The partial agonist activity of xamoterol (ICI 118,587) studied by heart rate response in pithed rats

The partial agonist activity of xamoterol was evaluated by measuring changes in heart rate (HR) in pithed rats. Xamoterol showed dose-dependent positive chronotropic effects in catecholamine-depleted pithed rats (HR: 199 +/- 6 beats/min) and dose-dependent negative chronotropic effects in sympathetic nerve-stimulated pithed rats (HR: 325 +/- 16 beats/min). In contrast, isoproterenol exerted dose-dependent positive chronotropic effects in either condition (HR: 189 +/- 7 and 332 +/- 5 beats/min) and propranolol exerted dose-dependent negative chronotropic effects in either condition (HR: 209 +/- 5 and 344 +/- 19 beats/min). When exogenous noradrenaline was continuously infused into catecholamine-depleted pithed rats, xamoterol showed dose-related positive chronotropic effects with noradrenaline at 0.5 micrograms/(kg min) (HR: 235 +/- 4 beats/min), virtually no effects at 1.5 micrograms/(kg min) (HR: 297 +/- 11 beats/min) and dose-related negative chronotropic effects at 5 micrograms/(kg min) (HR: 330 +/- 5 beats/min). During continuous infusion of xamoterol [100 ng/(kg min)] into pithed rats, the control HR before stimulation was increased and the maximum increase produced by the sympathetic nerve stimuli at 0.25 to 4 Hz decreased. The maximum increase in HR brought about by xamoterol was about 71% of that induced by isoproterenol, and those by pindolol and practolol were about 40% and 21% respectively. It is concluded that xamoterol is a partial agonist with a strong agonist action.