Adrenoceptors in the guinea pig's gallbladder

Adrenergic mechanisms in the control of gastrointestinal motility: from basic science to clinical applications

Over the years, a vast literature has accumulated on the adrenergic mechanisms controlling gut motility, blood flow, and mucosal transport. The present review is intended as a survey of key information on the relevance of adrenergic mechanisms modulating gut motility and will provide an outline of our knowledge on the distribution and functional role of adrenoceptor subtypes mediating motor responses. alpha1-Adrenoceptors are located postsynaptically on smooth muscle cells and, to a lesser extent, on intrinsic neurons; alpha2-adrenoceptors may be present both pre- and postsynaptically, with presynaptic auto- and hetero-receptors playing an important role in the modulation of neurotransmitter release; beta-adrenoceptors are found mainly on smooth muscle cells. From a clinical standpoint, adrenoceptor agonists/antagonists have been investigated as potential motility inhibiting (antidiarrheal/antispasmodic) or prokinetic agents, although at present their field of application is limited to select patient groups.

Contribution of prostaglandin to the contraction induced by catecholamines in the isolated gallbladder of the guinea-pig

Possible contribution of prostaglandins to the mechanical response produced by catecholamines was investigated in the isolated strips of the guinea-pig gallbladder. The contraction by catecholamine was blocked by an alpha-blocker, phentolamine (10(-6)M and relaxation was blocked by a beta-blocker, propranolol (10(-6)M. The rhythmic spontaneous motility and intrinsic tone were markedly inhibited by a prostaglandin synthesis inhibitor, indomethacin (10(-7)M. Furthermore, exogenous prostaglandins F2 alpha, E1 and E2 produced potent stimulant actions on gallbladder and the order of potency was found to be E2 > E1 > F2 alpha. The contractile response of catecholamines was abolished almost completely by indomethacin (10(-7)M. Even after the muscle tone was elevated by exogenous prostaglandin after treatment with indomethacin, catecholamine produced no contractile response, although it still relaxed the preparation through activation of beta-receptor. Based on these results, it was suggested that endogenous prostaglandins appear to be contributed to the alpha-action of catecholamines in the isolated guinea-pig gallbladder.