Prostaglandin E2 and the bovine sphincter pupillae

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Abstract
1 The bovine isolated sphincter pupillae incubated in Krebs solution releases a biologically active substance tentatively identified as prostaglandin E2. 2 The prostaglandin did not appear to be of neural origin or to result merely from tissue degeneration. 3 The spontaneous release of prostaglandin E2-like material was related to the tone of the sphincter. Output increased as tone was acquired after setting up the tissue and fell when various procedures were used to reduce the tone. 4 Low concentrations of E and F-type prostaglandins produced slow, well-sustained contractions of the atonic sphincter, prostaglandin E2 being the most potent of those tested. The responses to prostaglandin E2 were antagonized selectively by a prostaglandin antagonist SC-19220 (a dibenzoxazepine derivative) which in higher concentrations caused dose-dependent relaxations of the preparation. 5 Prostaglandins did not appear to modulate transmission from nerve to muscle in the sphincter. 6 The hypothesis that prostaglandin E2 might be produced to act as a local hormone causing tonic contraction of the sphincter pupillae is discussed.