An independent effect of osmolality on urea transport in rat terminal inner medullary collecting ducts.

Abstract

We have shown that urea transport across the terminal inner medullary collecting duct (terminal IMCD) is mediated by a vasopressin-stimulated, facilitated diffusion process exhibiting properties consistent with a transporter. To investigate whether hypertonic NaCI, as exists in vivo in the inner medulla, affects urea permeability, we studied isolated perfused rat terminal IMCD segments. Perfusate and bath osmolality were varied symmetrically by adding or removing NaCI or mannitol. Urea permeability rose progressively when osmolality was increased with NaCI or mannitol from 290 to 690 mOsm/kg H20 in the absence of vasopressin; there was no further increase at 890 mOsm/kg H20. In the presence of 10-8 M arginine vasopressin, urea permeability increased when NaCI was added to raise osmolality from 290 to 490 mOsm/kg H20 but there was no further increase at 690 mOsm/kg H20. When 1 mM 8-bromo cyclic AMP was added to the bath, raising NaCI still increased urea permeability. These results suggest that urea transport across the rat terminal IMCD is regulated both by vasopressin and by osmolality at values present in the renal inner medulla. Osmolality seems to activate urea transport across the rat terminal IMCD by mechanisms distinct from those of vasopressin or cyclic AMP. (J. Clin. Invest. 1991.

Changes in aquaporin-2 protein contribute to the urine concentrating defect in rats fed a low-protein diet.

“…Creatinine concentration in perfusate, bath, and collected fluid was measured using a continuous-flow ultramicro-colorimeter as described (7,14). P f was measured by increasing bath osmolality to 490 mOsm/kg H 2 O by adding NaCl (14,17). The perfusion rate (V o ) was calculated as: V o ϭ V l ( Cr l / Cr o ), where Cr o is the creatinine concentration in the perfusate, Cr l the creatinine concentration in the collected fluid, and V o and V l are as defined above.…”

Urea and Ammonia Metabolism and the Control of Renal Nitrogen Excretion

“…Urea transport across the IMCD is regulated independently by hypertonicity (11,16,17). Urea permeability increases rapidly in perfused IMCDs when osmolality is increased, even in the absence of vasopressin (30,31). Vasopressin and hyperosmolality have an additive stimulatory effect on urea permeability (11,16,17).…”

Urea transporters UT-A1 and UT-A3 accumulate in the plasma membrane in response to increased hypertonicity

“…The biotinylation buffer is slightly hypertonic at 450 mosM. To keep that protocol consistent with our previous biotinylation studies (8), but also to have a comparable increase in tonicity to that used for the other hypertonicity studies (14), the hypertonic solutions were made to twice the osmolality (900 mosmol/kgH 2O) of the control (450 mosmol/kgH2O) buffer rather than matching the absolute tonicity.…”

“…After the 3-h labeling period, IMCDs were incubated for a further 30 min with phosphate-free DMEM: 1) at 290 mosmol/kgH 2O, 2) with sucrose added to a final 600 mosmol/kgH 2O, 3) with urea added to a final 600 mosmol/ kgH2O, and 4) with NaCl added to a final 600 mosmol/kgH2O. These osmolalities were chosen to mimic those used in the original isolated perfused tubule studies (14). Following incubation in hypertonic solutions, IMCD cell lysates were prepared and UT-A1 (22) and UT-A3 (1) were immunoprecipitated as previously described.…”

“…Vasopressin stimulates urea transport across perfused rat terminal inner medullary collecting ducts (IMCD) (13). Urea transport is also stimulated by hyperosmolarity (resulting from addition of NaCl) in the absence of vasopressin and is further stimulated in the presence of vasopressin (14). While both vasopressin and hyperosmolarity stimulate urea transport, they do so through different second messenger pathways; vasopressin acts by increasing cAMP while hyperosmolarity acts by increasing intracellular calcium but does not increase cAMP (6,18).…”