UREA: APPARENT CARRIER-MEDIATED TRANSPORT BY FACILITATED DIFFUSION IN DOGFISH ERYTHROCYTES.

The exposure of erythrocytes from the elasmobranch, Squalus acanthias, to solutions isosmotic with plasma (IM) but containing urea or hydroxyurea as the sole solute does not produce hemolysis. Exposure of these cells to IM methylurea, thiourea and acetamide does produce hemolysis. Low concentrations of urea, which are associated with hemolysis, protect dogfish red cells against hemolysis by methylurea and thiourea. Dogfish red cells exposed to mediums containing high concentrations of urea, or no urea, reach 95 percent of their equilibrium concentration in less than 5 minutes.