Regulation of hormone secretion by acute cell volume changes: Ca(2+)-independent hormone secretion

Exocytosis of intravesicular material should help a cell meet a relative extracellular hyposmotic challenge by expanding the plasmalemma through fusion with vesicular membrane. Cell swelling evokes an immediate secretory burst of hormones stored in secretory vesicles with dynamics indistinguishable from those induced by specific secretagogues. Hormone secretion induced by cell swelling is not associated with a rise in cAMP, IP(3), or prostaglandins, and it is not depressed by inhibition of stretch mechano-receptors or aquaporin channels. In contrast to most types of regulated secretion, that induced by cell swelling in normal cells does not require a rise in intracellular Ca(2+) through opening L-type Ca(2+) channels. However, such Ca(2+) influx is essential for cell-swelling induced secretion in tumor-derived pituitary cells. Cell swelling induces universal secretion of exocytotic material. The response of cells specialized in osmoregulation is, however, different. Possible physiological significance: Consistent stimulation of secretion occurs with a 4% hyposmolar challenge. It is likely that fluctuations in osmotic pressure with resultant cell volume changes have a significant regulatory role in hormone secretion. Released hormones could also play an important role in the pathophysiology of ischemia. Exocytosis itself does not have an essential role in volume regulation.