| Abstract: | The inotropic response induced by beta-adrenergic and H1 histaminergic receptor stimulation was characterized in guinea pig left atria by obtaining dose-response relationships for isoproterenol and histamine under various experimental conditions. Conditions (hypothermia, high frequencies of stimulation, and large extracellular calcium concentrations) which enhanced the ability of cardiac muscle to develop force also increased the sensitivity of the left atrium to isoproterenol while decreasing its efficacy. On the other hand, conditions which enhanced the ability of cardiac muscle to develop force depressed the efficacy of histamine to such an extent that the sensitivity to histamine was also decreased. In addition, conditions which markedly depressed the ability of cardiac muscle to develop force also decreased the efficacy and sensitivity to histamine. The data indicate that while beta-adrenoceptor stimulation results in an inotropic response under all conditions studied, stimulation of H1 histaminergic receptors results in an inotropic response only within a narrow range of experimental conditions. |
