Coactivation of capacitative calcium entry and L-type calcium channels in guinea pig gallbladder.

We have evaluated the presence of capacitative Ca(2+) entry (CCE) in guinea pig gallbladder smooth muscle (GBSM), including a possible relation with activation of L-type Ca(2+) channels. Changes in cytosolic Ca(2+) concentration induced by Ca(2+) entry were assessed by digital microfluorometry in isolated, fura 2-loaded GBSM cells. Application of thapsigargin, a specific inhibitor of the Ca(2+) store pump, induced a transient Ca(2+) release followed by sustained entry of extracellular Ca(2+). Depletion of the stores with thapsigargin, cyclopiazonic acid, ryanodine and caffeine, high levels of the Ca(2+)-mobilizing hormone cholecystokinin octapeptide, or simple removal of external Ca(2+) resulted in a sustained increase in Ca(2+) entry on subsequent reapplication of Ca(2+). This entry was attenuated by 2-aminoethoxydiphenylborane, L-type Ca(2+) channel blockade, pinacidil, and Gd(3+). Accumulation of the voltage-sensitive dye 3,3'-dipentylcarbocyanine and direct intracellular recordings showed that depletion of the stores is sufficient for depolarization of the plasma membrane. Contractility studies in intact gallbladder muscle strips showed that CCE induced contractions. The CCE-evoked contraction was sensitive to 2-aminoethoxydiphenylborane, L-type Ca(2+) channel blockers, and Gd(3+). We conclude that, in GBSM, release of Ca(2+) from internal stores activates a CCE pathway and depolarizes plasma membrane, allowing coactivation of voltage-operated L-type Ca(2+) channels. This process may play a role in excitation-contraction coupling in GBSM.