Comparison of medroxalol and labetalol, drugs with combined alpha- and beta-adrenoceptor antagonist properties.

Medroxalol, a new drug which has been reported to possess both alpha- and beta-adrenoceptor antagonist properties, was compared with labetalol in single dose studies in normotensive males. Oral doses of 400 mg of each drug significantly reduced blood pressure, both supine and erect, without significant changes in heart rate. The chronotropic and diastolic depressor responses to isoprenaline were significantly antagonised by both drugs, for at least 6-8 h after drug administration. Medroxalol was associated with significantly greater beta-adrenoceptor antagonism than labetalol. There was no significant antagonism by either drug of the pressor response to noradrenaline. However, in a sub-group of subjects the responses to the selective alpha 1-adrenoceptor agonist, phenylephrine, were consistent with modest alpha 1-adrenoceptor antagonism. This was significant only for labetalol. The ratios of beta 1- to alpha 1-adrenoceptor antagonism as calculated from the relative shifts of the isoprenaline and phenylephrine dose-response curves, were 3 to 1 for labetalol and about 7 to 1 for medroxalol.