Urea and ethylene glycol-facilitated transport systems in the human red cell membrane. Saturation, competition, and asymmetry.

Abstract

The equilibrium exchange of [14C]ure and ethylene glycol was measured using a new type of fast flow system. Approximately equal volumes of saline and air were mixed to form a segmented fluid stream into which 14C-loaded red cells are injected. The stream flows through 3 filter chambers which allow sampling of the 14C in the extracellular fluid at 3 time points. The chambers are designed so that they do not disrupt the segmented bubble pattern. The alternating air and saline segments prevent laminar dispersion in the flowing stream and ensure good mixing at the injection and sampling sites. The equilibrium exchange of both urea and ethylene glycol showed saturation kinetics. The maximum permeability (Po) measured in the limit of zero solute concentration is 1.6 .times. 10-3 cm/s for urea and 4.8 .times. 10-4 cm/s for ethylene glycol (T = 23.degree. C). The Km was 218 mM for urea and 175 mM for ethylene glycol. The Po for thiourea is 2.3 .times. 10-6 cm/s and the Km is 19 mM. Urea and thiourea inhibit the transport of each other and the inhibition constant (KI) is approximately equal to the Km for both compounds. Other (53) analogues of urea were screened for their inhibition of urea or thiourea transport. Several analogs [e.g., 1-(3,4-dichloro-phenyl)-2-thiourea] had a KI in the range of 0.03 mM. The affinity of the inhibitor increased as it was made more hydrophobic. The urea analogs did not significantly inhibit the ethylene glycol or osmotic permeability. Glycerol inhibited ethylene glycol permeability with a KI of 1200 mM.