Unidirectional 14 C-labeled urea efflux under SE conditions at 1 mM in human RBCs from two donors. The ordinates express the fraction of tracer that remains in the cells at a given time, and the abscissa is time. Note the different time scaling of the abscissa. D21 represents a slow urea transport and D11 represents a fast urea transport in the present collection of donors in Table 1. (A and B) The experiments were carried out at 25°C, pH 7.2, by means of the continuous flow tube technique with control cells (A) and the Millipore-Swinnex filtering method with acute phloretin-exposed cells (B). The linearity of the efflux curves in the semilogarithmic plot indicates that the efflux is a monoexponential efflux process. The negative slope of the efflux curves equals the rate coefficient k (s−1) and T½ (ms or s) = ln2/k. The rate coefficient is used to calculate the permeability coefficient according to Eq. 3. (A) No inhibition. D21: Pu = 0.57 (SD ± 0.02, n = 3) × 10−4 cm/s. D11: Pu = 4.79 (SD ± 0.42, n = 4) × 10−4 cm/s. (B) Acute phloretin inhibition. D21: Pu,i = 0.60 (SD ± 0.09, n = 3) × 10−6 cm/s. D11: Pu,i = 0.76 (SD ± 0.24, n = 4) × 10−6 cm/s.