Urea transport in elasmobranch erythrocytes.

RABINOWITZ, LAWRENCE, AND ROBERT A. GUNTHER. Urea transport in elasmobranch erythrocytes. Am. J. Physiol. 224(5) : 11091115. 1973.-The postulated presence of carrier-mediated transport of urea in elasmobranch erythrocytes was examined using cells of the spiny dogfish Squalus suckleyi. Under steady-state conditions with no net solute or net water flux and the external medium urea concentration 25-1,000 mM, the unidirectional transmembrane flux of urea was proportional to the medium urea concentration and the urea permeability coefficient was constant. The presence of methylurea, 100-700 mM, did not alter the urea permeability coefficient. For cells in plasma, the relative values of the permeability coefficients for acetamide : methylurea : urea were approximately 4: 1 :0.4. The time required for hemolysis of cells placed in 1 M methylurea was substantially increased by the combination of a poorly permeating or nonpermeating solute in concentrations of .2 M or less with the 1 M methylurea. The retardative effect was in the order: sucrose > manitol > NaCl > urea. A model based on the Kedem-Katchalsky equations for penetration of nonelectrolytes and water and showing cell volume to increase in two distinct temporal phases is thought to explain the retardative effect of these solutes. The presence of carrier-mediated transport of urea or methylurea is not supported by these results.