Hydration status affects urea transport across rat urothelia

Abstract

Although mammalian urinary tract epithelium (urothelium) is generally considered impermeable to water and solutes, recent data suggest that urine constituents may be reabsorbed during urinary tract transit and storage. To study water and solute transport across the urothelium in an in vivo rat model, we instilled urine (obtained during various rat hydration conditions) into isolated in situ rat bladders and, after a 1-h dwell, retrieved the urine and measured the differences in urine volume and concentration and total quantity of urine urea nitrogen and creatinine between instilled and retrieved urine in rat groups differing by hydration status. Although urine volume did not change >1.9% in any group, concentration (and quantity) of urine urea nitrogen in retrieved urine fell significantly (indicating reabsorption of urea across bladder urothelia), by a mean of 18% (489 mg/dl, from an instilled 2,658 mg/dl) in rats receiving ad libitum water and by a mean of 39% (2,544 mg/dl, from an instilled 6,204 mg/dl) in water-deprived rats, but did not change (an increase of 15 mg/dl, P = not significant, from an instilled 300 mg/dl) in a water-loaded rat group. Two separate factors affected urea nitrogen reabsorption rates, a urinary factor related to hydration status, likely the concentration of urea nitrogen in the instilled urine, and a bladder factor(s), also dependent on the animal's state of hydration. Urine creatinine was also absorbed during the bladder dwell, and hydration group effects on the concentration and quantity of creatinine reabsorbed were qualitatively similar to the hydration group effect on urea transport. These findings support the notion(s) that urinary constituents may undergo transport across urinary tract epithelia, that such transport may be physiologically regulated, and that urine is modified during transit and storage through the urinary tract.

Dietary protein affects urea transport across rat urothelia

“…In contrast, the percentage of instilled urea reabsorbed was greater in the two dietary groups receiving the lowest protein (26 and 23%) than in those receiving higher protein (11 and 9%), suggesting the possibility that a bladder/urothelial factor, also affected by dietary protein, may have altered bladder permeability. These findings demonstrate significant regulated urea transport across the urothelium, resulting in alteration of urine excreted by the kidneys, and add to the growing evidence that the lower urinary tract may play an unappreciated role in mammalian solute homeostasis.solute transport across bladder epithelia; urothelial creatinine transport; regulation of urothelial urea transport ALTHOUGH MAMMALIAN LOWER URINARY tract functions primarily as a short-term transit and storage vehicle for urine made by the kidneys (13), recent data demonstrate that mammalian urothelia (the epithelial cell lining of the urinary tract from renal pelvis to proximal urethra) is a surprisingly dynamic and complex tissue that may play an unappreciated role in water and solute homeostasis (35)(36)(37)(38)(39) reviewed in Ref. 22).…”

“…solute transport across bladder epithelia; urothelial creatinine transport; regulation of urothelial urea transport ALTHOUGH MAMMALIAN LOWER URINARY tract functions primarily as a short-term transit and storage vehicle for urine made by the kidneys (13), recent data demonstrate that mammalian urothelia (the epithelial cell lining of the urinary tract from renal pelvis to proximal urethra) is a surprisingly dynamic and complex tissue that may play an unappreciated role in water and solute homeostasis (35)(36)(37)(38)(39) reviewed in Ref. 22).…”

“…Furthermore, Parsons and coworkers (30) demonstrated a net urea transport of only 1.2% across stretched rabbit urothelia in Ussing chambers but noted a 21% urea loss from urine dwelling over 45 min in an in vivo preparation of rabbit bladder. Utilizing a new in vivo isolated bladder model in rats that closely mimics the "physiological" condition, we recently reported changes in urine urea nitrogen (UN) and creatinine concentrations in urine instilled into and dwelling for 60 min in bladders of rats receiving either ad libitum water or after 48 h water deprivation or water loading (35). After 1 h, bladder dwell mean urine UN (and creatinine) concentrations fell significantly (for urea a mean of 18% in ad libitum rats and 37% in water-deprived rats) but did not change significantly in water-loaded rats.…”

“…The present experiments, utilizing the same in vivo rat bladder model, determined the effect of chronic dietary protein intake on urothelial urea and creatinine transport. Diets were designed to result in stepwise differences in urine UN (the largest metabolic breakdown product of protein) concentrations in the range previously achieved by acutely altering whole animal hydration status (35). We hypothesized that urine urea reabsorption across urothelium (reflected by decreases in concentration and quantity of urinary UN during bladder dwell) would be affected by dietary protein content.…”

“…We hypothesized that urine urea reabsorption across urothelium (reflected by decreases in concentration and quantity of urinary UN during bladder dwell) would be affected by dietary protein content. We included a group of rats fed a protein-deficient diet to test the hypothesis that urine urea reabsorption might be upregulated in states of dietary protein deficiency, thereby providing a mechanism for possible nitrogen conservation during such states (35,39). Finally, we also studied a group of water-deprived rats to directly compare urea reabsorption in two conditions associated with similarly high concentrations of urine urea, acute whole animal water deprivation and chronic high protein intake.…”