Urea transport by cotransporters

1. The rabbit Na+-glucose cotransporter (rbSGLT1) was expressed in Xenopus laevis oocytes and urea transport in rbSGLT1 and non-injected (control) ooc ytes was studied using [C-14]urea as a tracer. The level of rbSGLT1 express ion in these batches of oocytes was monitored by measuring the uptake of al pha -methyl-D-[C-14]glucopyranoside ([C-14]alpha MDG). 2. In rbSGLT1-expressing oocytes, there was a 4-fold increase in urea trans port in the absence of sugar relative to that in control oocytes. Urea upta ke was not Na+ dependent and was linear with both time of incubation (5-120 min) and increasing urea concentration (50 muM to 100 mM) in the bathing m edium. rbSGLT1 urea uptake was blocked by the rbSGLT1-specific inhibitor ph lorizin (K-i 1 muM) in 100 mM NaCl buffer, but was not affected in 100 mM c holine chloride buffer. Phloretin inhibited rbSGLT1 urea uptake with a low affinity (K-i > 1 mM) in the presence and absence of Na+. The uptake of 55 muM urea through rbSGLT1 was not blocked by 100 mM. urea analogues includin g thiourea, 1,3-dimethyl urea, 1,l-dimethyl urea and acetamide. 3. The activation energies (E-a) of urea transport for control and rbSGLT1- expressing oocytes were 14 +/- 3 and 6 +/- 1 kcal mol(-1), respectively. Th e low E-a for urea transport through rbSGLT1 is comparable to the E-a of pa ssive water transport through rbSGLT1. 4. Urea transport through rbSGLT1 was further increased when the cotranspor ter was activated by the addition of sugar to the external medium. The rate of sugar-dependent urea uptake was directly proportional to the rate of Na +-glucose-H2O cotransport such that the amount of urea transport was approx imately proportional to the molar concentration ratio of urea to H2O (55 mu M/55 M). 5. The low affinity Na+-glucose (pSGLT3), the Na+-iodide (rNIS) and the Na-Cl--GABA (hGAT1) cotransporters expressed in oocytes demonstrated similar urea transport properties. 6. These observations suggest that cotransporters behave as urea channels i n the absence of substrates. Furthermore, under substrate-transporting cond itions, the same cotransporters serve as urea cotransporters. This could ac count for urea transport in cells that appear not to have urea uniporters o r channels.