In circular muscle strips of the fundus and corpus of guinea-pig stomach, mechanical responses to catecholamines were studied mainly in the presence of a prostaglandin biosynthesis inhibitor, meclofenamate. 2. Normal preparations developed considerable muscle tone, and adrenaline (10-100 microM) in the presence of 3-5 microM propranolol produced a multiphasic response, generally consisting of transient relaxation and contraction, followed by slow relaxation and then contraction. Responses to phenylephrine were similar to those of adrenaline. 3. Meclofenamate (0.3 microM) nearly abolished the muscle tone and under this condition, both adrenaline and phenylephrine produced a simple contraction. This response was strongly inhibited by prazosin, but only weakly by yohimbine. 4. When muscle tone was maintained by prostaglandin E2 (10 nM) in the presence of meclofenamate, phenylephrine (30 microM) produced transient relaxation followed by slow contraction in most preparations. These were strongly inhibited by prazosin. Adrenaline produced a similar response, but the relaxation was only partially reduced by prazosin. The remaining relaxation was more dominant in the middle fundic region and this was considered to be mediated through beta-adrenoceptors. 5. It is concluded that in the circular muscle of the fundic region of guinea-pig stomach, endogenous prostaglandins are involved in maintaining muscle tone and in modifying the response to catecholamines and that both contraction and relaxation are mediated by alpha 1-adrenoceptors.
