ALPHA(1D)-ADRENOCEPTORS PLAY LITTLE ROLE IN THE POSITIVE INOTROPIC ACTION OF PHENYLEPHRINE

This study was designed to determine if the positive inotropic action of alpha(1)-adrenergic stimulation in rat heart is mediated via alpha( 1D)-adrenoceptors. Isolated left atrial and papillar muscle were suspe nded in oxygenated Krebs-Henseleit buffer (37 degrees C) containing 1 mu M nadolol and paced at 3.0 Hz. Isometric tension was continuously m onitored. Cumulative concentration-response curves for phenylephrine w ere obtained in the presence and absence of BMY 7378 erazinyl]ethyl]-8 -azaspirol[4.5]decane-7,9-dione), a selective competitive alpha(1D)-ad renoceptor antagonist. BMY 7378 at concentrations up to 30 nM did not significantly affect the positive inotropic response to phenylephrine. In contrast, as reported by other investigators, alpha(1D)-adrenocept or-selective concentrations of this antagonist (3 and 10 nM) did elici t a concentration-dependent right-ward shift in the vasoconstrictor re sponse to phenylephrine in rat abdominal aorta. These data suggest tha t alpha(1D)-adrenoceptors do not play a major role in the positive ino tropic action of alpha-adrenoceptor stimulation in rat cardiac muscle. (C) 1997 Elsevier Science B.V.