Volume-activated amino acid efflux from term human placental tissue: Stimulation of efflux via a pathway sensitive to anion transport inhibitors

Abstract

The effect of a hyposmotic challenge and hence cell-smelling upon the efflux of a variety of solutes from isolated human placental tissue has been examined. A hyposmotic shock increased the fractional release of taurine, the most abundant free amino acid in placental tissue, via a pathway sensitive to niflumic acid, DIDS (4,4′-Diisothiocyanatostilbene-2′,2′-disulphonic acid,) NPPB (5-Nitro-2(3-phenylpropylamino)benzoic acid) and DIOA (R(+)[2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5 -y)oxy] acetic acid). In contrast, tamoxifen was without effect. The cell-swelling induced efflux of taurine was attenuated (40 per cent) by replacing external Cl − with NO 3 −. The efflux of glutamic acid was also markedly increased by a hyposmotic challenge. Niflumic acid inhibited both basal and volume-activated glutamic acid efflux. A hyposmotic shock also increased α-aminoisobutyric acid efflux but not that of 3-0-methylglucose and SO 4 2−. The results suggest that the human placenta can respond to cell-swelling by releasing organic osmolytes such as amino acids via a pathway which is sensitive to anion transport inhibitors. However, it appears that the volume-activated amino acid transport system is independent from the placental anion-exchange pathways. The efflux of these compounds may act with K + and Cl − efflux to effect a regulatory volume decrease in placental tissue. In addition, volume-activated transport may play a role in transplacental amino acid transfer.