Effect of low extracellular calcium on gallbladder contraction in vitro.

Increases in intracellular calcium ion mediate smooth muscle contraction, but the requirement for extracellular calcium availability during this process is unclear. We studied the intact guinea pig gallbladder in an organ bath to define the effect of low bath calcium on contractile responses to varying doses of cholecystokinin-octapeptide, histamine, and acetylcholine. The contractile responses to cholecystokinin-octapeptide and acetylcholine were dependent on the presence of calcium in the bath, with a 40-100% reduction in contraction when buffer with no added calcium was used. In contrast, the contractile response to histamine was unchanged when stimulation was carried out in low calcium buffer. The contractions of the guinea pig gallbladder induced by cholecystokinin-octapeptide and acetylcholine, but not histamine, appear to require near-physiologic levels of extracellular calcium.