Release of noradrenaline by labetalol in the rat anococcygeus muscle

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Abstract
The effects of labetalol on the accumulation and spontaneous release of (-)-[3H]noradrenaline, and on contractile responses to exogenously applied (-)-noradrenaline were studied in the isolated anococcygeus muscle of the rat. Labetalol (3×10−7–10−4M) inhibited the accumulation of (-)-[3H]noradrenaline. Labetalol (10−6–10−4M) and guanethidine (6×10−6M) increased the spontaneous release of [3H] following incubation of the muscle with (-)-[3H]noradrenaline. Nortriptyline (10−6M) had no effect on the spontaneous release of [3H], antagonised the increased release of [3H] produced by 6×10−6 M guanethidine but not that observed with 10−5M labetalol. Labetalol (5×10−6M) markedly increased the loss of tritiated deaminated metabolites with little change in the loss of (-)-[3H]noradrenaline. Labetalol (10−7–10−5M), alone or in the presence of 10−6M nortriptyline, had no effect on contractile responses to (-)-noradrenaline. Following pretreatment with 6-hydroxydopamine (10−3M for 3 h) to deplete endogenous noradrenaline stores, labetalol (10−7–10−5M) inhibited responses to exogenously applied (-)-noradrenaline. These results suggest that, in the rat anococcygeus muscle, labetalol is a noradrenaline releasing agent and an α-adrenoceptor antogonist.