Some peculiarities of the Na(+)-Ca2+ exchange reaction in choline, tetramethylenammonium and Tris media.

N a + C a 2 + exchange has been well defined in the membranes of excitable tissues. Its main role is suggested to be the extrusion of calcium from the cells at the expense of t ransmembrane electrochemical sodium gradient. T h e system has been studied in detail m vitro in membrane vesicles loaded with sodium ions and transferred into media containing ions other t h a n sodium (e.g. Philipson 1985; DiPolo and Beaugé 1988; Reeves 1985; Sanchez-Armass and Blaustein 1987). The highest rates of the N a + C a 2 + exchange were observed in media containing potassium ions. W i t h a high probability, this is due to the fact t h a t in potassium medium two independent systems part icipate in calcium accumulation: voltage dependent calcium channels and N a + C a 2 + exchange (Coutinho et al. 1983; Duarte et al. 1991; Tagliatella et al. 1990). On the contrary, in our experiments with N a + C a 2 + exchanger from crayfish striated muscle reconstituted into asolectin phospholipids we observed higher 4 5 C a 2 + accumulation in vesicles incubated in choline medium as compared with potass ium medium (unpublished results). The experiments with the rat brain nerve endings and microsomes presented herein as well as those with the crayfish striated muscle membranes have shown t h a t not only choline, but also te t ramethylammonium and Tris ions in the external medium exhibited a higher 4 5 C a 2 + accumulation via N a + 4 5 C a 2 + exchange as compared with the external potassium medium.