Influence of progesterone on guinea pig gallbladder motility in vitro.

Experiments were designed to examine the in vitro effect of progesterone (1.6 X 10(-5) M; 8 X 10(-5) M) on the tone and contractility of guinea pig gallbladder smooth muscle. Muscle strips from animals of both sexes were stimulated with acetylcholine, cholecystokinin-octapeptide, histamine, and potassium prior to and 15 min after exposure to progesterone. The results were as follows: progesterone at all doses studied completely abolished spontaneous phasic contractions; progesterone produced a concentration-dependent decrease in resting tension; the hormone significantly decreased the maximal contractile response to each of the agonists but had no effect on either the ED50 of the acetylcholine and cholecystokinin dose-response curves; and estrogen priming is not necessary to demonstrate an inhibitory effect of progesterone. The data suggest that progesterone can directly affect the force-generating capacity of gallbladder smooth muscle both at rest and in response to stimulation. However, because of the relatively high concentration of hormone used, the physiological significance of the data must remain speculative.