Localization of Adrenergic Receptors in Guinea Pig Ileum and Rabbit Jejunum to Cholinergic Neurons and to Smooth Muscle Cells

Abstract

The localization of adrenergic receptors mediating a relaxing action was investigated in innervated and denervated longitudinal muscle strips from guinea pig ileum and rabbit jejunum. Denervated preparations were contracted by drugs that had a direct effect on smooth muscle cells, such as acetylcholine and histamine, but not by stimuli acting on cholinergic neurons, such as electrical field stimulation or nicotine. After blockade of .beta.-adrenoceptors, norepinephrine [NE] relaxed the innervated guinea pig ileum contracted by electrical field stimulation, by stimulating .alpha.-adrenoceptors. NE in low concentrations did not relax denervated preparations contracted by agents acting directly on smooth muscle. In high concentrations, it relaxed denervated preparations by a nonadrenergic mechanism, resistant to .alpha.- and/or .beta.-receptor blockade, but which was also activated by 1-(3,4-dihydroxyphenyl) ethanol. Phenylephrine only had a weak agonistic effect on the electrically stimulated innervated preparation and did not relax the denervated one. The denervated rabbit intestine contracted by acetylcholine was relaxed by NE and phenylephrine by stimulation of .alpha.-adrenoceptors. In the innervated preparations both drugs were more effective in inhibiting contractions induced by electrical field stimulation or eserine than those induced by exogenous acetylcholine. Both the denervated guinea pig and rabbit intestine were relaxed by stimulation of .beta.-adrenoceptors. In the guinea pig ileum .alpha.-adrenoceptors mediating relaxation were located only in cholinergic neurons; in rabbit jejunum they were located in these neurons and in the smooth muscle cells. .beta.-Adrenoceptors were located in the smooth muscle cells of both organs.