The time course of changes in renal tissue composition duruig water diuresis in the rat

Renal Deletion of LRRC8/VRAC Channels Induces Proximal Tubulopathy

BACKGROUND

Volume-regulated anion channels (VRACs) are heterohexamers of LRRC8A with LRRC8B, -C, -D, or -E in various combinations. Depending on the subunit composition, these swelling-activated channels conduct chloride, amino acids, organic osmolytes, and drugs. Despite VRACs' role in cell volume regulation, and large osmolarity changes in the kidney, neither the localization nor the function of VRACs in the kidney is known.

METHODS

Mice expressing epitope-tagged LRRC8 subunits were used to determine the renal localization of all VRAC subunits. Mice carrying constitutive deletions of Lrrc8b-e, or with inducible or cell-specific ablation of Lrrc8a, were analyzed to assess renal functions of VRACs. Analysis included histology, urine and serum parameters in different diuresis states, and metabolomics.

RESULTS

The kidney expresses all five VRAC subunits with strikingly distinct localization. Whereas LRRC8C is exclusively found in vascular endothelium, all other subunits are found in the nephron. LRRC8E is specific for intercalated cells, whereas LRRC8A, LRRC8B, and LRRC8D are prominent in basolateral membranes of proximal tubules. Conditional deletion of LRRC8A in proximal but not distal tubules and constitutive deletion of LRRC8D cause proximal tubular injury, increased diuresis, and mild Fanconi-like symptoms.

CONCLUSIONS

VRAC/LRRC8 channels are crucial for the function and integrity of proximal tubules, but not for more distal nephron segments despite their larger need for volume regulation. LRRC8A/D channels may be required for the basolateral exit of many organic compounds, including cellular metabolites, in proximal tubules. Proximal tubular injury likely results from combined accumulation of several transported molecules in the absence of VRAC channels.

Endogenous carbamylation of renal medullary proteins

Protein carbamylation is a post-translational modification that can occur in the presence of urea. In solution, urea is in equilibrium with ammonium cyanate, and carbamylation occurs when cyanate ions react with the amino groups of lysines, arginines, protein N-termini, as well as sulfhydryl groups of cysteines. The concentration of urea is elevated in the renal inner medulla compared with other tissues. Due to the high urea concentration, we hypothesized that carbamylation can occur endogenously within the rat inner medulla. Using immunoblotting of rat kidney cortical and medullary homogenates with a carbamyl-lysine specific antibody, we showed that carbamylation is present in a large number of inner medullary proteins. Using protein mass spectrometry (LC-MS/MS) of rat renal inner medulla, we identified 456 unique carbamylated sites in 403 proteins, including many that play important physiological roles in the renal medulla [Data can be accessed at https://helixweb.nih.gov/ESBL/Database/Carbamylation/Carbamylation_peptide_sorted.html]. We conclude that protein carbamylation occurs endogenously in the kidney, modifying many physiologically important proteins.

Tolvaptan as a tool in renal physiology

For decades, the Brattleboro rat has been a useful model in kidney physiology. These animals manifest central diabetes insipidus (lack of circulating vasopressin) due to a mutation in the vasopressin-neurophysin gene. V2 receptor-mediated vasopressin actions in the kidney can be assessed in these animals by infusing the V2-selective vasopressin analog 1-desamino-8-D-arginine vasopressin (dDAVP). However, the major commercial supplier in the United States has ceased production, creating the need for another reliable experimental model of V2 receptor-mediated vasopressin action in rodents. We designed an in vivo protocol to investigate vasopressin responses in the rat kidney using osmotic minipumps loaded with tolvaptan, a nonpeptide competitive inhibitor of the vasopressin V2 receptor. Tolvaptan-infused rats had a mean urinary osmolality of <300 vs. >2,000 mosmol/kgH₂O in vehicle-infused rats. The tolvaptan infusion produced large decreases in the renal abundance of aquaporin-2 (AQP2), aquaporin-3 (AQP3), the β-subunit of the epithelial sodium channel (β-ENaC), and γ-ENaC that were comparable to the differences seen in vehicle-infused vs. vasopressin-infused Brattleboro rats. Thus we conclude that tolvaptan infusion in rats provides an additional model (besides dDAVP-infusion in the Brattleboro rat) for the assessment of V2 receptor-mediated vasopressin actions in the kidney. We also provide ancillary in vitro data in rat inner-medullary-collecting-duct suspensions showing that tolvaptan can block vasopressin's effects on phosphorylation of the water channel AQP2 in vitro. Specifically, tolvaptan almost completely inhibited the ability of vasopressin to increase AQP2 phosphorylation at Ser256, Ser264, and Ser269, while strongly inhibiting a vasopressin-induced decrease in AQP2 phosphorylation at Ser261.

Hormonal regulation of salt and water excretion: a mathematical model of whole kidney function and pressure natriuresis

We present a lumped-nephron model that explicitly represents the main features of the underlying physiology, incorporating the major hormonal regulatory effects on both tubular and vascular function, and that accurately simulates hormonal regulation of renal salt and water excretion. This is the first model to explicitly couple glomerulovascular and medullary dynamics, and it is much more detailed in structure than existing whole organ models and renal portions of multiorgan models. In contrast to previous medullary models, which have only considered the antidiuretic state, our model is able to regulate water and sodium excretion over a variety of experimental conditions in good agreement with data from experimental studies of the rat. Since the properties of the vasculature and epithelia are explicitly represented, they can be altered to simulate pathophysiological conditions and pharmacological interventions. The model serves as an appropriate starting point for simulations of physiological, pathophysiological, and pharmacological renal conditions and for exploring the relationship between the extrarenal environment and renal excretory function in physiological and pathophysiological contexts.

