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

Dominant factors that govern pressure natriuresis in diuresis and antidiuresis: a mathematical model

We have developed a whole kidney model of the urine concentrating mechanism and renal autoregulation. The model represents the tubuloglomerular feedback (TGF) and myogenic mechanisms, which together affect the resistance of the afferent arteriole and thus glomerular filtration rate. TGF is activated by fluctuations in macula densa [Cl(-)] and the myogefnic mechanism by changes in hydrostatic pressure. The model was used to investigate the relative contributions of medullary blood flow autoregulation and inhibition of transport in the proximal convoluted tubule to pressure natriuresis in both diuresis and antidiuresis. The model predicts that medullary blood flow autoregulation, which only affects the interstitial solute composition in the model, has negligible influence on the rate of NaCl excretion. However, it exerts a significant effect on urine flow, particularly in the antidiuretic kidney. This suggests that interstitial washout has significant implications for the maintenance of hydration status but little direct bearing on salt excretion, and that medullary blood flow may only play a signaling role for stimulating a pressure-natriuresis response. Inhibited reabsorption in the model proximal convoluted tubule is capable of driving pressure natriuresis when the known actions of vasopressin on the collecting duct epithelium are taken into account.

Effects of "in vivo" administration of baclofen on rat renal tubular function

The effects of the in vivo administration of baclofen on renal tubular transport and aquaporin-2 (AQP2) expression were evaluated. In conscious animals kept in metabolic cages, baclofen (0.01-1mg/kg, s.c.) induced a dose-dependent increment in the urine flow rate (UFR) and in sodium and potassium excretion, associated with an increased osmolal clearance (Closm), a diminished urine to plasma osmolality ratio (Uosm/Posm) and a decrease in AQP2 expression. The above mentioned baclofen effects on functional parameters were corroborated by using conventional renal clearance techniques. Additionally, this model allowed the detection of a diminution in glucose reabsorption. Some experiments were performed with water-deprived or desmopressin-treated rats kept in metabolic cages. Either water deprivation or desmopressin treatment decreased the UFR and increased the Uosm/Posm. Baclofen did not change the Uosm/Posm or AQP2 expression in desmopressin-treated rats; but it increased the UFR and diminished the Uosm/Posm and AQP2 expression in water-deprived animals. These results indicate that in vivo administration of baclofen promotes alterations in proximal tubular transport, since glucose reabsorption was decreased. The distal tubular function was also affected. The increased Closm indicates an alteration in solute reabsorption at the ascending limb of the Henle's loop. The decreased Uosm/Posm and AQP2 expression in controls and in water-deprived, but not in desmopressin-treated rats, lead us to speculate that some effect of baclofen on endogenous vasopressin availability could be responsible for the impaired urine concentrating ability, more than any disturbance in the responsiveness of the renal cells to the hormone.

Regulation of aquaporin mRNA expression in rat kidney by water intake

Three aquaporins (AQP) are present in the membrane of the principal collecting duct cells. On the apical side, the levels of AQP2 protein are increased in response to both arginine vasopressin and water deprivation. However, whether this change parallels changes in the abundance of AQP3 and AQP4 in the basolateral membrane is less well known. This study evaluates the effect of either dehydration or water loading on the rat kidney mRNA expression of AQP2, AQP3, and AQP4. Poly(A+)RNA was prepared from renal cortex and medulla of control, water-deprived, well hydrated, and water-deprived rats treated with OPC31260, a V2 receptor antagonist. Northern blots were done and mRNA levels were quantified using a PhosphorImager system. Relative to control, water deprivation increased the expression of cortical AQP2, -3, and -4, whereas water loading decreased the cortical and medullar expression of AQP2, -3, and -4. Therefore, in addition to AQP2 and -3, AQP4 expression is also regulated by water intake. Treatment with OPC31260 (40 mg/kg of weight per d) inhibited up to 20 to 30% the upregulation of AQP-mRNA induced by water deprivation. Blood values of arginine vasopressin and aldosterone were significantly increased by water deprivation, whereas they were unchanged by water overloading. Taken together, these results indicate that renal AQP2, -3, and -4 expression is regulated in a coordinated manner. Simultaneous up- or downregulation of the three transcripts occurred upon either water deprivation or water loading of animals, respectively. However, the signaling mechanism for the two long-term adaptive processes may be different, and, in addition to arginine vasopressin, other factors may be involved in the transcriptional regulatory processes.

