Slow responses mediated by alpha-adrenoceptors in the cardiac muscles of the monkey (Macaca fuscata)

The effects of phenylephrine on force of contraction and on slow action potentials were studied in the cardiac muscles of the Japanese monkey (Macaca fuscata). 1) The concentration-response curves for the positive inotropic effect of phenylephrine in the left atria and right ventricular papillary muscles were shifted to the right by either prazosin (10(-7) M) or propranolol (10(-6) M), and were further shifted in the presence of both prazosin and propranolol. 2) Phenylephrine (10(-5) M) restored the tension development in the left atria and right ventricular papillary muscles which were depolarized and rendered inexcitable by elevating K+ concentration to 22 mM. The restored contractions with phenylephrine were abolished or diminished by either prazosin (10(-6) M), phenylephrine (10(-5) M) restored action potentials in 4 of 6 K+-depolarized right ventricular muscle preparations; the restored action potentials were abolished or diminished by prazosin (10(-6) M). These results indicate that alpha-adrenoceptors as well as beta-adrenoceptors are involved in inducing slow responses in the monkey heart.