Pre- and postjunctional alpha-adrenoceptors at sympathetic neuroeffector junction in bovine mesenteric lymphatics

We studied isolated bovine mesenteric lymphatics to elucidate the pharmacological characteristics of pre- and postjunctional alpha-adrenoceptors at the sympathetic neuroeffector junction. Cylindrical strips were incubated with [3H]-noradrenaline and mounted for superfusion. Electrical stimulation (2 Hz, 0.5 msec, 50 V) augmented the fractional release of labeled noradrenaline. Exogenous noradrenaline and clonidine caused a depression of the evoked tracer release. Phenoxybenzamine and yohimbine markedly enhanced the evoked overflow of adrenergic transmitter. Rings of lymphatic vessels were mounted for isometric tension recording in organ chambers filled with Krebs-Ringer bicarbonate solution. The vessels contracted when exposed to phenylephrine and clonidine. The ED50 of clonidine was significantly lower than that of phenylephrine. Prazosin caused a parallel shift to the right of the dose-response curve to phenylephrine. The antagonist, however, suppressed the magnitude of the maximum response to clonidine. Yohimbine caused parallel shift to the right of the dose-response curves to phenylephrine and clonidine, respectively. The Schild plots for yohimbine demonstrated that the drug was a competitive antagonist to phenylephrine and clonidine. The pA2 value of yohimbine to clonidine (7.6 +/- 0.4) was larger than that to phenylephrine (6.2 +/- 0.4). The pA2 value of prazosin to phenylephrine was 7.2 +/- 0.3. These results suggest that prejunctional alpha-adrenoceptors are involved in the negative feedback mechanism for autoregulation of noradrenaline release during postganglionic sympathetic nerve stimulation, and that both alpha 1- and alpha 2-like adrenoceptors do exist on lymphatic smooth muscle cells.