Mode of Action of (-)-pindolol on Feline and Human Myocardium

(-)-Pindolol antagonized competitively and to a similar extent the positive inotropic effects of both (-)-noradrenaline and (-)-adrenaline in human ventricular preparations. An equilibrium dissociation constant KD (-log mol 1(-1) = pKD) of 9.2-9.3 was estimated regardless of disease present or agonist used. (-)-Pindolol antagonized competitively the positive inotropic effects of (-)-adrenaline more than those of (-)-noradrenaline in human atrial preparations. pKD values of (-)-pindolol were 9.6 against (-)-adrenaline and 9.1 against (-)-noradrenaline. The results are consistent with a moderate selectivity of (-)-pindolol for beta 2-compared to beta 1-adrenoceptors in human atrium. (-)-Pindolol competed with [3H]-(-)-bupranolol with a pKD of 9.4 for beta-adrenoceptors of human ventricle. Positive inotropic effects of (-)-pindolol were not detected on human atrium or ventricle in a concentration range of 1-1000 nmol 1(-1). The affinity of (-)-pindolol estimated for human myocardial beta-adrenoceptors, its moderate beta 2-selectivity and its lack of intrinsic activity for contractile force agreed with similar characteristics in other species. (-)-Pindolol caused marked positive chronotropic effects in kitten right atria with an intrinsic activity of 0.5 with respect to catecholamines. On kitten left atria it caused only weak positive inotropic effects with an intrinsic activity of 0.1. (-)-Pindolol (0.6-6000 nmol-1) did not cause positive inotropic effects in kitten papillary muscle. The concentration-effect curve for (-)-pindolol on kitten right and left atria was biphasic. Its positive chronotropic and inotropic effects were not blocked by methysergide, suggesting that 5-hydroxytryptamine (5-HT)-receptors were not involved. Low concentrations of antagonists selective for beta 1- and beta 2-adrenoceptors blocked the high sensitivity component but not the low sensitivity component of the positive chronotropic and inotropic effects. The biphasic nature of the positive chronotropic effects of (-)-pindolol in kitten agreed with previous observations made on guinea-pig right atria and support the concept that 3 receptors in the sinoatrial pacemaker contribute to these chronotropic effects: beta 1, beta 2 and a low-affinity receptor for (-)-pindolol which is neither beta 1 nor beta 2. The partial agonistic activity of (-)-pindolol in the heart appears to be mainly (kitten) or completely (man) restricted to the sinoatrial pacemaker.