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 (Kmf) and fetomaternal (Kfm) clearances for 42K, 51Cr-labeled EDTA (used as a diffusion-limited paracellular marker), and 3H2O (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) microliters.min-1 x g placenta-1. Calculated K+ fluxes were asymmetric, being 0.75 +/- 0.12 and 0.41 +/- 0.12 mumol.min-1 x g placenta-1 for maternofetal and fetomaternal, respectively (mean +/- SE, n = 6; P < 0.01, paired t test). Although Kmf for 3H2O was 28% higher than Kfm, this could not completely account for the asymmetry in K+ fluxes. Kmf for 42K was 12-70 times higher than that for 51Cr-EDTA (presumed to be a paracellular marker), although its diffusion coefficient is only 2.5 times higher. An apparent Michaelis constant (Km) of 11.0 +/- 2.4 mM and maximum velocity (Vmax) of 3.8 +/- 0.33 mumol.min-1 x g placenta-1 was calculated by Michaelis-Menten analysis of the transcellular component of maternofetal flux (Jmf) for K+. Ouabain or barium (1 mM in maternal and fetal perfusate) reduced Kmf for 42K from 250 +/- 38 to 76 +/- 13 microliters.min-1 x g placenta-1 (n = 4; P < 0.01) and from 358 +/- 31 to 106 +/- 18 microliters.min-1 x g placenta-1 (n = 5; P < 0.001). Neither drug had any effect on Kmf for 51Cr-EDTA or 3H2O.(ABSTRACT TRUNCATED AT 250 WORDS)