Na+-H+ exchanger of human placental brush-border membrane: identification and characterization

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Abstract
Syncytiotrophoblast brush-border membrane vesicles isolated from full-term human placentas were shown to transport Na+ against a concentration gradient in the presence of an outward proton gradient [( H+i] greater than [H+]o). This proton gradient-coupled Na+ uptake was markedly inhibited and the uphill transport abolished when the electrochemical proton gradient was dissipated by carbonylcyanide 4-(trifluoromethoxy) phenylhydrazone. The presence of nigericin also eliminated the concentrative uptake of Na+ in these vesicles. Dimethylamiloride and harmaline inhibited the proton gradient-induced Na+ uptake. The apparent inhibition constant for this process was 0.32 microM for dimethylamiloride was freely reversible and the inhibitor reduced the Na+ uptake by directly interacting with the exchanger protein rather than by dissipating the H+ gradient. The dimethylamiloride-sensitive Na+ uptake was saturable with respect to Na+. The affinity constant for Na+ was 7.8 +/- 1.2 mM and the maximal velocity was 38.7 +/- 2.4 nmol X mg protein-1 X min-1. The dimethylamiloride-insensitive Na+ uptake was not saturable and probably represented simple diffusion. The diffusional component accounted for only 10% of the total uptake. Li+ strongly competed with Na+ for the uptake process and the apparent inhibition constant was 3.6 +/- 0.4 mM. Tetraethylammonium also caused significant inhibition of Na+ uptake, whereas K+, Rb+, Cs+, and choline had no effect. These data provide evidence for the existence of a Na+-H+ exchanger in human placental brush-border membrane, and the properties of this exchanger are similar to those of the Na+-H+ exchanger identified in the brush-border membrane of mammalian kidney and small intestine.(ABSTRACT TRUNCATED AT 250 WORDS)