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 ouabai n-sensitive Rb-86(+) transport in rat brain synaptosomes was studied. A decreased osmolality from 310 to 250 mOsm increased the rate of Rb-8 6(+) uptake from 3.72 to 6.23 nmol/mg of protein min. To evaluate the involvement of cytoplasmic sodium in sodium pump stimulation inhibitor s of ion channels and transport pathways able to increase [Na+](i), we re used. Tetrodotoxin (1 mu M), amiloride (0.5 mM) and verapamil (0.1 mM) had no influence on the osmotic response of the sodium pump. The d ecrease of sodium concentration in incubation medium to 15 mM, leading to a practical loss of its transmembrane gradient, did not abolish st imulation of pump. No increase in Na-22(+) influx or intrasynaptosomal sodium content was registered at hypotonic conditions. it is suggeste d that osmotic regulation of Na+,K+-ATPase is not connected with an in crease of internal sodium through opening of sodium channels, or with activation of other membrane sodium-transporting systems.