Effects of osmotic swelling on voltage-gated calcium channel currents in rat anterior pituitary cells

Decrease in extracellular osmolarity (Os]_e) results in stimulation of hormone secretion from pituitary cells. Different mechanisms can account for this stimulation of hormone secretion. In this study we examined the possibility that hyposmolarity directly modulates voltage-gated calcium influx in pituitary cells. The effects of hyposmolarity on L-type (I_L) and T-type (I_T) calcium currents in pituitary cells were investigated by using two hyposmotic stimuli, moderate (18-22% decrease in Os]_e) and strong (31-32% decrease in Os]_e). Exposure to moderate hyposmotic stimuli resulted in three response types in I_L (a decrease, a biphasic effect, and an increase in I_L) and in increase in I_T. Exposure to strong hyposmotic stimuli resulted only in increases in both I_L and I_T. Similarly, in intact pituitary cells (perforated patch method), exposure to either moderate or strong hyposmotic stimuli resulted only in increases in both I_L and I_T. Thus it appears that the main effect of decrease in Os]_e is increase in calcium channel currents. This increase was differential (I_L were more sensitive than I_T) and voltage independent. In addition, we show that these hyposmotic effects cannot be explained by activation of an anionic conductance or by an increase in cell membrane surface area. In conclusion, this study shows that hyposmotic swelling of pituitary cells can directly modulate voltage-gated calcium influx. This hyposmotic modulation of I_L and I_T may contribute to the previously reported hyposmotic stimulation of hormone secretion. The mechanisms underlying these hyposmotic effects and their possible physiological relevance are discussed. L-type channels; mechanosensitivity; somatotrophs; lactotrophs