| Abstract: | In light of recent evidence that carrier-mediated transport of urea occurs in the mammalian kidney, this study examined the renal handling of urea in freshwater rainbow trout (Oncorhynchus mykiss). Fish were fitted with indwelling arterial and urinary bladder catheters for the measurement of plasma and urine composition (urea, Na+, Cl-, glucose, H2O), glomerular filtration rate (3H]PEG-4000), and urine flow rate, thereby allowing quantification of tubular reabsorption rates. The fractional reabsorption of urea (72%) was greater than that of H2O (50%) but less than that of Na+, Cl-, or glucose (95–100%) and occurred against an apparent concentration gradient, suggesting active reabsorptive transport; urea] in the urine was only 59% of that in blood plasma. When fish were infused with exogenous urea loads, these patterns remained largely unchanged, and urea reabsorption increased in direct proportion to the filtered urea load. There was no evidence for saturation of the reabsorptive transport mechanism at urea filtration rates up to 4-fold above the normal range, representing urea loading rates that proved toxic. Extra-renal excretion, presumably through the gills, increased markedly, almost keeping pace with urea loading. This evidence suggests that carrier-mediated reabsorptive transport of urea occurs in the kidney and that plasma urea levels are normally subject to tight homeostatic control in freshwater trout. |