Maximum urine concentrating capability in a mathematical model of the inner medulla of the rat kidney

In a mathematical model of the urine concentrating mechanism of the inner medulla of the rat kidney, a nonlinear optimization technique was used to estimate parameter sets that maximize the urine-to-plasma osmolality ratio (U/P) while maintaining the urine flow rate within a plausible physiologic range. The model, which used a central core formulation, represented loops of Henle turning at all levels of the inner medulla and a composite collecting duct (CD). The parameters varied were: water flow and urea concentration in tubular fluid entering the descending thin limbs and the composite CD at the outer-inner medullary boundary; scaling factors for the number of loops of Henle and CDs as a function of medullary depth; location and increase rate of the urea permeability profile along the CD; and a scaling factor for the maximum rate of NaCl transport from the CD. The optimization algorithm sought to maximize a quantity E that equaled U/P minus a penalty function for insufficient urine flow. Maxima of E were sought by changing parameter values in the direction in parameter space in which E increased. The algorithm attained a maximum E that increased urine osmolality and inner medullary concentrating capability by 37.5% and 80.2%, respectively, above base-case values; the corresponding urine flow rate and the concentrations of NaCl and urea were all within or near reported experimental ranges. Our results predict that urine osmolality is particularly sensitive to three parameters: the urea concentration in tubular fluid entering the CD at the outer-inner medullary boundary, the location and increase rate of the urea permeability profile along the CD, and the rate of decrease of the CD population (and thus of CD surface area) along the cortico-medullary axis.

Hyperfiltration and inner stripe hypertrophy may explain findings by Gamble and coworkers

Simulations conducted in a mathematical model were used to exemplify the hypothesis that elevated solute concentrations and tubular flows at the boundary of the renal outer and inner medullas of rats may contribute to increased urine osmolalities and urine flow rates. Such elevated quantities at that boundary may arise from hyperfiltration and from inner stripe hypertrophy, which are correlated with increased concentrating activity (Bankir L, Kriz W. Kidney Int. 47: 7-24, 1995). The simulations used the region-based model for the rat inner medulla that was presented in the companion study (Layton AT, Pannabecker TL, Dantzler WH, Layton HE. Am J Physiol Renal Physiol 298: F000-F000, 2010). The simulations were suggested by experiments which were conducted in rat by Gamble et al. (Gamble JL, McKhann CF, Butler AM, Tuthill E. Am J Physiol 109: 139-154, 1934) in which the ratio of NaCl to urea in the diet was systematically varied in eight successive 5-day intervals. The simulations predict that changes in boundary conditions at the boundary of the outer and inner medulla, accompanied by plausible modifications in transport properties of the collecting duct system, can significantly increase urine osmolality and flow rate. This hyperfiltration-hypertrophy hypothesis may explain the finding by Gamble et al. that the maximum urine osmolality attained from supplemental feeding of urea and NaCl in the eight intervals depends on NaCl being the initial predominant solute and on urea being the final predominant solute, because urea in sufficient quantity appears to stimulate concentrating activity. More generally, the hypothesis suggests that high osmolalities and urine flow rates may depend, in large part, on adaptive modifications of cortical hemodynamics and on outer medullary structure and not entirely on an extraordinary concentrating capability that is intrinsic to the inner medulla.

Role of UTB urea transporters in the urine concentrating mechanism of the rat kidney

A mathematical model of the renal medulla of the rat kidney was used to investigate urine concentrating mechanism function in animals lacking the UTB urea transporter. The UTB transporter is believed to mediate countercurrent urea exchange between descending vasa recta (DVR) and ascending vasa recta (AVR) by facilitating urea transport across DVR endothelia. The model represents the outer medulla (OM) and inner medulla (IM), with the actions of the cortex incorporated via boundary conditions. Blood flow in the model vasculature is divided into plasma and red blood cell compartments. In the base-case model configuration tubular dimensions and transport parameters are based on, or estimated from, experimental measurements or immunohistochemical evidence in wild-type rats. The base-case model configuration generated an osmolality gradient along the cortico-medullary axis that is consistent with measurements from rats in a moderately antidiuretic state. When expression of UTB was eliminated in the model, model results indicated that, relative to wild-type, the OM cortico-medullary osmolality gradient and the net urea flow through the OM were little affected by absence of UTB transporter. However, because urea transfer from AVR to DVR was much reduced, urea trapping by countercurrent exchange was significantly compromised. Consequently, urine urea concentration and osmolality were decreased by 12% and 8.9% from base case, respectively, with most of the reduction attributable to the impaired IM concentrating mechanism. These results indicate that the in vivo urine concentrating defect in knockout mouse, reported by Yang et al. (J Biol Chem 277(12), 10633-10637, 2002), is not attributable to an OM concentrating mechanism defect, but that reduced urea trapping by long vasa recta plays a significant role in compromising the concentrating mechanism of the IM. Moreover, model results are in general agreement with the explanation of knockout renal function proposed by Yang et al.

Effect of peristaltic contractions of the renal pelvic wall on solute concentrations of the renal inner medulla in the hamster

The mechanism of solute accumulation in the renal inner medulla remains an unresolved issue. Experiments were carried out in hamsters to address the possibility that the peristaltic contractions of the renal pelvic wall surrounding the inner medulla play a role in the inner medullary concentrating process. The right renal pelvis was subjected to one of four manipulations (surgical removal of the pelvic wall, paralysis of the pelvic wall with xylocaine, inhibition of pelvic contractions by direct application of heat, or sham treatment) followed by analysis of the inner medullary solute concentrations in the right kidney vs. the untouched left kidney. Removal of the pelvic wall resulted in a marked reduction in inner medullary osmolality, confirming prior observations. Paralysis of the pelvic wall with xylocaine produced a similar decrease in inner medullary osmolality, despite the fact that urine flow was maintained. In contrast, sham treatment (surgical exposure of the right renal pelvic wall without any further manipulation) did not decrease inner medullary osmolality. To test whether the decrease in urinary osmolality following xylocaine treatment could have been due to a side effect of the drug, pelvic peristaltic contractions were eliminated in another way, by direct application of heat to denature the smooth muscle of the pelvic wall. This procedure also significantly decreased inner medullary osmolality. We conclude that elimination of the contractions of the renal pelvic wall in the hamster significantly impairs inner medullary concentrating ability.

A region-based mathematical model of the urine concentrating mechanism in the rat outer medulla. II. Parameter sensitivity and tubular inhomogeneity

In a companion study (Layton AT and Layton HE. Am J Physiol Renal Physiol 289: F1346-F1366, 2005), a region-based mathematical model was formulated for the urine concentrating mechanism (UCM) in the outer medulla (OM) of the rat kidney. In the present study, we quantified the sensitivity of that model to several structural assumptions, including the degree of regionalization and the degree of inclusion of short descending limbs (SDLs) in the vascular bundles of the inner stripe (IS). Also, we quantified model sensitivity to several parameters that have not been well characterized in the experimental literature, including boundary conditions, short vasa recta distribution, and ascending vasa recta (AVR) solute permeabilities. These studies indicate that regionalization elevates the osmolality of the fluid delivered into the inner medulla via the collecting ducts; that model predictions are not significantly sensitive to boundary conditions; and that short vasa recta distribution and AVR permeabilities significantly impact concentrating capability. Moreover, we investigated, in the context of the UCM, the functional significance of several aspects of tubular segmentation and heterogeneity: SDL segments in the IS that are likely to be impermeable to water but highly permeable to urea; a prebend segment of SDLs that may be functionally like thick ascending limb (TAL); differing IS and outer stripe Na(+) active transport rates in TAL; and potential active urea secretion into the proximal straight tubules. Model calculations predict that these aspects of tubular of segmentation and heterogeneity generally enhance solute cycling or promote effective UCM function.

