Homogeneous and non-homogeneous distribution of inhibitory and excitatory adrenoceptors in the longitudinal muscle of the guinea-pig ileum

1 The effects of adrenoceptor agonists and antagonists on spontaneous and evoked membrane activities of longitudinal muscle cells from different parts of the guinea-pig ileum were observed, using microelectrode methods.

2 Isoprenaline inhibited the generation of spikes in cells in the terminal (0-3 cm from the ileocaecal valve) and proximal (more than 50 cm from the ileocaecal valve) regions of the ileum, with no change on the membrane potential and ionic conductance of the membrane. These actions of isoprenaline were abolished by propranolol.

3 Noradrenaline and phenylephrine depolarized the membrane and increased both the spike frequency and ionic conductance of cell membranes of the terminal ileum, whereas noradrenaline and clonidine hyperpolarized the membrane, increased the ionic conductance of the membrane and inhibited the spontaneously generated spikes from cells of the proximal ileum. The excitatory effect of phenylephrine in the cells of the proximal ileum and the inhibitory effect of clonidine on cells of the terminal ileum were less pronounced.

4 The excitatory actions of noradrenaline or phenylephrine were antagonized by prazosin and phentolamine, but not by yohimbine, whereas the inhibitory actions of noradrenaline or clonidine were antagonized by yohimbine and phentolamine but not by prazosin.

5 The cholinergic e.j.ps evoked by field stimulation to the tissue were not affected by isoprenaline or phenyleprine but were inhibited by noradrenaline and clonidine, in both the terminal and proximal regions of the ileum. These actions of noradrenaline and clonidine were antagonized by yohimbine but not by prazosin.

6 The results indicate that in the myenteric plexus and longitudinal muscle tissues of the guinea-pig ileum there are prejunctional inhibitory (α₂), postjunctional inhibitory (α₂ and β) and postjunctional excitatory (α₁) adrenoceptors. The homogeneous distributions of prejunctional α₂- and postjunctional β-adrenoceptors in the ileum are responsible for inhibitions of cholinergic excitatory junction potentials (e.j.ps) and spontaneous spike activities, respectively. The density of distribution of the postjunctional α₁-adrenoceptors is higher in the terminal than in the proximal regions, and these distributions are reversed in the case of the postjunctional α₂-adrenoceptors. The postjunctional α₁-adrenoceptors are probably responsible for the membrane depolarization and α₂-adrenoceptors for the hyperpolarization induced by catecholamines.