The dose dependency of the alpha- and beta-adrenoceptor antagonist activity of carvedilol in man.

1. The alpha- and beta-adrenoceptor antagonist activity of carvedilol, a beta-adrenoceptor antagonist with vasodilating properties, and labetalol were investigated in 10 healthy male subjects. They received infusions with serially increasing concentrations of isoprenaline and phenylephrine before and after single oral doses of carvedilol 6.25, 12.5 and 25 mg, labetalol 400 mg and placebo at weekly intervals in a double-blind randomised manner. An exercise step test was performed at the end of the infusions. 2. The dose of isoprenaline required to increase heart rate by 25 beats min-1 (I25) and the dose of phenylephrine required to increase systolic and diastolic blood pressure by 20 mm Hg (PS20 and PD20) were calculated using a quadratic fit to individual dose-response curves. Comparisons were made with placebo and P < 0.05 was considered significant. 3. The I25 was increased by carvedilol 25 mg and labetalol 400 mg (P < 0.05). The dose ratios at I25 were: carvedilol 6.25 mg 2.1 +/- 1.6, carvedilol 12.5 mg 3.1 +/- 1.9, carvedilol 25 mg 6.4 +/- 4.9 and labetalol 400 mg 8.8 +/- 4.4. 4. The PS20 was increased by labetalol 400 mg (P < 0.05). The dose ratios at PS20 were: carvedilol 6.25 mg 1.0 +/- 0.2; 12.5 mg, 1.2 +/- 0.2; 25 mg, 1.3 +/- 0.4 and labetalol 400 mg 2.2 +/- 0.8. 5. The PD20 was increased by labetalol 400 mg (P < 0.05). The dose ratios at PD20 were: carvedilol 6.25 mg 1.1 +/- 0.3; 12.5 mg, 1.3 +/- 0.3; carvedilol 25 mg 1.3 +/- 0.4 and labetalol 400 mg 2.1 +/- 0.8.(ABSTRACT TRUNCATED AT 250 WORDS)