MECHANISMS OF POTASSIUM TRANSFER ACROSS THE DUALLY PERFUSED RAT PLACENTA

The purpose of this study was to directly investigate the mechanisms of K+ transfer across the rat placenta, which was isolated and perfused through both its maternal and fetal circulations. Unidirectional maternofetal (K(mf)) and fetomaternal (K(fm)) clearances for K-42, Cr-51-labeled EDTA (used as a diffusion-limited paracellular marker), and (H2O)-H-3 (used as a flow-limited marker) were respectively 232 +/- 36, 12 +/- 4, and 1,020 +/- 260 (mf) and 96 +/- 26, 18 +/- 6, and 737 +/- 176 (fm) mul . min-1 . g placenta-1. Calculated K+ fluxes were asymmetric, being 0.75 +/- 0.12 and 0.41 +/- 0.12 mumol . min-1 . g placenta-1 for maternofetal and fetomaternal, respectively (means +/- SE, n = 6; P < 0.01, paired t test). Although K(mf) for (H2O)-H-3 was 28% higher than K(fm), this could not completely account for the asymmetry in K+ fluxes. K(mf) for K-42 was 12-70 times higher than that for Cr-51-EDTA (presumed to be a paracellular marker), although its diffusion coefficient is only 2.5 times higher. An apparent Michaelis constant (K(m)) of 11.0 +/- 2.4 mM and maximum velocity (V(max)) of 3.8 +/- 0.33 mumol . min-1 g placenta-1 was calculated by Michaelis-Menten analysis of the transcellular component of maternofetal flux (J(mf)) for K+. Ouabain or barium (1 mM in maternal and fetal perfasate) reduced K(mf) for 42 K from 250 +/- 38 to 76 +/- 13 mul . min-1 . g placenta-1 (n = 4; P < 0.01) and from 358 +/- 31 to 106 +/- 18 mul . min-1 . g placenta-1 (n = 5; P < 0.001). Neither drug had any effect on K(mf) for Cr-51-EDTA or (H2O)-H-3. It is concluded that K+ transfer across the rat placenta is predominantly an asymmetric transcellular process that utilizes Na+-K+-ATPase- and barium-sensitive K+ channels. maternofetal exchange; Na+-K+-ATPase; K+ channels