A Study on the Modes of Action of Angiotensin on Isolated Smooth muscle Preparations

Abstract

Isolated preparations of ilea from guinea-pigs, rats and rabbits responded to angiotensin with a biphasic contraction, No biphasic contractions were observed in guinea-pig taenia coli, rat stomach fundus strip, rat colon, rabbit aortic strip or rat portal vein preparations. The biphasic actions of angiotensin were examined using known pharmacological blocking agents in selective concentrations. The results obtained suggest that the receptors for angiotensin are probably different in the same tissues from different species and in different tissues of the same species. Using a modified sucrose-gap bath, the effect of angiotensin on the electrical and mechanical activity of the guinea-pig taenia coli was studied. The main action of angiotensin appeared to be an increase in permeability to sodium ions. Although the permeability to K+ Ca++ and C1- ions also appeared to be affected. Hyperpolarization was observed following angiotensin stimulation. The possible significance of this observation was discussed. The effect of prostaglandin biosynthesis inhibitors, aspirin and indomethacin, on angiotensin-induced contraction was examined in several smooth muscle preparations. Indomethacin selectively reduced angiotensin-induced contraction in all preparations used with the exception of the rat colon. Attempts were made to determine the relationship between prostaglandin and angiotensin action in the guinea-pig ileum. The results suggest that at least part of the contractile response to angiotensin in the guinea-pig ileum involves the release of or requires the presence of prostaglandin. Indomethacin selectively reduced the pressor response to angiotensin in the pithed rat. These results are discussed in the light of recent evidence that angiotensin can release prostaglandins from several isolated organs. It is concluded that  angiotensin besides being able to release acetylcholine, may also release prostaglandins which in turn can modulate the contractile actions of angiotensin. No single mode of action can be used to explain the action of angiotensin in causing contraction of a relatively small number of preparations of smooth muscle taken from a few similar species.