Urea inhibits Na-K-2Cl cotransport in medullary thick ascending limb cells

We examined the effect of physiological concentrations of urea (100-500 mM) on Na-K-2Cl cotransport in cultured cells from mouse medullary thick ascending limb (mTAL). Urea acutely inhibited bumetanide-sensitive K influx in mTAL cells in a concentration-dependent fashion, with a statistically significant inhibition (19%) at 100 mM and 86% inhibition at 500 mM. The effect of urea was entirely reversible and was blocked by prior treatment with okadaic acid, a phosphatase inhibitor, suggesting that urea exerts its action upstream of the phosphorylation-dephosphorylation step. Cell volume was unchanged in the presence of 500 mM urea. The number of [3H]bumetanide binding sites, a measure of the number of functioning cotransporter sites, was decreased in the presence of urea, and the decrease in bumetanide binding was proportional to the decrease in bumetanide-sensitive K influx. Urea also stimulated the Ba-sensitive swelling-activated K efflux from mTAL cells. Thus urea, in concentrations that prevail in the renal medulla, alters ion transport in mTAL cells.

Amino acids as volume-regulatory osmolytes in mammalian cells

1. This review summarizes current knowledge relating to the volume-regulatory and osmoprotective functions of amino acids in mammalian cells exposed to anisosmotic fluids. 2. Experiments in vivo and in vitro have established that they play a significant role in regulating brain cell volume under these conditions, and that taurine may be of particular importance in this respect. 3. Their possible role in renal medulla is discussed, and it is suggested that they may protect cells against acute (but not long-term) osmotic variation. 4. Evidence is briefly presented regarding adaptive changes in amino acid content of other cell types.

Basolateral membrane Na(+)-independent Cl-/HCO3- exchange in the inner stripe of the rabbit outer medullary collecting tubule

The inner stripe of the outer medullary collecting tubule is a major distal nephron segment in urinary acidification. To examine the mechanism of basolateral membrane H+/OH-/HCO3- transport in this segment, cell pH was measured microfluorometrically in the inner stripe of the rabbit outer medullary collecting tubule perfused in vitro using the pH-sensitive fluorescent dye, (2',7')-bis(carboxyethyl)-(5,6)-carboxyfluorescein. Decreasing peritubular pH from 7.4 to 6.8 (changing [HCO3-] from 25 to 5 mM) caused a cell acidification of 0.25 +/- 0.02 pH units, while a similar luminal change resulted in a smaller cell acidification of only 0.04 +/- 0.01 pH units. Total replacement of peritubular Cl- with gluconate caused cell pH to increase by 0.18 +/- 0.04 pH units, an effect inhibited by 100 microM peritubular DIDS and independent of Na+. Direct coupling between Cl- and base was suggested by the continued presence of peritubular Cl- removal-induced cell alkalinization under the condition of a cell voltage clamp (K(+)-valinomycin). In addition, 90% of basolateral membrane H+/OH-/HCO3- permeability was inhibited by complete removal of luminal and peritubular Cl-. Peritubular Cl(-)-induced cell pH changes were inhibited two-thirds by removal of exogenous CO2/HCO3- from the system. The apparent Km for peritubular Cl- determined in the presence of 25 mM luminal and peritubular [HCO3-] was 113.5 +/- 14.8 mM. These results demonstrate that the basolateral membrane of the inner stripe of the outer medullary collecting tubule possesses a stilbene-sensitive Cl-/HCO3- exchanger which mediates 90% of basolateral membrane H+/OH-/HCO3- permeability and may be regulated by physiologic Cl- concentrations.

Are ninhydrin-positive substances volume-regulatory osmolytes in rat renal papillary cells?

