Influence of hypotonic swelling on 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 microM), CoCl2 (IC50 = 58 microM) 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 omega-conotoxin GVIA; (iii) insensitivity to preliminary depolarization by high potassium. 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 existence in synaptosomal plasma membrane of volume-dependent calcium-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.