Effects of osmotic shrinkage on voltage-gated Ca channel currents in rat anterior pituitary cells

Ben-Tabou De-Leon, Shlomo, Galia Ben-Zeev, and Itzhak Nussinovitch. Effects of osmotic shrinkage on voltage-gated Ca channel currents in rat anterior pituitary cells. Am J Physiol Cell Physiol 290: C222–C232, 2006. First published September 7, 2005; doi:10.1152/ajpcell.00118.2005.—Increased extracellular osmolarity ([Os]e) suppresses stimulated hormone secretion from anterior pituitary cells. Ca influx may mediate this effect. We show that increase in [Os]e (by 18–125%) differentially suppresses L-type and T-type Ca channel currents (IL and IT, respectively); IL was more sensitive than IT. Hyperosmotic suppression of IL depended on the magnitude of increase in [Os]e and was correlated with the percent decrease in pituitary cell volume, suggesting that pituitary cell shrinkage can modulate L-type currents. The hyperosmotic suppression of IL and IT persisted after incubation of pituitary cells either with the actindisrupter cytochalasin D or with the actin stabilizer phalloidin, suggesting that the actin cytoskeleton is not involved in this modulation. The hyperosmotic suppression of Ca influx was not correlated with changes in reversal potential, membrane capacitance, and access resistance. Together, these results suggest that the hyperosmotic suppression of Ca influx involves Ca channel proteins. We therefore recorded the activity of L-type Ca channels from cellattached patches while exposing the cell outside the patch pipette to hyperosmotic media. Increased [Os]e reduced the activity of Ca channels but did not change single-channel conductance. This hyperosmotic suppression of Ca currents may therefore contribute to the previously reported hyperosmotic suppression of hormone secretion.