1. A study has been made of the concentrations and contents of ninhydrin-positive substances (n.p.s.), presumed to be predominantly but not exclusively amino acids, in the cells of rat renal papillary slices incubated in variously modified Krebs phosphate-bicarbonate Ringer solution. 2. When the medium osmolality was increased from 710 (control) to 2000 mosmol/kg H2O by additional NaCl and urea, the steady-state cellular n.p.s. concentration rose from 42.3 +/- 0.6 (mean +/- S.E. of mean; n = 36) to 105 +/- 2 (n = 68) mmol/l (glycine equivalent). Cell fluid content fell from 5.11 +/- 0.09 (n = 36) to 4.16 +/- 0.11 (n = 68) microliter/mg solute-free dry weight. Hence cell n.p.s. content increased from 211 +/- 4 (n = 36) to 421 +/- 10 (n = 68) nmol/mg solute-free dry weight. 3. A comparable loss of cell fluid was observed when urea was replaced by sucrose or sorbitol. No increase in cell n.p.s. occurred, and there was a marked cell Na+-for-K+ exchange. 4. The extent of the increase in cell n.p.s. in the presence of 2000 mosmol/kg H2O (NaCl + urea) was sensitive to the presence of external anions in the sequence acetate less than Cl- less than NO3- less than or equal to SCN-. 5. Cell n.p.s. concentration increased progressively as the medium osmolality was increased by the addition of urea, but Na+ at a concentration above 330 mmol/l had an inhibitory effect. The increase in n.p.s. concentration was also significantly reduced in hyperosmotic media in which Na+ was replaced by choline. 6. The increase in cell n.p.s. content due to hyperosmotic NaCl + urea was completely inhibited by pre-incubation in control medium containing trimethylamine N-oxide. 7. On transference of slices from control to hyperosmotic media (NaCl + urea) the steady-state increase in cell n.p.s. concentration was complete within 20 min and followed a time course similar to that for cell fluid loss. The n.p.s. concentration and cell fluid content returned to control levels, with similar time courses, following re-immersion in control medium. 8. Efflux of alpha-amino[1-14C]isobutyric acid (AIB) from slices pre-loaded in control medium containing 1 mmol AIB/l was slightly but significantly slower into AIB-free hyperosmotic NaCl + urea than into AIB-free control medium. The rate of efflux was greatly increased by the presence of hyperosmotic sucrose or very high Na+ (935 mmol/l).(ABSTRACT TRUNCATED AT 400 WORDS)

Mechanisms of escape from desmopressin in the rat

The mechanism(s) of renal escape from the hydro-osmotic effect of vasopressin is unknown. We therefore studied escape in conscious, unrestrained rats receiving continuous intravascular infusions of 1-deamino-8-arginine-vasopressin (desmopressin) and hypotonic fluid over 5 days. Escape from desmopressin started 8 hours after exposure and was characterized by a progressive increase in urine flow and decreases in urine osmolality and free water reabsorption. When positive water balance was prevented by matching the rate of infusion of hypotonic fluid to urine flow while maintaining the dose of desmopressin constant, escape did not occur. This suggested that water retention, rather than chronic exposure to desmopressin, mediated the escape. To elucidate the mechanism whereby water retention induces escape from desmopressin, urinary prostaglandin E2 excretion was measured and found to be increased concomitant with the onset of escape. Prevention of this increase in urinary prostaglandin E2 excretion with indomethacin resulted in additional water retention and a delay in the onset of escape. During the maintenance of escape, after significant water retention occurred, increases in mean arterial pressure, renal blood flow, and glomerular filtration rate were observed. Renal interstitial solute concentration remained constant through escape. Basal and vasopressin-stimulated collecting tubular and thick ascending limb adenylate cyclase did not differ when control and escape animals were compared. These results suggest that enhanced renal synthesis of prostaglandin E2 facilitates the early phase of escape; later, water retention results in plasma volume expansion with increases in cardiac index, arterial pressure, renal blood flow, and glomerular filtration rate. These systemic and renal hemodynamic alterations may be important in maintaining escape from desmopressin.

