Reaction of isolated pig iris sphincter to electrical stimulation and acetylcholine

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Abstract
The isolated iris ring of the pig irrigated with Tyrode's solution was suspended on two platinum hooks which served as stimulating electrodes. Isometric contractions of the iris sphincter were elicited by injections of acetylcholine (ACh) into the bathing medium or by short periods of electrical stimulation. The results obtained with atropine, Prostigmine, diisopropylfluorophosphate and Xylocaine suggested that electrical stimulation produced sphincter contractions indirectly, i.e., via activation of postganglionic nerve fibers and release of ACh. The effects of electric stimulation or of administration of ACh differed in many respects from those induced by stretch. A dose of atropine or iodoacetate which prevented the effect of the former two procedures did not attenuate the latter. Stretch enhanced the reactivity to injections of ACh and electrical stimulation; continuous administration of ACh initially increased and then decreased the contractile responses to strong electrical stimuli or single superimposed injections of ACh. Prolonged electrical stimulation usually decreased the effect of superimposed administration of ACh.