Measurement of partial agonist activity of pindolol

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Abstract
The partial agonist activity of pindolol was assessed by examining the action of cumulative doses on the heart rates of resting, standing and exercising healthy men and by studying the interaction of pindolol with metoprolol, a .beta.-blocker devoid of partial agonist activity. Pindolol did not affect resting or standing heart rates (RHR, SHR) but reduced the heart rate after vigorous exercise by .apprx. 25%. The flatter dose-response curve of pindolol for exercise heart rate (EHR) has been reported for practolol and oxprenolol, which also exert partial agonist activity. After extremely large doses of pindolol there was no evidence of enhancement of agonist activity on RHR, nor was there any evidence of dominance of agonist activity over antagonist activity on EHR. Metoprolol did not alter RHR but reduced SHR by .apprx. 20% and EHR by .apprx. 31%. The effects of pindolol on SHR and EHR were not enhanced by metoprolol, even though the drug by itself induced greater reductions of both. The reduction of SHR by metoprolol was reversed by pindolol. Pindolol appears to have greater affinity than metoprolol for atrial .beta.-adrenoceptors in man.