OSMOTIC SWELLING-INDUCED CHANGES IN CYTOSOLIC CALCIUM DO NOT AFFECT REGULATORY VOLUME DECREASE IN RAT CULTURED SUSPENDED CEREBELLAR ASTROCYTES

Hyposmotic swelling-induced changes in intracellular Ca2+ concentratio n ([Ca2+](i)) and their influence on regulatory volume decrease (RVD) were examined in rat cultured suspended cerebellar astrocytes, Hyposmo tic media (50 or 30%) evoked an immediate rise in [Ca2+](i) from 117 n M to a mean peak increase of 386 (50%) and 220 nM(30%), followed by a maintained plateau phase. Ca2+ influx through the plasmalemma as well as release from internal stores contributed to this osmosensitive [Ca2 +](i) elevation. Omission of external Ca2+ or addition of Cd2+, Mn2+, or Gd3+ did not reduce RVD, although it was decreased by La3+ (0.1-1 m M), Verapamil did not affect either the swelling-evoked [Ca2+](i) or R VD, Maneuvers that deplete endoplasmic reticulum (ER) Ca2+ stores, suc h as treatment (in Ca2+-free medium) with 0.2 mu M thapsigargin (Tg), 10 mu M 2,5-di-tert-butylhydroquinone, 1 mu M ionomycin, or 100 mu M A TP abolished the increase in [Ca2+](i) but did not affect RVD. However , prolonged exposure to 1 mu M Tg blocked RVD regardless of ER Ca2+ co ntent or cytosolic Ca2+ levels. Ryanodine (up to 100 mu M) and caffein e (10 mM) did not modify [Ca2+](i) or RVD, BAPTA-acetoxymethyl ester ( 20 mu M) abolished [Ca2+](i) elevation without affecting RVD, but at h igher concentrations BAPTA prevented cell swelling and blocked RVD. We conclude that the osmosensitive [Ca2+](i) rise occurs as a consequenc e of increased Ca2+ permeability of plasma and organelle membranes, bu t it appears not relevant as a transduction signal for RVD in rat cult ured cerebellar astrocytes.