The influence of hyaluronidase on urinary and renal medullary composition following antidiuretic stimulus in the rat

1. The influence of urinary hyaluronidase (believed to be predominantly of renal origin) on the urinary concentrating process has been studied in rats subjected to antidiuretic stimulus. 2. Antiserum against a partially purified preparation of this enzyme has been raised in rabbits. Urinary volume, solute excretion and medullary composition have been investigated in rats treated with this antiserum (0.2 ml./100 g body weight, i.v.) before water deprivation for 48 hr or infusion for up to 4 hr with arginine-vasopressin. Control rats were pre-treated with normal rabbit serum. 3. Pre-treatment with antiserum against rat urinary hyaluronidase (AUase) caused water-deprived rats to excrete urine at a rate significantly greater, and of osmolality significantly lower, than that recorded in control rats. 4. The increase in medullary solute gradient which typically accompanies antidiuresis was significantly reduced in water-deprived rats pre-treated with AUase. 5. In rats treated with AUase and infused for 4 hr with arginine-vasopressin, there was no significant increase in the medullary solute gradient, whereas this increased markedly in control rats. 6. During the first 24 hr of water deprivation there weas an increase in the rate of Ca excretion by control rats which was abolished by pre-treatment with AUase. 7. The effects of antiserum against a partially purified preparation of rat testicular hyaluronidase (ATase) were studied in water-deprived rats. No evidence was obtained that this enzyme has any influence on renal function. 8. It is concluded that urinary hyaluronidase, but not testicular hyaluronidase, plays an important role in facilitating the urinary concentrating process following antidiuretic stimulus.

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.

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 effect of chlorpropamide on water balance in pitressin-treated Brattleboro rats

1 The daily administration of a 5% glucose solution to the heterozygous Brattleboro rat produced an experimental model in a comparable state of polydipsia and polyuria to the homozygous rat with diabetes insipidus (DI).2 The effect of chlorpropamide on water metabolism was then examined in both the homozygous DI rat treated with submaximal doses of pitressin tannate in oil (Pitressin), and in the glucose-hydrated heterozygous rat with and without simultaneous pitressin therapy.3 A dose-response curve for chlorpropamide (5, 10, and 20 mg/24 h) in DI rats treated with Pitressin (25 mu/24 h) indicated that the drug decreased fluid intake further, but only by a maximum of 13.8% (at the 20 mg/24 h dose of chlorpropamide), differing markedly from results obtained in patients with diabetes insipidus. A second experiment in which chlorpropamide (5 mg/24 h) was administered to DI rats treated with Pitressin (either 25 or 50 mu/24 h) confirmed the lack of any significant drug-effect on water metabolism in these animals.4 Chlorpropamide (20 mg/24 h), when administered alone or simultaneously with a submaximal dose of Pitressin (25 mu/24 h), had no obvious effect on the fluid intake of glucose-hydrated heterozygous rats. The absence of any action by chlorpropamide on water metabolism was confirmed in these experimental animals using 5 mg/24 h of the drug together with Pitressin (either 25 or 50 mu/24 hours).5 Indirect evidence for the slower growth-rate in the DI rat being due to an insufficient daily calorific intake was obtained from the study on glucose-hydrated heterozygous rats.

The effect of vasopressin (Pitressin) administration and dehydration on the concentration of solutes in renal fluids of rats with and without hereditary hypothalamic diabetes insipidus

1. The method of sequential centrifugation has been used to obtain fluid samples from both the renal papilla and inner medulla of the rat.2. Experiments were carried out on Brattleboro rats with hereditary hypothalamic diabetes insipidus (DI; homozygous recessive), and on their (heterozygous) litter-mates with normal quantities of neurohypophysial vasopressin. Initial classification of the animals by measurement of urine volume and osmolality was confirmed by post-mortem bio-assay of the pituitary glands, in thirty-five out of forty-seven animals.3. In rats with DI, urine osmolalities comparable to those of heterozygous rats were obtained after four daily injections of 1 u. Pitressin Tannate in Oil (PTO). Under these conditions, when dehydration was superimposed for 72 hr, urine osmolality did not increase markedly. In heterozygous rats dehydrated for the same period of time, urine osmolality increased by some twofold.4. In rats with DI, the administration of PTO induced a rise of both sodium and urea concentrations in renal fluids and in urine. Dehydration during PTO administration caused a further rise of urea concentrations only. Dehydration per se significantly raised urinary and renal fluid urea concentrations, but sodium concentrations did not rise.5. In heterozygous rats dehydration per se increased both urea and sodium concentrations in renal fluids and urine.6. The evidence is discussed that the action of vasopressin involves factors apart from increasing the permeability of the distal nephron to water and urea.

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.