Effect of labetalol on the accumulation and release of noradrenaline in rat ventricle.

In rat ventricular tissue, labetalol inhibited the accumulation of (-)-[3H]noradrenaline and released [3H] following preloading with (-)[3H]noradrenaline. Cocaine (30 micrometer) inhibited the release observed with tyramine (5 micrometer) and beta-phenethylamine (5 micrometer) but not that observed with labetalol (5 micrometer). Reserpine pretreatment of the animals abolished the release observed with labetalol (5 and 50 micrometer). Labetalol primarily increased the loss of deaminated metabolites of noradrenaline. It is suggested that labetalol may release (-)-noradrenaline from the vesicles.