Calcium and gallbladder smooth muscle contraction in the guinea pig: effect of pregnancy.

Experiments were designed to evaluate the relative contributions of extracellular and intracellular calcium to the contractile process in gallbladder smooth muscle. Tissue strips from nonpregnant and pregnant guinea pigs were stimulated in vitro with maximally effective concentrations of potassium (80 mM), acetylcholine (10(-4) M), and cholecystokinin-octapeptide (80 nM), both in the presence and absence of extracellular calcium. The results were as follows. (a) Potassium-induced contractions were completely abolished in the absence of extracellular calcium and in the presence of verapamil (10 microM). Tissues from nonpregnant and pregnant animals were affected equally. (b) Removal of extracellular calcium or the addition of verapamil reduced but did not abolish the acetylcholine and cholecystokinin-octapeptide responses. Acetylcholine-induced contractions were more sensitive to the elimination of extracellular calcium. (c) For both acetylcholine and cholecystokinin-octapeptide, the contractile responses of tissues from pregnant animals were reduced when compared with control responses. These data suggest that contractions induced by acetylcholine and cholecystokinin-octapeptide utilize calcium from both extracellular and intracellular stores, and that pregnancy affects the response to these agonists by reducing the contribution of the intracellular calcium pool.