Anticholinergic properties of progesterone in the isolated ileum of the guinea‐pig

To determine if the cholinergic system is involved in the intestinal smooth muscle relaxant action of progesterone, segments of guinea‐pig ileum were mounted in 20 ml chambers containing Krebs solution. In one group of experiments, we studied the effect of various doses of progesterone and atropine on the twitch‐like contraction of the ileum caused by electrical stimulation. In the second group of experiments, dose‐response curves were constructed for acetylcholine, histamine, and barium chloride alone and in the presence of progesterone or atropine. Progesterone, like atropine, decreased in a dose‐dependent manner the phasic contractions and tone of the unstimulated intestinal segments, and exerted a rapid, reversible, and dose‐dependent inhibitory action on intestinal responses to electrical stimulation. In addition, in the presence of the hormone or atropine, the dose‐response curve to acetylcholine was clearly shifted to the right in a parallel manner with no change in maximal response. At the concentrations tested, neither progesterone nor atropine blocked the contractile effect of barium chloride on the ileum and only slightly reduced the spasmogenic action of histamine. These results suggest that the relaxant action of progesterone on the intestinal smooth muscle may be mediated through muscarinic receptors, indicating that progesterone is an endogenous product with anticholinergic activity. © 1996 Wiley‐Liss, Inc.