A region-based mathematical model of the urine concentrating mechanism in the rat outer medulla. I. Formulation and base-case results

We have developed a highly detailed mathematical model for the urine concentrating mechanism (UCM) of the rat kidney outer medulla (OM). The model simulates preferential interactions among tubules and vessels by representing four concentric regions that are centered on a vascular bundle; tubules and vessels, or fractions thereof, are assigned to anatomically appropriate regions. Model parameters, which are based on the experimental literature, include transepithelial transport properties of short descending limbs inferred from immunohistochemical localization studies. The model equations, which are based on conservation of solutes and water and on standard expressions for transmural transport, were solved to steady state. Model simulations predict significantly differing interstitial NaCl and urea concentrations in adjoining regions. Active NaCl transport from thick ascending limbs (TALs), at rates inferred from the physiological literature, resulted in model osmolality profiles along the OM that are consistent with tissue slice experiments. TAL luminal NaCl concentrations at the corticomedullary boundary are consistent with tubuloglomerular feedback function. The model exhibited solute exchange, cycling, and sequestration patterns (in tubules, vessels, and regions) that are generally consistent with predictions in the physiological literature, including significant urea addition from long ascending vasa recta to inner-stripe short descending limbs. In a companion study (Layton AT and Layton HE. Am J Physiol Renal Physiol 289: F1367-F1381, 2005), the impact of model assumptions, medullary anatomy, and tubular segmentation on the UCM was investigated by means of extensive parameter studies.

Two modes for concentrating urine in rat inner medulla

We used a mathematical model of the urine concentrating mechanism of rat inner medulla (IM) to investigate the implications of experimental studies in which immunohistochemical methods were combined with three-dimensional computerized reconstruction of renal tubules. The mathematical model represents a distribution of loops of Henle with loop bends at all levels of the IM, and the vasculature is represented by means of the central core assumption. Based on immunohistochemical evidence, descending limb portions that reach into the papilla are assumed to be only moderately water permeable or to be water impermeable, and only prebend segments and ascending thin limbs are assumed to be NaCl permeable. Model studies indicate that this configuration favors the targeted delivery of NaCl to loop bends, where a favorable gradient, sustained by urea absorption from collecting ducts, promotes NaCl absorption. We identified two model modes that produce a significant axial osmolality gradient. One mode, suggested by preliminary immunohistochemical findings, assumes that aquaporin-1-null portions of loops of Henle that reach into the papilla have very low urea permeability. The other mode, suggested by perfused tubule experiments from the literature, assumes that these same portions of loops of Henle have very high urea permeabilities. Model studies were conducted to determine the sensitivity of these modes to parameter choices. Model results are compared with extant tissue-slice and micropuncture studies.

Down-regulation of urea transporters in the renal inner medulla of lithium-fed rats

BACKGROUND

Lithium is commonly used to treat bipolar psychiatric disorders but can cause reduced urine concentrating ability.

METHODS

To test whether lithium alters UT-A1 or UT-B urea transporter protein abundance or UT-A1 phosphorylation, rats were fed a standard diet supplemented with LiCl for 10 or 25 days, and then compared to pair-fed control rats. To investigate another potential mechanism for decreased urea transport, inner medullary collecting duct (IMCD) suspensions from lithium-fed or control rats were incubated with 32P-orthophosphate to measure the phosphorylation of UT-A1.

RESULTS

In lithium-fed rats (25 days), UT-A1 abundance was reduced to 50% of control rats in IM tip and to 25% in IM base, and UT-B abundance was reduced to 40% in IM base. Aquaporin-2 (AQP2) protein abundance was reduced in both IM regions. Vasopressin (100 pmol/L) increased UT-A1 phosphorylation in IMCD suspensions from control but not from lithium-fed rats; a higher vasopressin concentration (100 nmol/L) increased UT-A1 phosphorylation in control and lithium-fed rats.

CONCLUSIONS

Decreases in UT-A1, UT-B, and AQP2 protein abundance, and/or vasopressin-stimulated phosphorylation of UT-A1, can contribute to the reduced urine concentrating ability that occurs in lithium-treated rats.

Autoradiographic analysis and regulation of angiotensin receptor subtypes AT(4), AT(1), and AT((1-7)) in the kidney

Receptor autoradiography revealed that angiotensin AT(4) receptors were abundantly expressed in normal mammalian (mouse, rat, gerbil, guinea pig, rabbit) and avian (sparrow, chicken, turkey) kidneys and were more extensively distributed than previously reported (including proximal and distal segments of the nephron, interstitium, renal artery, vein, and ureter). Angiotensin AT(4) receptors were generally found to be more abundant than angiotensin AT(1) receptors in mammalian kidneys, whereas angiotensin AT((1-7)) receptors were not detected in either mammalian or avian kidneys. Rats subjected to various chronic treatments were found to preferentially decrease kidney AT(4) receptor density (furosemide, puromycin aminonucleoside, nitro-L-arginine methyl ester), decrease kidney AT(1) receptor density (bilateral ureteral obstruction), or increase kidney AT(1) receptor distribution in the inner medulla (water diuresis). These results indicate that the AT(4) receptor can be expressed in numerous cell types within the normal kidney of several species. Furthermore, several models of renal dysfunction and injury have been identified that selectively alter kidney AT(4) density and may potentially aid in elucidating the role of this novel angiotensin receptor system in renal function.

