Intrinsic .BETA.-sympathomimetic activity of dilevalol, R,R-isomer of labetalol.

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Abstract
The potency and selectivity of the intrinsic .beta.-sympathomimetic activity (ISA) of dilevalol were studied with isolated guinea-pig trachea and pithed rat. This drug relaxed the contracture of isolated guinea-pig tracheal preparation induced by histamine in a dose-dependent manner. The dose-response curves of dilevalol and labetalol were shifted to the right with propranolol pretreatment. The relaxing potency of dilevalol was approximately 4.7 times more potent than that of labetalol. Dilevalol markedly reduced the diastolic blood pressure with only a slight increase of heart rate in pithed rats, while isoproterenol and pindolol caused moderate to marked positive chronotropic effects in proportion to their hypotensive effects. These results suggest that dilevalol has more potent ISA than labetalol and shows more .beta.2-adrenoceptor selectivity than pindolol.