WB4101-SENSITIVE AND CEC-SENSITIVE POSITIVE INOTROPIC ACTIONS OF PHENYLEPHRINE IN RAT CARDIAC-MUSCLE

This study was designed to determine the role of the alpha(1)-adrenergic receptor (AR) subtypes in the positive inotropic action of alpha(1)-adrenergic agonists in rat myocardium. Isolated left atrial and papillary muscle were suspended in oxygenated Krebs-Henseleit buffer (37 degrees C) containing 3 mu M nadolol and paced at 3.3 Hz. Isometric tension was continuously monitored. Cumulative concentration-response curves for phenylephrine (3 x 10(-7) to 3 x 10(-4) M) were obtained in the presence and absence of WB4101 (4 and 10 nM) and with and without treatment with chloroethylclonidine (CEC; 10, 100, and 300 mu M). WB4101 antagonized the effect of phenylephrine in both tissues, increasing half-maximal effective concentration (EC(50)) values in a concentration-dependent manner. CEC pretreatment also increased EC(50) values in both tissues, and 300 mu M CEC reduced the maximal positive inotropic effect of phenylephrine by similar to 48 and 38% in left atrial and papillary muscle, respectively. CEC alone elicited significant increases in contractile force that were not readily reversible. These data suggest that the positive inotropic effect of alpha(1)-adrenergic agonists in rat atrial and ventricular myocardium results from stimulation of both WB4101- and CEC-sensitive alpha(1)-ARS.