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

Academic Article

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)] > [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 μM for dimethylamiloride and 100 μM for harmaline. The inhibition by dimethylamiloride was freely reversible and the inhibitor reduced the Na+ uptake by directly interacting with the exchange 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 · mg protein-1 · 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. The data also show that virtually all of the carrier-mediated Na+ uptake observed in placental brush-border membrane vesicles under the experimental conditions occurs via the Na+-H+ exchange mechanism.
  • Digital Object Identifier (doi)

    Author List

  • Balkovetz DF; Leibach FH; Mahesh VB; Devoe LD; Cragoe EJ; Ganapathy V
  • Volume

  • 251
  • Issue

  • 6 (20/6)