[Studies of the effect of 2,3-dioxoindolin (isatin) in comparison with serotonin on the isolated guinea pig ileum].

In preparations of the isolated guinea-pig ileum 2,3-dioxoindoline shows a dose-dependent stimulating activity. In a contraction range between 5 x 10(-5) to 2 x 10(-3) moles/l this compound is spasmogenic and causes concentration (pD2 = 3.3). Prolonged contact time of higher concentrations of isatin results in a reduction of excitation and a relaxation of the ileum. An additive synergism is seen in combined treatment with serotonin. Pretreatment with serotonin antagonists reduces the stimulating effect of isatin. Quantitative differences in inhibition by dihydroergotamine and benzyloxygramine in contrast to morphine or atropine actions provided evidence that the stimulating activity of isatin is mainly mediated through the "D" receptors on the smooth muscle cells. Besides its stimulating effect isatin also exhibits blocking activity when tested against 5-HT (ACh). In high concentration (> 2 mmoles/l) isatin is able to antagonize--after only 2 min of pre-incubation--responses to 5-HT completely. In this range dose-response curves become progressively flatter and their maxima decline in a non-competitive way.