Differentiation between pre‐ and postjunctional α‐receptors in guinea pig ileum and rabbit aorta

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Abstract
Electric stimulation of proximal and terminal guinea pig ileum induced contractions which were mediated by stimulation of cholinergic neurons. After blocking β‐receptors, prejunctional α‐receptors mediating inhibition could be studied in these neurons. In proximal ileum the order of potencies of the following agonists was: clonidine (8)* < epinephrine (3) < oxymetazoline (2)< norepinephrine (1)< phenylephrine (0.02). The intrinsic activities of clonidine and phenylephrine were not maximal. In electrically stimulated terminal ileum practically identical results were obtained on the prejunctional α‐receptor except that phenylephrine was ineffective; the drug instead acted as a competitive blocker against norepinephrine. After blocking cholinergic receptors with atropine, excitatory α‐receptors located postjunctionally in the smooth muscle cells of terminal ileum could be studied. Under these conditions the order of potencies of the agonists was: oxymetazoline (40)< epinephrine (7) < phenylephrine (4) clonidine (0.04). On the postjunctional α‐receptor in terminal guinea pig ileum phentolamine was 55 fold more effective in blocking the response of norepinephrine than it was on blocking the response on the prejunctional receptor of the same preparation. Phenoxybenzamine (4 times 10‐8M) was ineffective on the prejunctional α‐receptor of the cholinergic neuron, while it blocked the postjunctional α‐receptor of smooth muscle in guinea pig terminal ileum noncompetitively. It is suggested that the differences in the activities of clonidine and phenylephrine between prejunctional α‐receptors located to cholinergic neurons and postjunctional α‐receptors located to smooth muscle cells was due to a pharmacological difference between the receptors. The results obtained with the blockers support this suggestion.

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