Progesterone inhibits the contractile motility of the guinea pig gallbladder

We studied the effects of progesterone on the contractile motility of the guinea pig gallbladder in vitro. Carbachol (10(-6) M) induced contractions were reduced by the pretreatment with progesterone (10(-8)-10(-6) M) in a dose-dependent manner. Concentration-response curves for carbachol, histamine and CCK-OP showed inhibition by progesterone (5 X 10(-7) M). These results suggest that progesterone has a direct inhibitory effect on gallbladder smooth muscle. Contractile responses to potassium (10-60 mM) or calcium (0.4-3.2 mM), which were thought to activate the contractile machinery by increasing the influx of extracellular calcium, were not affected by the pretreatment of progesterone. The direct inhibitory effects of progesterone on gallbladder smooth muscle might be explained by the inhibition of calcium release from the intracellular storage sites.