Pergamon PII: S0197-0186(97)00033-8 Neurochem. Int. Vol. 31, No. 6, pp. 835-843,1997 © 1997 Elsevier Science Ltd Printedin Great Britain.All rightsreserved 0197~)186/97$17.00+0.00 HYPOOSMOTIC SHOCK ACTIVATES Ca 2+ CHANNELS IN ISOLATED NERVE TERMINALS ALEXANDER A. MONGIN 1., SERGEI L. AKSENTSEW, SERGEI N. ORLOV 2 and SERGEI V. KONEW qnstitute of Photobiology, Belarussian Academy of Sciences, Minsk, 220072, Belarus and 2Centre de Recherche, Hotel-Dieu de Montreal, University of Montreal, Montreal, Canada H2W 1T8 (Received 28 October 1996; accepted 15 April 1997) Abstract--Influence of hypotonic swellingon Ca2 ÷ (45Ca2+) uptake in rat brain synaptosomes was studied. A decrease in medium osmolality from 310 to 260-180 mOsm led to a progressive stimulation of 45Ca2 ÷ accumulation. The effect was blocked by verapamil (IC50 = 5/~M), COC12 (ICs0 = 58/~M) and retained at a fixed concentration of external sodium indicating the involvement of Ca2 ÷ channels rather than Na ÷/Ca2+ exchange in swelling-induced Ca2+ influx. The populations of calcium channels observed in hypoosmotic and depolarizing conditions are different in three aspects: (i) kinetics of 45Ca2+ entry; (ii) insensitivity to dihydropyridines and co-conotoxin GVIA; (iii) insensitivity to preliminary depolarization by high potass- ium. The effects of swelling and depolarization on Ca2+ uptake were additive. No change in membrane potential monitored with diS-C3-(5) was recorded during synaptosome hypotonic swelling. The results suggest the existencein synaptosomal plasma membrane of volume-dependentcalcium-permeable channels with properties distinct from those of the voltage-dependent calcium channels. Activation of these channels may constitute an early event in volume regulation of nerve terminals in anisoosmotic conditions. © 1997 Elsevier Science Ltd. All rights reserved There is evidence showing a compensatory adjustment of vertebrate brain volume to disturbance of body fluid osmolality (Gullans and Verbalis, 1993; Law, 1994). Mechanisms of brain adaptation to aniso- osmotic conditions are extensively investigated using cell models (for review see Kimelberg and Ransom, 1986; Ballanyi and Grafe, 1988; Kimelberg, 1995). As many other mammalian cell types studied neuronal cells and isolated nerve terminals are capable of returning their volume to normal after swelling under hypotonic conditions due to activation of K ÷ and anion conductances that results in efflux of intra- cellular K ÷, CI- and various amino acids (Hoffmann and Simonsen, 1989; Falke and Misler, 1989; Babila et al., 1990; Lang et al., 1993; Pasantes-Morales et al., 1993, 1994). In the nerve terminals an additional role in volume regulation may be played by the sodium pump (Fraser and Sarnaki, 1989; Mongin et al., 1992; Aksentsev et al., 1994). Both extracellular and intracellular Ca 2+ has been shown to influence regulatory cell volume decrease *To whom all correspondence should be addressed. Fax: +375-172-685359; E-mail: mongin@bas07.basnet. minsk.by. (RVD) and volume-dependent ion efflux in a number of cell types (for review see Pierce and Politis, 1990; McCarty and O'Neil, 1992) including brain astrocytes (O'Connor and Kimelberg, 1993; Bender and Nor- enberg, 1994) and neuroblastoma ceils (Falke and Misler, 1989). The entry of extracellular Ca 2+ into the cell during hypoosmotic stress may constitute an initial step in the mechanism of RVD (Christensen, 1987; McCarty and O'Neil, 1992; O'Connor and Kimelberg, 1993; Bender et al., 1994). Several alter- native Ca 2 ÷ permeation pathways have been proposed for swelling-induced Ca 2÷ entry, including Ca 2+- selective channels, voltage-dependent Ca 2+ channels, and stretch-activated Ca2+-permeating channels (McCarty and O'Neil, 1992). The present study was aimed at investigating the nature of swelling-induced Ca 2+ uptake in rat brain synaptosomes. Some of this work has been presented in a preliminary form (Mon- gin et al., 1995). EXPERIMENTAL PROCEDURES Materials Nitrendipine, verapamil, to-conotoxin GVIA, veratrine, COC12,CdC12, 3,3'-dipropylthiacarbocyanine iodide (diS-C3- 835