Effect of labetalol on adrenoceptors in the isolated perfused rat mesentery.

Isolated perfused rat mesentery has been used to define alpha- and beta-adrenoceptors. Norepinephrine was used as a reference substance to induce vasoconstrictor responses (increase in perfusion pressure). Log-dose response curves were obtained in the dose range between 0.3-10 mug. Labetalol, a known competitive inhibitor of both alpha- and beta-adrenoceptors, when administered as a single bolus injection (3 mug) initially induced predominant alpha-blockade lasting upto 75 min followed by potentiation of the vasoconstrictor responses to norepinephrine used at multiple dose levels. This potentiation was maximum between 150-195 min after injection of labetalol. This potentiating effect of labetalol observed in the later phase of the experiment might be related to its inhibitory mechanism of neuronal uptake.