HYPER-OSMOTIC REGULATION OF VOLTAGE-GATED CALCIUM CURRENTS IN RAT ANTERIOR PITUITARY CELLS. Matzner, Orna, Shlomo Ben-Tabou and Itzhak Nussinovitch. Department of Anatomy and Embryology, Hebrew-University Hadassah Medical School, Jerusalem, Israel.
APStracts 2:0335N, 1995.
SUMMARY AND CONCLUSIONS
1. The sensitivity of voltage-gated calcium currents to hyper-osmotic media containing mannitol or sucrose (373-723 mOsm) and to the dihydropyridine (DHP) calcium channel agonist Bay K 8644 was examined in enriched populations of rat anterior pituitary somatotrophs using the whole-cell mode of the patch clamp technique. 2. Hyper-osmotic media reduced the amplitude of voltage-gated calcium currents. With 61.9% increase in extracellular medium osmolarity (523 mOsm) low voltage-activated (LVA) calcium currents were reduced to 67.9+/- 17.8% of control size and high voltage-activated (HVA)calcium currents were reducrd to 57.0+/-5.7% (mean+/-SD) of control size. The hyper-osmotic suppression of HVA calcium currents was usually accompanied with a negative shift of 6.0+/-2.9 mV (mean+/-SD) in the activation curve of HVA currents. 3. The DHP calcium channel agonist Bay K 8644 (10[mu]M), which stimulates hormone secretion from somatotrophs, increased the amplitude of HVA calcium currents to 212.6+/-67.2% (mean+/-SD) of their control size, prolonged their tail currents and negatively shifted the activation curve of HVA calcium currents by 6.2+/-2.8 mV. 4. Hyper-osmotic media reduced the amplitude of DHP sensitive HVA calcium currents and their associated prolonged tail currents, thus providing direct evidence for hyper-osmotic suppression of DHP sensitive currents. 5. Hence, exposure of pituitary cells to hyper-osmotic media reduced voltage sensitive calcium influx through LVA and DHP sensitive HVA calcium channels. The inhibition of calcium influx through DHP sensitive channels, which are implicated in regulation of hormone secretion in these cells, suggest that inhibitory hyper-smotic effects on hormone secretion from pituitary cells may stem from inhibition of calcium influx, prior to the exocytotic process. These results may also be relevant to effects of hypertonicity on neurosecretion at the nervous system. 

Received 26 June 1995; accepted in final form 30 October 1995.
APS Manuscript Number J403-5.
Article publication pending J. Neurophysiol.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 November 95