated by the K+/ouabain antagonism, addition of K+ accentuating the heterogeneity. These in vitro results associated with in vivo experiments on isolated rat heart working under isovolumic conditions suggested that lowering Ca2+ has qualitative and quantitative effects. Low Ca2+ concentrations increased the sensitivities to ouabain and the amplitudes of both the enzyme inhibition and the positive inotropic effects.

Responsiveness of Hypertrophied Rat Heart to Digitalis. In Vivo and in Vitro studies

In response to a sustained pressure overload the cardiac myocytes hypertrophy (for a review, see ref. 1). At the sarcolemma level, there are numerous structural, electrophysiological and biochemical alterations associated with hypertrophy. The T. tubules are particularly enlarged (2,3). Depending on both the species and the experimental model, the electrophysiological properties of the membrane are differently affected (for a review, see 4). In pressure-overloaded rat heart, the model used here, there is considerable evidence (4–7) that the duration of the action potential is lengthened, such a prolongation becoming more marked as the degree of hypertrophy becomes more severe (5).