title: Pre- and postjunctional adrenoceptor types in the circular muscle of the guinea-pig caecum.
abstract: 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.
mesh_terms: D000818:Animals; D002432:Cecum / Q000502:physiology; D004347:Drug Interactions; D004558:Electric Stimulation; D005071:Evoked Potentials / Q000187:drug effects; D005260:Female; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007545:Isoproterenol / Q000494:pharmacology; D008297:Male; D009130:Muscle, Smooth / Q000502:physiology*; D009638:Norepinephrine / Q000494:pharmacology; D010646:Phentolamine / Q000494:pharmacology; D010656:Phenylephrine / Q000494:pharmacology; D011224:Prazosin / Q000494:pharmacology; D011433:Propranolol / Q000494:pharmacology; D011941:Receptors, Adrenergic / Q000187:drug effects*; D013696:Temperature; D015016:Yohimbine / Q000494:pharmacology
chemical_list: D011941:Receptors, Adrenergic; D010656:Phenylephrine; D015016:Yohimbine; D011433:Propranolol; D007545:Isoproterenol; D009638:Norepinephrine; D011224:Prazosin; D010646:Phentolamine