Comparative analysis of beta-1 adrenoceptor agonist and antagonist potency and selectivity of cicloprolol, xamoterol and pindolol

The partial beta adrenoceptor agonist properties of cicloprolol, xamoterol and pindolol have been compared in vivo (anesthetized catecholamine-depleted or pithed rats) and in vitro (guinea pig or rat right atria and guinea pig tracheal muscle preparations) conditions. All three compounds increased heart rate in the former preparations, and their intrinsic activities relative to isoproterenol were 0.7, 0.65 and 0.45, respectively. The positive chronotropic effects of cicloprolol or xamoterol were competitively antagonized by betaxolol or propranolol; however, part of those induced by pindolol were resistant to these beta adrenoceptor antagonists. None of these compounds increased the spontaneous beating rate of isolated guinea pig atria; however, xamoterol only increased heart rate in isolated rat atria, and its intrinsic activity with respect to isoproterenol was 0.4. Pindolol, xamoterol and cicloprolol behaved as competitive beta-1 adrenoceptor antagonists against isoproterenol-induced tachycardia in a pithed rat model. In order to mimic the intrinsic effects of the partial agonist drugs, control dose-response curves for isoproterenol were determined in pithed rats in which the base-line heart rate was elevated by thoracic spinal cord stimulation. In this in vivo preparation, xamoterol and pindolol were more potent beta-1 adrenoceptor antagonists than cicloprolol; however, cicloprolol and xamoterol, in contrast to pindolol, were selective for beta-1 adrenoceptors. In isolated spontaneously beating guinea pig right atria, cicloprolol and xamoterol were equipotent beta-1 adrenoceptor antagonists but were about 50 times less potent than pindolol. In isolated rat atria, the beta-1 adrenoceptor antagonist potency of xamoterol was greater (pA2 = 8.7) than in guinea pig atria (pA2 = 7.8). The potencies of cicloprolol and pindolol did not vary between these species. In catecholamine-depleted rats, high i.v. doses of cicloprolol had vasodilator activity that was partly mediated by beta-2 adrenoceptors. In carbachol-contracted guinea pig trachea, cicloprolol and xamoterol, in contrast to pindolol, were relatively inactive against isoproterenol-induced relaxation. In conclusion, cicloprolol and xamoterol, similarly to pindolol, behave as agonists and antagonists of beta-1 adrenoceptors. However, only cicloprolol and xamoterol show an elevated degree of selectivity toward the beta-1 adrenoceptor subtype.