Osmotic regulation of sodium pump in rat brain synaptosomes: the role of cytoplasmic sodium

The effect of hypoosmolality of incubation medium on the rat of ouabain-sensitive 86Rb + transport in rat brain synaptosomes was studied. A decreased osmolality from 310 to 250 mOsm increased the rate of 86Rb + uptake from 3.72 to 6.23 nmol/mg of protein min. To evaluate the involvement of cytoplasmic sodium in sodium pump stimulation inhibitors of ion channels and transport pathways able to increase [Na +] in were used. Tetrodotoxin (1 μM), amiloride (0.5 mM) and verapamil (0.1 mM) had no influence on the osmotic response of the sodium pump. The decrease of sodium concentration in incubation medium to 15 mM, leading to a practical loss of its transmembrane gradient, did not abolish stimulation of pump. No increase in 22Na + influx or intrasynaptosomal sodium content was registered at hypotonic conditions. It is suggested that osmotic regulation of Na +,K +-ATPase is not connected with an increase of internal sodium through opening of sodium channels, or with activation of other membrane sodium-transporting systems.