Urea transport processes are induced in rat IMCD subsegments when urine concentrating ability is reduced

Infusing urea into low-protein-fed mammals increases urine concentration within 5-10 min. To determine which urea transporter may be responsible, we measured urea transport in perfused IMCD3 segments [inner medullary collecting duct (IMCD) segments from the deepest third of the IMCD] from low-protein-fed rats. Basal facilitated urea permeability increased 78%, whereas active urea secretion was completely inhibited. This suggests that upregulation of facilitated urea transport may mediate the rapid increase in urine concentration. Next, expression of active urea transporter(s) in perfused IMCDs was determined in rats with other causes of reduced urine concentrating ability. In untreated and water diuretic rats, IMCD1 segments showed no active urea transport, nor did IMCD2 segments from untreated or hypercalcemic rats. In IMCD1 segments from hypercalcemic rats, active urea reabsorption was induced. The induced active urea reabsorption was completely inhibited by replacing perfusate Na+ with N-methyl-D-glucamine (NMDG+). Active urea secretion was completely inhibited in IMCD3 segments from hypercalcemic rats. In contrast, water diuresis stimulated active urea secretion in IMCD2 segments. The induced active urea secretion was inhibited by phloretin, ouabain, triamterene, or replacing perfusate Na+ with NMDG+. In conclusion, the response of active urea transporters to reductions in urine concentrating ability follows two paradigms: one occurs with hypercalcemia or a low-protein diet, and the second occurs only in water diuresis.

Evidence for sodium-dependent active urea secretion in the deepest subsegment of the rat inner medullary collecting duct

Active reabsorption of urea appears in the initial IMCD (IMCD1) of rats fed a low-protein diet. To determine whether active urea transport also occurs in the deepest IMCD subsegment, the IMCD3, we isolated IMCDs from the base (IMCD1), middle (IMCD2), and tip (IMCD3) regions of the inner medulla from rats fed a normal protein diet and water ad libitum. IMCDs were perfused with identical perfusate and bath solutions. A significant rate of net urea secretion was present only in IMCD3s. Replacing perfusate Na+ with NMDG+ reversibly inhibited net urea secretion but replacing bath Na+ with NMDG+ or perfusate Cl- with gluconate- had no effect. Net urea secretion was significantly inhibited by: (a) 250 microM phloretin (perfusate); (b) 100 nM triamterene (perfusate); (c) 1 mM ouabain (bath); and (d) cooling the tubule to 23 degrees C. Net urea secretion was significantly stimulated by 10 nM vasopressin (bath). Next, we perfused IMCD3s from water diuretic rats (given food ad libitum) and found a significant, fivefold increase in net urea secretion. In summary, we identified a secondary active, secretory urea transport process in IMCD3s of normal rats which is upregulated in water diuretic rats. This new urea transporter may be a sodium- urea antiporter.

The role of organic osmolytes in osmoregulation: from bacteria to mammals

Cells of marine species are known to establish osmotic balance with their environment by adjusting the concentrations of organic osmolytes rather than inorganic osmolytes such as sodium, potassium, and chloride. These organic osmolytes fall into three classes: polyhydric alcohols such as sorbitol, amino acids and amino acid derivatives, and urea and trimethylamines. Substantial evidence is available for a central role of each of these classes in osmoregulation in marine species. In this chapter information on the importance of organic osmolytes is extended to a study of isolated mammalian kidney cells. The intracellular concentration of organic osmolytes in these cells responds dramatically to changes in the osmotic environment. The release of sorbitol following hypoosmotic exposure appears to be triggered by calcium, possibly via a mechanism involving membrane recycling. The summarized experiments provide a basis for further work in marine species.

Renal hemodynamics and urinary concentrating capacity in protein deprivation: role of antidiuretic hormone

The role of antidiuretic hormone (ADH) in the renal concentration defect and hemodynamic changes in protein malnutrition was evaluated in rats with diabetes insipidus (DI) after 2 weeks of low protein feeding. Free water reabsorptive capacity (TcH2O), glomerular filtration rate (GFR), and renal plasma flow (RPF) were measured in the protein deprived rats and in DI rats fed a normal protein diet. The effect of urea supplementation of the low protein diet on renal concentrating capacity was also evaluated. In addition, the renal hemodynamic response to acute administration of ADH was measured and correlated with changes in plasma renin concentration and renal renin content (RRC). Protein deprivation in DI rats resulted in reduced urine osmolality and urea excretion, differences which were reversed by urea supplementation. Protein deprivation did not affect free water reabsorptive capacity but did reduce GFR and RPF. Acute ADH administration significantly increased GFR and RPF in protein-deprived rats; these changes were associated with a reduction in RRC and release. These results suggest that dietary protein restriction does not directly affect the tubular capacity to generate and reabsorb free water. The hemodynamic changes seen in protein deprivation are not mediated by ADH and may be secondary to increased intrarenal angiotensin II.

Sugar transport in isolated rat kidney papillary collecting duct cells

D-Glucose is an important substrate of energy metabolism and osmolyte synthesis in the renal papillary collecting duct. In order to characterize the cellular entry of D-glucose in this tubular segment, collecting duct cells were isolated from rat kidney papilla and the rate of D-glucose uptake was measured indirectly by monitoring the D-glucose-dependent O2 uptake in the presence of the uncoupler CCCP. D-Glucose uptake was found to be sodium-independent and not sensitive to phlorizin even at a concentration of 10(-3) M. Uptake was, however, completely inhibited by 10(-5) M cytochalasin B and 10(-4) M phloretin. The apparent Ki for cytochalasin B was 1.5 x 10(-6) M and for phloretin 2.0 x 10(-5) M. Studies on the substrate specificity revealed that at 1 mM D-mannose is taken up and metabolized to the same extent as D-glucose. A 50-fold higher concentration of 2-deoxy-D-glucose and 2-amino-2-deoxy-D-glucose inhibited D-glucose uptake completely whereas alpha-methyl-D-glucoside, D-allose, and D-galactose were without effect. Under conditions where D-glucose utilization was maximally stimulated an apparent Km of 1.2 mM and a Vmax of 1 mmol D-glucose/g protein.hour was found for D-glucose uptake. These results indicate that the D-glucose uptake into papillary collecting duct cells is probably mediated by a transport system similar to the one found in basal-lateral membranes of polarized renal, intestinal, and liver cells as well as in nonpolarized fat cells and erythrocytes.

