Vasodilating mechanism and response to physiological pressor stimuli of acute doses of carvedilol compared with labetalol, propranolol and hydralazine.

There is conflicting evidence regarding the main mechanism of the vasodilating effect with carvedilol at therapeutic doses, and to examine this, single doses of carvedilol 50mg and 100mg were compared with labetalol 400mg, propranolol 160mg, propranolol 80mg plus hydralazine 50mg and placebo in healthy subjects. Dose-response studies (required to increase heart rate or systolic blood pressure by 25 beats/min and 20mm Hg, respectively) were performed with phenylephrine, angiotensin and isoprenaline after each drug, and placebo administration and the effects of physiological pressor stimuli were compared. Phenylephrine systolic pressure dose-response curves were shifted by labetalol (dose ratio 2.4) and both carvedilol doses (dose ratios 50mg 1.9, 100mg 20.2). The slight shift to the right of the angiotensin dose-response curves with hydralazine plus propranolol (dose ratio 1.4) and carvedilol 50mg (dose ratio 1.4) was not significant. beta-Blockade was greatest with propranolol 160mg, followed by carvedilol 100mg, propranolol 80mg plus hydralazine 50mg, carvedilol 50mg and was least with labetalol 400mg (isoprenaline dose ratios required to increase heart rate by 25 beats/min were 55.2, 27.2, 20.2, 14.2, 11.5, respectively). Blood pressure rise with cold pressor and isometric exercise was inhibited most by labetalol. At these acute doses carvedilol displayed some alpha-blockade, but the lower ratio of alpha-blockade to beta-blockade differed from that seen with labetalol, which may account for the different haemodynamic responses at rest and during physiological pressor stimuli with the 2 drugs. There was no definite evidence of direct vasodilator effect.