The dose in humans at which ICI 118,551 (a selective β2-adrenoceptor blocking agent) demonstrates blockade of β1-adrenoceptors

The effects of four single oral doses of ICI 118,551 (a selective beta 2-adrenoceptor blocking agent: doses 10, 20, 50, and 100 mg) have been compared with placebo in five normal, healthy volunteers on some cardiovascular responses to intravenous infusions of dobutamine. Increasing infusions of dobutamine produced reproducible dose-dependent reductions in systolic time intervals and increases in systolic blood pressures, these responses representing positive inotropic effects of dobutamine. These effects of dobutamine were unaffected 2 hours after administration by 10 mg ICI 118,551 and minimally by 20 mg; the 50 mg dose attenuated the systolic time interval effect whereas the 100 mg dose attenuated further the systolic time interval reduction and also the increase in systolic blood pressure. These results allow a conclusion that at unit doses of 50 mg and above, ICI 118,551 will produce demonstrable effects on beta 1-adrenoceptors.