Cellular osmoregulation in the renal papilla

The cells of the renal papilla are subject to extreme variations in extracellular tonicity. To obtain more insight into the mechanisms whereby these cells adapt osmotically to these unique environmental conditions, elements were measured in individual cells of the rat renal papilla in antidiuresis and after prolonged furosemide administration. In antidiuresis cell sodium, chloride and potassium concentrations did not differ fundamentally from those observed in tubule cells exposed to isotonic surroundings such as in proximal tubule cells. The marked fall in extracellular electrolyte concentrations induced by furosemide was paralleled by a far less pronounced decline in intracellular sodium, chloride and potassium concentrations. These data indicate that papillary cells achieve osmoadaptation to widely differing extracellular tonicities mainly by varying the intracellular concentrations of osmotically active substances other than inorganic electrolytes. Since high concentrations of organic osmolytes (sorbitol, inositol, glycerophosphorylcholine and other trimethylamines) have been detected in the papilla and since the tissue contents of these compounds have been shown to vary in parallel with urine osmolality, it may be concluded that metabolically inert, organic osmolytes play a dominant role in the osmoregulation of renal papillary cells.

The influence of renal papillary urea concentrations and of plasma vasopressin on the urinary prostaglandins E2 and F2 alpha excretion in conscious rats in steady state of urine formation

Urinary excretion rates of PGE2 and PGF2 alpha were measured radioimmunologically in four different groups of unanaesthetized rats: water diuretic rats (I), rats with free access to water (II), water deprived rats (III) and 1.5% saline-loaded rats (IV). The animals were decapitated when steady states of urine formation were ascertained for three to six spontaneously delivered urine portions. Plasma vasopressin was measured radioimmunologically and urea, sodium, potassium concentrations and osmolalities of papillary fluids and of bladder urines were determined. Group means of urinary prostaglandin excretion, papillary urea concentration and logarithmic-transformed plasma vasopressin values vary in parallel for three of the groups (I, II and III) but a dissociation of the effects of papillary urea and vasopressin on the prostaglandin excretion was obtained for group IV. Statistical analyses indicated that the differences in prostaglandin excretion rates between group I and the three other groups are accounted for by the combined effects of vasopressin and papillary urea. The results support the hypothesis that vasopressin stimulates release of arachidonic acid in the papilla and that urea inhibits the trapping of this prostaglandin precursor in cellular lipids. The ratio of urinary PGF2 alpha to PGE2 varied greatly between groups but no consistent dependency on the measured parameters was found.

Pathogenesis of nephrogenic diabetes insipidus due to chronic administration of lithium in rats

A polyuric syndrome with nephrogenic diabetes insipidus (NDI) is a frequent consequence of prolonged administration of lithium (Li) salts. Studies in the past, mainly the acute and in vitro experiments, indicated that Li ions can inhibit hydroosmotic effect of [8-arginine]vasopressin (AVP) at the step of cAMP generation in vitro. However, the pathogenesis of the NDI due to chronic oral administration of low therapeutic doses of Li salts is not yet clarified. We conducted a comprehensive study to clarify the mechanism by which Li administered orally for several weeks induces polyuria and NDI in rats. Albino rats consuming a diet which contained Li (60 mmol/kg) for 4 wk developed marked polyuria and polydipsia; at the end of 4 wk the plasma Li was 0.7 +/- 0.09 mM (mean +/- SEM; n = 36). Li-treated rats had a significantly decreased (-33%) tissue osmolality in papilla and greatly reduced cortico-papillary gradient of urea (cortex--43%; medulla--64%; papilla--74%). Plasma urea was significantly (P less than 0.001) lower in Li-treated rats (5.4 +/- 0.2 mM) compared with controls (6.8 +/- 0.3 mM). Medullary collecting tubules (MCT) and papillary collecting ducts (PCD) microdissected from Li-treated animals had higher content of protein than MCT and PCD from the control rats. The cAMP accumulation in response to AVP added in vitro was significantly (delta = -60%) reduced. Also, the cAMP accumulation in MCT and PCD after incubation with forskolin was markedly lower in Li-treated rats. Addition of 0.5 mM 1-methyl,3-isobutyl-xanthine did not restore the cAMP accumulation in response to AVP and forskolin in MCT from Li-treated animals. In collecting tubule segments from polyuric rats with hypothalamic diabetes insipidus (Brattleboro homozygotes) the AVP-dependent cAMP accumulation was not diminished. The activity of adenylate cyclase (AdC) in MCT of Li-treated rats, both the basal and the activity stimulated by AVP, forskolin, or fluoride, was significantly (delta approximately equal to -30%) reduced, while the activity of cAMP phosphodiesterase (cAMP-PDIE) in the same segment showed no significant difference from the controls. Also, the content of ATP in MCT microdissected from Li-treated rats and incubated in vitro did not differ from controls. The rate of [14C]succinate oxidation to 14CO2 in MAL was inhibited (-77%) by 1 mM furosemide, which indicates that this metabolic process is coupled with NaCl cotransport in MAL. The rate of (14)CO(2) production from [14C]succinate in MAL was not significantly different between control and Li-treated rats. In MCT of control rats, the rate of [14C]succinate oxidation was approximately 3 times lower than in MAL. The rate of (14)CO(2) production from [(14)C]succinate in MCT of Li-treated rats was significantly (delta +33%) higher than in MCT dissected from control rats. Based on these results, we conclude that at least two factors play an important role in the pathogenesis of NDI consequent to chronic oral administration of Li: (a) decreased ability of MCT and PCD to generate and accumulate cAMP in response to stimulation by AVP; this defect is primarily due to diminished activity of AdC in these tubular segments caused by prolonged exposure to Li; and (b) lower osmolality of renal papillary tissue, due to primarily to depletion of urea, which decreases osmotic driving force for water reabsorption in collecting tubules. On the other hand, NaCI reabsorption in MAL is apparently not affected by chronic Li treatment.

Osmoregulation of renal papillary cells

Element concentrations were determined in various extra- and intracellular compartments of the rat renal papilla in antidiuresis and after furosemide-induced diuresis using electron microprobe analysis to elucidate further how the cells adapt osmotically to different osmolalities. In antidiuresis and diuresis the sum of intracellular cations (sodium and potassium), accompanying anions and urea was insufficient in both cases to provide cell osmolalities similar to those in extracellular compartments. This finding provides further evidence that the papillary cells achieve osmoadaptation to widely differing extracellular electrolyte concentrations mainly by varying the cellular concentrations of osmotically-active substances other than urea and electrolytes.

