We studied the effects of prelytic copper concentrations on cell volume, intracellular pH, and ion transport in lamprey erythrocytes. Ion fluxes and pH were measured by radioactive tracer technique, patch clamp, and flame photometry. Prelytic CuSO4 concentration of 100 mu M caused anion-dependent intracellular acidification and increase in Cl- influx after 2 min lag-phase. In the presence of ascorbate copper effect was amplified and lag-phase was skipped. Pretreatment of the cells with N-phenyl maleimide abolished copper-induced changes completely. Copper treatment caused an increase in Na+ fluxes in both directions and a net Na+ uptake. Copper-induced Na+ transport was partially amiloride(MIA)-sensitive representing Na+/H+ exchange. The nature of the amiloride-insensitive fraction of copper-activated Na+ influx remains unknown. Cell swelling after 15 min of copper exposure induced regulatory volume decrease response involving KCI extrusion via K+ and Cl- volume-sensitive channels. We suggest that the effects of copper on ion transport fit the following sequence of events: (i) cupric ions are reduced to cuprous state on the membrane surface, (ii) electroneutral pairs CuCl and CuOH mediate chloride/hydroxyl exchange, as shown before for trialkyltin, dissipating transmembrane pH gradient, and (iii) changes in intracellular pH result in the activation of the Na+/H+ exchange and consecutive volume changes cause the RVD response. (C) 1999 Academic Press.