Urea gradient-associated fluid absorption with sigma urea = 1 in rat terminal collecting duct

It has been proposed that inner medullary collecting ducts (IMCDs) can absorb fluid in the absence of a transepithelial osmolality gradient if a perfusate-to-bath urea gradient is present. Such a process has been suggested to be caused by a nonunity reflection coefficient for urea (sigma urea less than 1). However, our recent measurements of sigma urea yielded values not significantly different from 1.0. The present study was done to readdress the possibility of direct coupling of water and urea transport in the rat IMCD. Isolated rat terminal IMCD segments were studied in the presence of 10(-10) M vasopressin with the osmolality of the perfusate equal to that of the peritubular bath but with a perfusate-to-bath urea gradient (bath osmolality balanced with NaCl). We measured both fluid absorption rate and urea concentration in collected fluid and calculated the osmolality of the collected fluid. We observed rapid fluid absorption associated with substantial urea absorption. The urea absorption caused a large fall in the osmolality of the collected fluid with respect to the bath. Simulations with a mathematical model of an isolated perfused tubule revealed that the transepithelial osmolality gradient generated along the length of tubule (caused by urea absorption) was large enough to account for the fluid absorption. Measurement of sigma urea with the "zero-flux" (or null point) method revealed a value of 1.00 +/- 0.02. Thus we conclude that the observed fluid absorption is the result of a transepithelial osmolality gradient generated by rapid urea absorption and does not require sigma urea less than one.