IMPAIRED G-PROTEIN FUNCTION IN GALLBLADDER MUSCLE FROM PROGESTERONE-TREATED GUINEA-PIGS

This study was designed to elucidate the mechanism of action of proges terone on gallbladder smooth muscle in guinea pigs. Adult male pigs we re treated with either progesterone (2 mg.kg(-1).day(-1)) or saline fo r 7 days. Gallbladder muscle cells were isolated by enzymatic digestio n with collagenase. Contractile responses td agonists were expressed a s percent shortening from control cell length. [S-35]guanosine 5'-O-(3 -thiotriphosphate) ([S-35]GTP gamma S)-binding properties of G protein s were assessed in crude membranes of gallbladder muscle with or witho ut cholecystokinin octapeptide (CCK-8) stimulation. Gallbladder muscle cells from progesterone-treated guinea pigs exhibited an impaired con tractile response to CCK-8, GTP gamma S, or aluminum fluoride but a no rmal response to potassium chloride or D-myo-inositol 1,4,5-trisphosph ate compared with controls. Western blot analysis of gallbladder muscl e revealed the presence of G(i1-2), G(i3), G(q/11), and G(s) proteins. The maximal contraction induced by CCK-8 was blocked by pertussis tox in and G(i) alpha(3)-specific antibodies, but not by G(i) alpha(1-2) o r C(q/11)alpha antibodies. CCK-8 caused a significant increase in [S-3 5]GTP gamma S binding to G(i) alpha(3), but not to G(q/11)alpha or G(i ) alpha(1-2) The stimulation of G(i) alpha(3) binding, however, was si gnificantly reduced in gallbladder muscle membranes from progesterone- treated guinea pigs compared with that in control animals. In conclusi on, progesterone might cause gallbladder hypomotility by downregulatin g G(i3) proteins.