Renal chemoreceptors

A study of the renal receptors and types of stimuli which give origin to supraspinal and spinal-mediated autonomic reflexes is presented. Multiunit and single unit recordings from the afferent renal nerves of male Sprague-Dawley rats have revealed two groups of renal chemosensitive receptors (chemoreceptors). These we have called renal R1 and R2 "chemoceptive" receptors. R1 receptors do not have a resting discharge but are activated after 38.7 +/- 3.3 (S.E) sec (n = 40) of complete renal ischemia (occlusion of the renal artery). Other activating stimuli are associated with a marked impairment in renal blood flow (prolonged occlusion of the renal vein and the hypotension of systemic asphyxia or hemorrhage). Their discharge is characterized by trains of impulses which cease abruptly upon re-entry of blood into the kidney. They are not responsive to increases or decreases in renal perfusion pressure or to increases in renal venous or ureteral pressure. In contrast, R2 receptors have a resting discharge and respond vigorously to backflow of normal urine (nondiuretic) into the renal pelvis. The results of the backflow into the pelvis of different test solutions (diuretic and nondiuretic urine, 1 M urea, 1 M mannitol and solutions of NaCl and KCl) indicate that this response is dependent upon the composition of the fluid bathing the renal pelvis rather than the increase in pelvic pressure or pelvic distension. The resting discharge rate is highest in nondiuretic conditions and declines substantially after diuresis is induced by extracellular volume expansion. R2 receptors are also activated by renal ischemia produced by clamping the renal artery. It is concluded that these two groups of afferent sensory units are renal chemosensitive receptors, (chemoreceptors) which respond to the chemical environment of renal interstitium.

Application of scanning electron microscopy to x-ray analysis of frozen-hydrated sections. III. Elemental content of cells in the rat renal papillary tip

The electrolyte and water content of cellular and interstitial compartments in the renal papilla of the rat was determined by x-ray microanalysis of frozen-hydrated tissue sections. Papillae from rats on ad libitum water were rapidly frozen in a slush of Freon 12, and sectioned in a cryomicrotome at -30 to -40 degrees C. Frozen 0.5-micrometer sections were mounted on carbon-coated nylon film over a Be grid, transferred cold to the scanning microscope, and maintained at -175 degrees C during analysis. The scanning transmission mode was used for imaging. Structural preservation was of good quality and allowed identification of tissue compartments. Tissue mass (solutes + water) was determined by continuum radiation from regions of interest. After drying in the SEM, elemental composition of morphologically defined compartments (solutes) was determined by analysis of specific x-rays, and total dry mass by continuum. Na, K, Cl, and H2O contents in collecting-duct cells (CDC), papillary epithelial cells (PEC), and interstitial cells (IC) and space were measured. Cells had lower water content (mean 58.7%) than interstitium (77.5%). Intracellular K concentrations (millimoles per kilogram wet weight) were unremarkable (79-156 mm/kg wet weight); P was markedly higher in cells than in interstitium. S was the same in all compartments. Intracellular Na levels were extremely high (CDC, 344 +/- 127 SD mm/kg wet weight; PEC, 287 +/- 105; IC, 898 +/- 194). Mean interstitial Na was 590 +/- 119 mm/kg wet weight. CI values paralleled those for Na. If this Na is unbound, then these data suggest that renal papillary interstitial cells adapt to their hyperosmotic environment by a Na-uptake process.

Effects of osmolality and oxygen availability on soluble cyclic AMP-dependent protein kinase activity of rat renal inner medulla

The renal inner medulla is ordinarily exposed to osmolalities that are much higher and to O2 tensions that are lower than those in other tissues. The effects of media osmolality and O2 availability on basal and arginine vasopressin(AVP)-responsive soluble cyclic (c)AMP-dependent protein kinase activity were examined in slices of rat inner medulla. Increasing total media osmolality from 305 to 750 or 1,650 mosM by addition of urea plas NaCl to standard Krebs-Ringer bicarbonate buffer significantly reduced basal cAMP content and protein kinase activity ratios. This occurred in the presence or absence of O2. Incubation of slices in high osmolality buffer also blunted increases in inner medullary slice cAMP and protein kinase activity ratios induced by O2. These changes reflected predominantly an action of the urea rather than the NaCl content of high osmolality buffers. In contrast to effects on basal activity, high media osmolality significantly enhanced activation of inner medullary protein kinase by AVP. Conversely, increases in media O2 content suppressed AVP stimulation of enzyme activity. This inhibitory effect of O2 was best expressed at low osmolality. Naproxen and ibuprofen, inhibitors of prostaglandin biosynthesis, reduced basal kinase activity ratios and increased AVP responsiveness in the presence, but not in the absence, of O2. Exogenous prostaglandins (PG) modestly increased (PGE2 and PGE1) or did not change (PGF2alpha) cAMP and protein kinase activity ratios in O2-deprived inner medullary slices. Protein kinase activation by PGE2 was not observed in oxygenated inner medulla with high basal activity ratios. The stimulatory effects of PGE2 and PGE1 on protein kinase activity observed in O2-deprived slices were additive with those of submaximal or maximal AVP. PGE2, PGE1, and PGF2alpha all failed to suppress AVP activation of protein kinase. Thus, enhanced endogenous PGE production may contribute to the higher basal protein kinase activity ratios induced by O2. However, the results do not support a role for PGE2, PGE1, or PGF2alpha in O2-mediated inhibition of AVP responsiveness. The present data indicate that both solute content and O2 availability can alter the expression of AVP action on cAMP-dependent protein kinase activity in inner medulla. AVP activation of protein kinase is best expressed when osmolality is high and O2 availability is low, conditions that pertain in inner medulla during hydropenia.

Acute compensatory adaptation of renal function following contralateral kidney exclusion in Brattleboro rats with diabetes insipidus

1. The function of the remaining kidney following ligation of a single renal pedicle in anaesthetized rats with hereditary hypothalamic diabetes insipidus (DI) was studied using clearance methods. Values were compared with those obtained in DI rats which had been sham operated. 2. A small increase in the glomerular filtration rate of the remaining kidney was observed, which was statistically significant after 150 min. Increases in effective renal plasma flow were also observed, but these were not statistically significant. 3. The fractional excretion rates of Na and K increased immediately after contralateral kidney ligation; as a result total Na and K excretion rates remained similar to values in sham operated animals with both kidneys intact. 4. Fractional urine flow (V/GFR) increased by approximately 40% after contralateral ligation. It is argued that, since water reabsorption in the diluting segments of the nephron was unlikely to have been reduced, the increase in V/GFR was an indication that fractional fluid reabsorption in the proximal nephron had been inhibited. 5. Clearance determinations showed that the fractional reabsorption of Na in the proximal nephron was similarly reduced after contralateral ligation. However, Na reabsorption in the distal nephron increased, though not by enough to prevent a doubling of fractional sodium excretion. 6. The osmolalities of interstitial fluids obtained from the medulla and papilla of the contralateral kidney 2 hr after unilateral renal pedicle ligation were slightly, but significantly, higher than corresponding values in sham operated rats. This raises the possibility that salt reabsorption in the ascending limb of Henle might be enhanced immediately after contralateral renal exclusion.

