Species differences in responses to hyperosmolality and D600 in cat and rat heart

Abstract

The direct inotropic effect of hypertonic mannitol was compared in isolated rat and cat papillary muscles. The inotropic effects of paired electrical stimulation and D600 [.alpha.-isopropyl-.alpha.-[N-methyl-N-homoveratryl-.gamma.-aminopropyl]3,4,5-trimethoxyphenylacetonitrile] were also evaluated in the same species. At extracellular Ca concentrations of 2.5 mM hypertonic mannitol (25-100 mosmol/kg H2O above normal) depressed contractility in isolated rat myocardium; hyperosmolality exerted a positive effect only when extracellular Ca2+ was low (e.g., 0.3 mM). Paired pacing exerted a small but significant inotropic effect in rat heart when extracellular Ca2+ was 2.5 mM, and a larger effect at lower Ca2+. Hypertonic mannitol and paired pacing both produced significant positive effects in isolated cat heart at an extracellular Ca2+ concentration of 2.5 mM. D600 exerted less of a depressant effect on contractility in rat than in cat heart at concentrations of 10-6-10-7 M. In contrast to results in cat heart, the positive inotropic effect of hyperosmolality in isolated rat cardiac muscle is apparent only when extracellular Ca concentration is reduced. The inotropic effect of paired pacing in rat heart is greatest at low Ca2+ levels, but persists to a lesser degree at extracellular Ca concentrations of 2.5 mM. D600-inhibitable Ca channels appear to be relatively less important in the maintenance of cardiac contractility in rat than in cat cardiac muscle.