The effect of an inhibitory factor from the bovine retractor penis on the gastro-intestinal tract and gall bladder of the guinea-pig.

1 The effect of a smooth muscle inhibitory factor extracted from the bovine retractor penis has been examined on a variety of in vitro smooth muscle preparations from the guinea-pig alimentary canal and on the guinea-pig gall bladder. 2 The inhibitory factor caused relaxation of spontaneous and carbachol-induced tone in the taenia coli, the stomach fundal strip and the duodenum and colon. There was little effect on the ileum. Sensitivity was highest in the taenia coli where the response to the inhibitory factor mimicked the response to stimulation of the non-adrenergic, non-cholinergic (NANC) inhibitory nerves. 3 In the taenia coli the inhibitory response to stimulation of the NANC nerves and to ATP was abolished by apamin 5 x 10(-8) M, whereas this or higher concentrations had no effect on the response to the inhibitory factor. This makes it unlikely that the latter is the neurotransmitter in these NANC nerves. 4 The inhibitory factor had no effect on the gall bladder. Inhibitory responses to field stimulation were obtained in this tissue but these were insensitive to tetrodotoxin in concentrations greater than those needed to block the motor cholinergic nerves.