Submitted on March 14, 2005
Accepted on August 31, 2005
Effects of osmotic shrinkage on voltage-gated calcium channel currents in rat anterior pituitary cells
Shlomo Ben-Tabou De-Leon1, Galia Ben-Zeev1, and Itzhak Nussinovitch1*
1 Anatomy and Cell Biology, Hebrew Univresity, Hadassah Medical School, Jerusalem 91120, Israel
* To whom correspondence should be addressed. E-mail: nussin{at}md.huji.ac.il.
Increased extracellular osmolarity [Os]e suppresses stimulated hormone secretion from anterior-pituitary cells. Calcium influx may mediate this effect. We show that increase in [Os]e (by 18-125%) differentially suppresses L-type and T-type calcium 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 % 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 actin-disrupter Cytochalasin-D or with the actin-stabilizer phalloidin, suggesting that the actin cytoskeleton is not involved in this modulation. The hyperosmotic suppression of calcium influx was not correlated with changes in reversal-potential, membrane-capacitance and access-resistance. Together these results suggest that the hyperosmotic suppression of calcium influx involves calcium channel proteins. We therefore recorded the activity of L-type calcium channels from cell-attached patches while exposing the cell outside the patch pipette to hyperosmotic media. Increased [Os]e reduced the activity (NPo) of calcium channels but did not change single channel conductance. This hyperosmotic suppression of calcium currents may therefore contribute to the previously reported hyperosmotic suppression of hormone secretion.
