Hyperosmotic regulation of voltage-gated calcium currents in rat anterior pituitary cells

1. The sensitivity of voltage-gated calcium currents to hyperosmotic 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 by using the whole cell mode of the patch-clamp technique. 2. Hyperosmotic media reduced the amplitude of voltage-gated calcium currents. With a 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 reduced to 57.0 +/- 5.7% (mean +/- SD) of control size. The hyperosmotic 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 microM), which stimulates hormone secretion from somatotrophs, increased the amplitude of HVA calcium currents to 212.6 +/- 67.2% 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. Hyperosmotic media reduced the amplitude of DHP-sensitive HVA calcium currents and their associated prolonged tail currents, thus providing direct evidence for hyperosmotic suppression of DHP-sensitive currents. 5. Hence, exposure of pituitary cells to hyperosmotic 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, suggests that inhibitory hyperosmotic effects on hormone secretion from pituitary cells may stem from inhibition of calcium influx, before the exocytotic process. These results may also be relevant to effects of hypertonicity on neurosecretion in the nervous system.