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

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Abstract
We have evaluated the presence of capacitative Ca2+entry (CCE) in guinea pig gallbladder smooth muscle (GBSM), including a possible relation with activation of L-type Ca2+channels. Changes in cytosolic Ca2+concentration induced by Ca2+entry were assessed by digital microfluorometry in isolated, fura 2-loaded GBSM cells. Application of thapsigargin, a specific inhibitor of the Ca2+store pump, induced a transient Ca2+release followed by sustained entry of extracellular Ca2+. Depletion of the stores with thapsigargin, cyclopiazonic acid, ryanodine and caffeine, high levels of the Ca2+-mobilizing hormone cholecystokinin octapeptide, or simple removal of external Ca2+resulted in a sustained increase in Ca2+entry on subsequent reapplication of Ca2+. This entry was attenuated by 2-aminoethoxydiphenylborane, L-type Ca2+channel blockade, pinacidil, and Gd3+. 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 Ca2+channel blockers, and Gd3+. We conclude that, in GBSM, release of Ca2+from internal stores activates a CCE pathway and depolarizes plasma membrane, allowing coactivation of voltage-operated L-type Ca2+channels. This process may play a role in excitation-contraction coupling in GBSM.