HETEROGENEITY OF L-ALANINE TRANSPORT-SYSTEMS IN BRUSH-BORDER MEMBRANE-VESICLES FROM RAT PLACENTA DURING LATE GESTATION

The placental uptake Of L-alanine was studied by using purified brush-border membrane vesicles from rat trophoblasts. Saturation curves were carried out at 37-degrees-C in buffers containing 100 mm (zero-trans) -NaSCN, -NaCl, -KSCN, -KCl, or -N-methyl-D-glucamine gluconate. The uncorrected uptake results were fitted by non-linear regression analysis to an equation involving one diffusional component and either one or two saturable Michaelian transport terms. In the presence of NaCl, two distinct L-alanine transport systems were distinguished, named respectively System 1 (S-1; V(m1) about 760 pmol/s per mg of protein, K(T1) = 0.5 mM) and System 2 (S-2; V(m2) about 1700 pmol/s per mg; K(T2) = 9 mm). In contrast, in the presence of K+ (KCl = KSCN) or iii the absence of any alkali-metal ions (N-methyl-D-glucamine gluconate), only one saturable system was apparent, which we identify as S-2. When Na+ is present, S-1. but not S-2, appears to be rheogenic, since its maximal transport capacity significantly increases in the presence of an inside-negative membrane potential, created either by replacing Cl with the permeant anion thiocyanate (NaSCN > NaCl) or by applying an appropriate K- gradient and valinomycin. Alpha-(Methylamino)isobutyrate (methyl)-AIB) appears to be a substrate of S-1, but not of S-2. For reasons that remain to be explained, however, methyl-AIB inhibits S-2. We conclude that S-1 represents a truly Na+-dependent mechanism, where Na+ behaves as an obligatory activator, whereas S-2 cannot discriminate between Na+ and K+, although its activity is higher in the presence-of alkali-metal ions than in their absence (Na+ = K+ > N-methyl-D-glucammonium ion). S-2 appears to be fully developed 2 days before birth, whereas S-1 undergoes a capacity-type activation between days 19.5 and 21.5 of gestation, i.e. its apparent V(max.) nearly doubles, whereas its K(T) remains constant.