Lability of renal papillary tissue composition in the rat

1. The acute effects of (a) a minor operative procedure using ether as the anaesthetic, and (b) the administration of 0.9% saline as a single I.V. injection in the conscious rat, on renal tissue composition were studied in hydropenic and normally hydrated rats. 2. The operative procedure and anaesthesia induced a rapid and transient decrease in papillary osmolality in both hydropenic and normally hydrated animals, the important contributing factor being a significant decrease in urea content. 3. Administration of a small volume of saline caused a rapid decrease in urea content, and an increase in water content. 4. It is concluded that papillary composition is extremely labile, large changes being produced by relatively minor experimental procedures.

Collecting duct dlow rate as a determinant of equilibration between urine and renal papilla in the rat in the presence of a maximal antidiuretic hormone concentration

1. Antidiuretic hormone (ADH) was infused into normal male rats at a rate of 60 muu./min. 100 g body wt., to maintain an effectively constant maximal circulating level. Four groups of rats were used; they were water-loaded by receiving together with the ADH, I.V. infusions of hypotonic dextrose (2.5 g/100 ml.) at different rates (1.0, 4.5, 9.0 and 12 ml./hr, respectively), over an infusion period of 4 hr.2. Urine flow rate increased in all groups, the rate and extent of the increase being related to the volume rate of infusion. The differences in urine flow rates between the four groups were due almost entirely to increases in free water clearance, with no consistent differences in osmolal clearance between the groups. At the end of the 4 hr infusion period, osmolal clearances were closely similar in the four groups.3. Papillary and medullary tissue solute concentrations were progressively reduced at the higher rates of infusion. The changes were due to small increases in the water content, together with a profound decrease in urea concentration and a smaller decrease in sodium concentration. However, papillary osmolality was consistently higher than urine osmolality at the three highest rates of dextrose infusion.4. As urine flow rate increased, there was a progressive reduction in the degree of osmotic equilibration between the final urine and the papillary tip. For urea, however, the degree of equilibration remained high.5. It is concluded that, in the rat, the rate of flow per se, along the collecting duct, is an important determinant of final urine concentration; even if there is an osmotic driving force for water re-absorption in the renal medulla, and the collecting duct walls are permeable to water, osmotic equilibration is restricted by tubular flow rate.

The inulin space, solute concentrations, and weight changes in rat renal medullary slices incubated in iso-osmolal media, and their modification during anoxia and hypothermia

1. The volume of distribution of [14-C]carboxyl inulin has been studied in slices of outer and inner medulla from rat kidney incubated in Krebs phosphate-bicarbonate Ringer, modified to render it iso-osmolal with the tissue fluids in these zones, under three conditions, (a) aerobically at 37 degrees C (control), (b) anoxically at 37 degrees C, and (c) aerobically at 0 degrees C. 2. Under control conditions near steady-state volumes of approximately 24 and 42 mul./100 mg wet weight slice were obtained for outer and inner medulla respectively during the period 10-30 min from the start of incubation. In the outer medulla the volumes of distribution in anoxic and hypothermic slices exceeded that in control slices during this time, but control values increased from 30 to 100 min so that after 100 min the distribution volumes were approximately 30 mul./100 mg under each set of conditions. 3. In the inner medulla control and anoxic slices had inulin distribution volumes of approximately 42 mul./100 mg during 10-30 min, rising to over 50 mul./u99 mg by 100 min. Slices incubated hypothermically reached a steady-state value of approximately 40 mul./100 mg by 30 min, which did not increase further for up to 100 min. 4. All slices lost about 10% of their initial weight during the first 3 min of incubation. Thereafter control slices maintained weight constancy for at least 30 min (outer medulla) or 100 min (inner medulla); slices incubated anoxically or hypothermically gained weight, the gains being greatest in anoxic outer and hypothermic inner medulla. 5. The K concentration within control slices (both zones), hypothermic outer and anoxic inner medulla attained equilibrium when slice [K] was approximately 8 times medium [K] (5-9 mM). In anoxic outer and hypothermic inner medullary slices [K] fell to a significantly greater extent, but interpretation of these findings in terms of slice K loss is subject to modification in respect of the increases in slice weight (water content) accompanying the [K] decreases. 6. There was a transient (1-3 min) rise in [Na] in all slices. This was followed by a [Na] decrease, which was most apparent in control slices, and finally a gradual increase towards medium [Na] (141 and 180 mM for outer and inner medulla respectively).

Effects of ethacrynic acid on the isolated collecting tubule

Effects of the diuretic ethacrynic acid on osmotic water permeability were investigated in the isolated perfused collecting tubule of the rabbit kidney. The base-line water permeability of the collecting tubule was not affected when the drug (10(-4)M) alone was added to the bathing medium. Vasopressin alone in the bathing medium (2, 5 muU/ml) elicited a significant increase in osmotic water absorption. With vasopressin kept in the bathing medium, the addition of 10(-5)M ethacrynic acid depressed the hydro-osmotic effect of vasopressin by 50%. This inhibitory effect of low concentrations of ethacrynic acid could be surmounted by high, supramaximal dosage levels of vasopressin. When 10(-4)M ethacrynic acid was added to the bathing medium before vasopressin, the hydro-osmotic effect of vasopressin and the diuretic in combination was insignificant.Dibutyryl adenosine 3'5'-monophosphate (10(-4)-10(-2)M) alone in the bathing medium significantly increased baseline osmotic water flow, mimicing the effect of antidiuretic hormone. When ethacrynic acid was added together with the nucleotide, the permeability remained at the same high level. Theophylline, like the nucleotide and vasopressin, produced a significant hydro-osmotic effect. The magnitude of this response was not affected by further addition of ethacrynic acid (10(-4)M). It was concluded that ethacrynic acid is an antagonist of antidiuretic hormone. The antagonism probably occurs at the level of the receptor site of the hormone on the peritubular membrane. Antagonism to circulating antidiuretic hormone may therefore be one of the factors involved in the loss of renal concentrating ability brought about by ethacrynic acid diuresis.

