Pre- and Postjunctional Adrenoceptor Types in the Circular Muscle of the Guinea-pig Caecum

The sucrose gap technique was used to study the effects of applied adrenoceptor agonists on membrane potential and non-adrenergic, non-cholinergic inhibitory junction potentials (IJPs), in the smooth muscle of the circular coat of guinea-pig caecum. Noradrenaline (10 nmol/l-100 mumol/l), phenylephrine (1-100 mumol/l) and isoprenaline (0.1-100 mumol/l) caused hyperpolarisations of the smooth muscle membrane which were rapid in onset and effect. The magnitudes of hyperpolarisations elicited by noradrenaline were significantly reduced by the non-specific alpha-adrenoceptor antagonist phentolamine (10 mumol/l), the alpha 1-adrenoceptor antagonist prazosin (1 mumol/l) and the beta-adrenoceptor antagonist propranolol (5 mumol/l). The alpha 2-antagonist yohimbine (10 mumol/l) did not significantly reduce the magnitude of the hyperpolarisations induced by noradrenaline. Noradrenaline application caused a reduction in IJP amplitude, during hyperpolarisation, by up to 50%. The reduction of the IJP amplitude elicited by noradrenaline was significantly antagonised by yohimbine and phentolamine, but not by prazosin or propranolol. Clonidine caused a reduction of IJP amplitude by up to 20%, but neither phenylephrine nor isoprenaline caused any significant reduction in IJP amplitude. It is concluded that the hyperpolarising responses to exogenous noradrenaline in the circular muscle of guinea-pig caecum are mediated by postjunctional alpha 1- and beta-adrenoceptors, and that the inhibition of IJP amplitude is mediated by prejunctional alpha 2-adrenoceptors.