Biochemical properties of the Na+/H+ exchange system in rat brain synaptosomes. Interdependence of internal and external pH control of the exchange activity.
Properties of the Na+/H+ exchange system in synaptosomes have been studied primarily by using acridine orange fluorescence to follow H+ efflux. Results obtained from 22Na+ uptake experiments and [3H]ethylpropylamiloride binding experiments are also presented for comparison. The basal properties of the Na+/H+ antiport in synaptosomes are similar to those found in other systems; (i) the stoichiometry of Na+/H+ exchange is 1:1; (ii) Li+ can be successfully substituted for Na+; its affinity for the exchanger (KLi+ = 3 mM) is higher than that of Na+ (KNa+ = 12 mM), but the maximal rate of H+ efflux in the presence of Li+ is about 3 times lower than the maximal rate of H+ efflux in the presence of Na+; and (iii) the Na+/H+ antiport is inhibited by amiloride derivatives with the rank order:ethylisopropylamiloride greater than ethylpropylamiloride greater than amiloride greater than benzamil. The most important finding of this paper is that the external pH dependence of the synaptosomal Na+/H+ antiport is controlled by the value of internal pH and vice versa. For example apparent pHo values for half-maximum activation of the Na+/H+ exchanger are pHo = 7.12 when pHi = 6.4 and pHo = 7.95 when pHi = 7.3. Therefore, a 0.9 pH unit increase in internal pH produces a shift of at least a 0.83 pH unit in the external pH dependence. In addition, changing pHo from 7.75 to 8.50 also shifts the half-maximum pHi value for activation of the Na+/H+ antiport from 6.67 to 7.54.
Jean
T
T
Frelin
C
C
Vigne
P
P
Barbry
P
P
Lazdunski
M
M
D000165:Acridine Orange; D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001921:Brain / Q000378:metabolism / Q000648:ultrastructure*; D002352:Carrier Proteins / Q000378:metabolism*; D004553:Electric Conductivity; D005456:Fluorescent Dyes; D006863:Hydrogen-Ion Concentration; D008094:Lithium / Q000378:metabolism; D008433:Mathematics; D011188:Potassium / Q000378:metabolism; D051381:Rats; D012964:Sodium / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers; D013574:Synaptosomes / Q000378:metabolism*
D002352:Carrier Proteins; D005456:Fluorescent Dyes; D017923:Sodium-Hydrogen Exchangers; D000584:Amiloride; D008094:Lithium; D012964:Sodium; D000165:Acridine Orange; D011188:Potassium