A bumetanide-sensitive, potassium carrier-mediated transport system in excitable tissues

Anionic selectivity sequence of the Cl(-)-H+ symporter in the synaptosomal preparation from rat brain cortex

The Na(+)/H(+) exchanger has been the only unequivocally demonstrated H(+)-transport mechanism in the synaptosomal preparation. We had previously suggested that a Cl(-)-H(+) symporter (in its acidifying mode) is involved in cytosolic pH regulation in the synaptosomal preparation. Supporting this suggestion, we now show that: (1) when synaptosomes are transferred from PSS to either gluconate or sulfate solutions, the Fura-2 ratio remains stable instead of increasing as it does in 50 mM K solution. This indicates that these anions do not promote a plasma membrane depolarization. (2) Based in the recovery rate from the cytosolic alkalinization, the anionic selectivity of the Cl(-)-H(+) symporter is NO(3)(-) > Br(-) > Cl(-) >> I(-) = isethionate = sulfate = methanesulfonate = gluconate. (3) PCMB 10 muM inhibits the gluconate-dependent alkalinization by 30 +/- 6%. (4) Neither Niflumic acid, 9AC, Bumetanide nor CCCP inhibits the recovery from the cytosolic alkalinization.

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 microM), 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.

Volume regulation of nerve terminals

Pinched-off presynaptic nerve terminals (synaptosomes) possess significant regulatory volume increase (RVI) and regulatory volume decrease (RVD) capabilities. Following a swelling induced by a hypotonic challenge, the synaptosomes regulate their volume and adjust it, in 2 min, to within 5% of its initial value (RVD) at an initial rate of -0.77 +/- 0.10%/s (mean +/- SEM). Following a shrinking induced by a hypertonic challenge, the synaptosomes also regulate their volume at an initial rate of 0.18 +/- 0.02%/s (RVI), resulting in a new steady state, reached within 5-10 min, with a synaptosomal volume below the original volume. The omission of Na+ or K+ ions from the extrasynaptosomal medium reduces the initial rate of RVI by 72.5 and 66.5%, respectively. The "loop diuretics" bumetanide and furosemide significantly inhibited the RVI of the synaptosomes. In contrast, ouabain, amiloride, or 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid did not have any significant effect on RVI parameters. Furthermore, bumetanide-sensitive 86Rb uptake by rat brain synaptosomes was stimulated threefold by a hypertonic perturbation of 30%. Thus we conclude that the RVI of synaptosomes is mainly due to a stimulation of the Na+, K+, Cl- co-transport system induced by the synaptosomal shrinking following the hypertonic challenge.

Regulation of ion uptake in membrane vesicles from rat brain by thiamine compounds

We examined the effects of thiamine derivatives on ion uptake in rat brain membrane vesicles. Thiamine triphosphate (1 mM) and pyrithiamine (0.1 mM) increase chloride uptake. Preincubation of crude homogenate with thiamine or pyrithiamine increases chloride uptake while oxythiamine has the reverse effect. Thiamine and oxythiamine also affect 22Na+ and 86Rb+ uptake in the same way as for 36Cl- but to a lesser extent. Thiamine-dependent 36Cl- uptake is activated by sodium bicarbonate (10 mM) and partially inhibited by bumetanide (0.1 mM) and 2,4-dinitrophenol (0.1 mM). Preincubation with thiamine increases the thiamine triphosphate content of the vesicles. The hypothesis that TTP is the activator of a particular chloride uptake mechanism is discussed.