The sympathomimetic activity of (±)-pindolol at β 1- and β 2-adrenoceptor sites

Abstract

In isolated right atrial and stilboestrol-pretreated uterine preparations from both guinea-pigs and rats, pindolol elicited propranolol-sensitive positive chronotropic and smooth muscle relaxant actions. Although the pD 2 values for pindolol (8.4–9.2) and (−)-isoprenaline (ISO, 8.4–8.7) fell within the same range in these preparations, the maximum responses to pindolol were less than 15% of those to the catecholamine. Thus, pindolol did not display any selectivity for agonistic actions at β 1- or β 2-adrenoceptors. In uteri taken from progesterone-pretreated rats, the pD 2 value for (−)-isoprenaline was 9.5 and that of pindolol 8.5. In these preparations the maximal relaxant effect of pindolol (approximately 50% E max ISO) was greater than that found in oestrogen-pretreated uteri. Thus, it appears that the maximal response of pindolol in vitro can be related to the pD 2 value for (−)-isoprenaline. In anaesthetized cats, intravenous pindolol elicited non-β-adrenoceptor-mediated increases in heart rate and decreases in soleus muscle contractility. The mechanism(s) underlying the latter actions are unknown.