Urea and renal concentrating ability in the rabbit

Abstract

The hypotheses of passive salt accumulation predict an enhancement of renal concentrating ability by urea. We tested this prediction in rabbits, a species whose nephons when studied in vitro show tansport properties that support these hypotheses. We used calm, unanesthetized, hydropenic, vasopressin-treated rabbits with intact kidneys fed a 16% protein diet, and we observed the effect of urea administration at two rates of solute excretion (60 and 190 microOsm/min . kg body wt; N = 10 and 5, respectively). After an i.v. mannitol infusion, when urea was infused, the i.v. solute excretion rate was unchanged, the changes in urine urea concentration were large (a change of 767 and 408 mumoles/ml), but only small and variable changes in urine osmolality occured (a change of 78 +/- 146, and 36 +/- 50 microOsm/g H20). In additional experiments, we removed the kidneys from antidiuretic, or urea- or mannitol-infused rabbits and measured the intrarenal distribution of sodium, potassium, urea, and chloride. When the urine urea level was greater than 400 mmoles, the urine-to-papilla ratios for urea were 1.6 to 3.6. This suggested that a low collecting duct permeability to urea could explain the absence of a marked enhancement of concentrating ability during urea administration. Further analysis, based on a model of inner medullary solute compartments, indicated that sodium chloride was the major (86%) osmotically active solute in the medullary central core of these rabbits and that it was not influenced by changes in urinary urea concentration. The results of tissue analysis were consonant with either active or passive sodium chloride reabsorption from the thin ascending limb of Henle's loop in these rabbits.

Renal countercurrent system: Role of collecting duct convergence and pelvic urea predicted from a mathematical model

“…However, these measurements were made in an exposed papilla and do therefore not agree with the assumptions of the present model. The computed values for the final urine concentrations are within the range of measurements for both salt and urea (Gunther and Rabinowitz, 1980;Forrest and Stanier, 1966).…”

“…Because of these experimental findings they suggested a low urea permeability of the collecting duct epithelium in the inner medulla, a conclusion which is in conflict with measurements of Rocha and Kokko (1974). The present model shows that the observations by Roch-Ramel et al (1978) and Gunther and Rabinowitz (1980) do not contradict the high urea permeability reported by Rocha and Kokko (1974).…”

“…Let the weighting factors w¡(x) describe the ratio of the cross-sectional area of the ith region to the total cross-sectional area of the medulla at depth x. Then the slice concentrations SC^JC) (salt) and SC 2 (x) (urea) predicted by the model are given by Figure 4.3 compares SC 2 (x) with measurements in the rabbit (Gunther and Rabinowitz, 1980) and in the rat (Hai and Thomas, 1969) and shows good agreement. An analogous comparison for salt with the experimental findings of the same authors (Fig.…”

“…For details see the simulation studies in Section 5. Recently, Roch-Ramel et al (1978) and Gunther and Rabinowitz (1980) have reported non-equilibrium of urea between urine and papillary tissue water of rabbits. Because of these experimental findings they suggested a low urea permeability of the collecting duct epithelium in the inner medulla, a conclusion which is in conflict with measurements of Rocha and Kokko (1974).…”

“…2) The mathematical model study of the collecting duct system demonstrates that the measured non-equilibrium of urea between urine and papillary tissue water (Roch-Ramel et al, 1978;Gunther and Rabinowitz, 1980) does not contradict a relatively high urea permeability of the papillary collecting duct (Rocha and Kokko, 1974).…”