Influence of lysine-vasopressin dosage on the time course of changes in renal tissue and urinary composition in the conscious rat

1. The changes in urinary and renal tissue composition induced by continuous, intravenous infusion of lysine-vasopressin (2.5, 5, 15 and 60 mu-u./min. 100 g body wt.) for up to 4(1/2) hr in water-loaded, conscious rats were determined.2. Both the magnitude of, and the time required to attain, maximal and stable responses, in respect to both urinary and tissue composition, varied with the dose.3. The dose-dependent changes in medullary composition were compounded of graded decreases in water content and graded increases in solute (mainly Na and urea) content.4. The relative contribution of the changes in water, Na and urea contents varied with time and with dose. Significant increases in papillary urea content occurred with all doses. The range of change in urea content was wider than that for any other solute.5. At low doses, the changes in urinary flow and osmolality were ascribable, almost entirely, to large decreases in free-water clearance, with minor changes in medullary composition; at higher doses, the increases in urinary osmolality were accompanied by steep increases in medullary solute concentrations.6. A variable, dose-dependent, transient natriuresis occurred during the phase of increasing medullary Na concentration; the peak natriuresis preceded the times of maximal osmolal and Na concentrations in the papilla and urine.7. The differences in osmolal, urea and Na concentrations between papilla and urine also changed with time.8. Both the transitional and steady-state changes induced by lysine-vasopressin are discussed in terms of intrarenal mechanisms. It is concluded that the data are most reasonably interpreted on the basis that several hormone-sensitive loci exist in the kidneys, each with individual dose-response and kinetic characteristics.

Influence of variations in hydration and in solute excretion of the effects of lysine-vasopressin infusion on urinary and renal tissue composition in the conscious rat

1. The changes in urinary and renal tissue composition induced by continuous, intravenous infusion of lysine-vasopressin (60 mu-u./min. 100 g body wt. until steady-state conditions prevailed) in normally hydrated, hydropaenic, saline-loaded (0.9%, w/v) and mannitol-loaded (15%, w/v) rats were determined and compared with those induced in water-loaded rats.2. Previous reports that the urinary responses to antidiuretic hormones vary both with hydration status and with concurrent solute excretion rate were confirmed.3. The data show that variations in urinary responses were accompanied by differences in the papillary responses to lysine-vasopressin.4. The results are discussed in terms of the effects of hydration and concurrent solute excretion on factors influencing (a) medullary accumulation of water and solute, (b) osmotic water reabsorption and (c) osmotic equilibration across the collecting duct; and of the effects of lysine-vasopressin on these factors.5. It is concluded that the effects of hydration and solute excretion on the antidiuretic responses to lysine-vasopressin may be interpreted by differences in (a) the medullary composition prevailing at the start and (b) any further changes in medullary composition that can be induced under the experimental circumstances.

Unilateral lesions of the auditory cortex and the 'precedence effect'

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Effects of 0-9 per cent saline infusion on urinary and renal tissue composition in the hydropaenic, normal and hydrated conscious rat

1. Changes in water and solute outputs of hydropaenic, normal and hydrated conscious rats were determined during intravenous infusion (0.2 ml./min) of isotonic (0.9%) saline for 4 hr; renal tissue composition was determined before, and after 1 or 2 hr, infusion.2. In normal and hydrated rats increased excretion of water and sodium was such that urinary output matched intravenous input from about 2 hr. In hydropaenic rats, the diuretic and natriuretic response was much reduced; a retention of infused saline, equivalent to 15% body weight, occurred over 4 hr.3. A considerable increase in urea output and clearance, and a smaller increase in potassium and ammonium outputs, occurred in all groups.4. The corticomedullary osmolal gradients characteristic of non-diuretic rats were largely dissipated during saline infusion: by 1 hr in normal and hydrated rats, and by 2 hr in the hydropaenic group.5. These changes were ascribable mainly to an increase in tissue water content in all segments, particularly in the hydropaenic group; and to a profound decrease in urea content in all groups.6. Changes in tissue sodium content were smaller, and differed between segments and between the differently hydrated groups. A decrease in papillary content occurred in hydropaenic and normal groups and an increase in cortical and outer medullary content occurred in all groups.7. After 2 hr saline infusion, incomplete papillary-urinary osmotic equilibration was evident in all groups.8. These changes in medullary osmolality and in papillary-urinary osmotic equilibration preceded the maximal diuresis, and must contribute to the diuresis induced by saline infusion, as in water and osmotic diureses.

Influence of prehydration on the changes in renal tissue composition induced by water diuresis in the rat

1. The composition of renal tissue was determined in rats before and immediately after intravenous infusion of dextrose (2.5 g/100 ml.) in amounts sufficient to administer a positive fluid load of 4% body weight over 2 hr. The rats were classified into three groups, according to the preinfusion urine osmolality: hydropaenia, normal and moderately diuretic (over 2400, 800-1500 and below 800 mu-osmoles/g H(2)O, respectively).2. In non-infused rats, the steepness of the corticomedullary osmolal gradient varied, due to differences in both water and solute (sodium and urea) contents, and was related to urinary osmolality. Whereas differences in medullary and papillary solute contents occurred between all three groups, papillary water content was significantly higher only in the moderately diuretic animals.3. Dextrose infusion caused the induction of water diuresis, the lowest urinary osmolalities being produced in the previously moderately diuretic animals.4. Dextrose infusion caused a considerable reduction in the steepness of the corticomedullary osmolal gradient in all rats, particularly in the previously hydropaenic animals, due to changes in both solute (sodium and urea) and water contents. Whereas reductions in medullary and papillary solute contents occurred in all three groups, there was no further increase in papillary water content from the already high values seen in the noninfused diuretic animals.5. Thus, dextrose infusion largely abolished any previous differences in tissue water content, whereas significant, though small, differences in osmolal (particularly urea) content persisted.6. These data are discussed in terms of changes and differences in endogenous antidiuretic hormone (A.D.H.) release.7. Changes in the magnitude and direction of the urinary-papillary urea concentration difference are discussed in terms of passive transport, with probable A.D.H.-induced changes in nephron urea permeability.