Osmolality of distal tubular fluid in the dog

Chemical evidence for the tradeoff-in-the-nephron hypothesis to explain secondary hyperparathyroidism

Background

Secondary hyperparathyroidism (SHPT) complicates advanced chronic kidney disease (CKD) and causes skeletal and other morbidity. In animal models of CKD, SHPT was prevented and reversed by reduction of dietary phosphate in proportion to GFR, but the phenomena underlying these observations are not understood. The tradeoff-in-the-nephron hypothesis states that as GFR falls, the phosphate concentration in the distal convoluted tubule ([P]_DCT]) rises, reduces the ionized calcium concentration in that segment ([Ca⁺⁺]_DCT), and thereby induces increased secretion of parathyroid hormone (PTH) to maintain normal calcium reabsorption. In patients with CKD, we previously documented correlations between [PTH] and phosphate excreted per volume of filtrate (E_P/C_cr), a surrogate for [P]_DCT. In the present investigation, we estimated [P]_DCT from physiologic considerations and measurements of phosphaturia, and sought evidence for a specific chemical phenomenon by which increased [P]_DCT could lower [Ca⁺⁺]_DCT and raise [PTH].

Methods and findings

We studied 28 patients (“CKD”) with eGFR of 14–49 mL/min/1.73m² (mean 29.9 ± 9.5) and 27 controls (“CTRL”) with eGFR > 60 mL/min/1.73m² (mean 86.2 ± 10.2). In each subject, total [Ca]_DCT and [P]_DCT were deduced from relevant laboratory data. The Joint Expert Speciation System (JESS) was used to calculate [Ca⁺⁺]_DCT and concentrations of related chemical species under the assumption that a solid phase of amorphous calcium phosphate (Ca₃(PO₄)₂ (am., s.)) could precipitate. Regressions of [PTH] on eGFR, [P]_DCT, and [Ca⁺⁺]_DCT were then examined. At filtrate pH of 6.8 and 7.0, [P]_DCT was found to be the sole determinant of [Ca⁺⁺]_DCT, and precipitation of Ca₃(PO₄)₂ (am., s.) appeared to mediate this result. At pH 6.6, total [Ca]_DCT was the principal determinant of [Ca⁺⁺]_DCT, [P]_DCT was a minor determinant, and precipitation of Ca₃(PO₄)₂ (am., s.) was predicted in no CKD and five CTRL. In CKD, at all three pH values, [PTH] varied directly with [P]_DCT and inversely with [Ca⁺⁺]_DCT, and a reduced [Ca⁺⁺]_DCT was identified at which [PTH] rose unequivocally. Relationships of [PTH] to [Ca⁺⁺]_DCT and to eGFR resembled each other closely.

Conclusions

As [P]_DCT increases, chemical speciation calculations predict reduction of [Ca⁺⁺]_DCT through precipitation of Ca₃(PO₄)₂ (am., s.). [PTH] appears to rise unequivocally if [Ca⁺⁺]_DCT falls sufficiently. These results support the tradeoff-in-the-nephron hypothesis, and they explain why proportional phosphate restriction prevented and reversed SHPT in experimental CKD. Whether equally stringent treatment can be as efficacious in humans warrants investigation.

Novel insights into TRPV4 function in the kidney

Kidneys are complex highly organized paired organs of nearly one million nephrons each. They rigorously process about 180 l of plasma daily to keep whole body homeostasis. To effectively perform such a titanic work, kidneys rely on mechanisms able to sense dynamic changes in composition and flow rates of protourine along the renal tubule. It is envisioned that Ca(2+)-permeable transient receptor potential (TRP) channels, and specifically mechanosensitive TRPV4, can serve to interpret these external mechanical cues in the form of elevated intracellular Ca(2+) concentration. This, in turn, initiates multiple cellular responses and adaptation mechanisms. The current review summarizes up-to-date knowledge about the sites of TRPV4 expression in renal tissue as well as discusses the functional role of the channel in cellular responses to hypotonicity and tubular flow. We will also provide insights as to how TRPV4 fits into classical polycystin mechanosensory complex in cilia and will speculate about previously underappreciated clinical implication of pharmacological TRPV4 targeting in treatment of polycystic kidney disease.

The Effects of Angiotensin II on Renal Water and Electrolyte Excretion in Normal and Caval Dogs

The effects of intravenous administration of angiotensin II on renal water and electrolyte excretion were examined during hydropenia, water diuresis, and hypotonic saline diuresis in anesthetized normal dogs and dogs with thoracic inferior vena cava constriction and ascites (caval dogs). The effects of unilateral renal artery infusion of a subpressor dose were also examined.During hydropenia angiotensin produced a decrease in tubular sodium reabsorption, with a considerably greater natriuresis in caval dogs, and associated with a decrease in free water reabsorption (T(c) (H(2)O)). Water and hypotonic saline diuresis resulted in an augmented angiotensin natriuresis, with a greater effect still observed in caval dogs. In these experiments free water excretion (C(H(2)O)) was limited to 8-10% of the glomerular filtration rate (GFR), although distal sodium load increased in every instance. In the renal artery infusion experiments a significant ipsilateral decrease in tubular sodium reabsorption was induced, particularly in caval dogs.These findings indicate that angiotensin has a direct effect on renal sodium reabsorption unrelated to a systemic circulatory alteration. The attenuation or prevention of the falls in GFR and effective renal plasma flow (ERPF) usually induced by angiotensin may partially account for the greater natriuretic response in caval dogs and the augmentation during water or hypotonic saline diuresis. However, a correlation between renal hemodynamics and the degree of natriuresis induced was not always present and, furthermore, GFR and ERPF decreased significantly during the intrarenal artery infusion experiments. Therefore, the present experiments indicate that another mechanism is operative in the control of the angiotensin natriuresis and suggest that alterations in intrarenal hemodynamics may play a role.The decrease in T(c) (H(2)O) and the apparent limitation of C(H(2)O) associated with an increase in distal sodium load localize the site of action of angiotensin to the ascending limb of Henle's loop and the proximal tubule.

Micropuncture study of nephron function in the rhesus monkey

The function of the proximal and distal tubule was studied in the rhesus monkey during antidiuresis and during the diuresis after furosemide administration (during which extracellular fluid volume was maintained).In the proximal tubule, fluid to plasma ratios for sodium, potassium, and osmolality approximated unity. During antidiuresis, about 30% of the filtered water remained at the end of the accessible portion of this segment (92% of length). Fluid was hypotonic to plasma throughout the distal tubule. 25% of the filtered water was present in the early distal tubule. Small but significant water reabsorption (about 8% of filtered) occurred in remainder of this segment. Tubule fluid to plasma potassium concentration ratios tended to increase along the distal tubule, and the amount of potassium, relative to the amount filtered, increased from 13% in the early portion of this segment to 26% in the late portion.After furosemide was administered animals excreted about one-third of the filtered sodium and water. Despite this diuresis, electrolyte and water reabsorption along the proximal tubule did not differ from values obtained in control animals. Osmolality and sodium concentration of fluid from the distal tubule approached those of plasma. 22% of the filtered sodium (twice the control values) reached the distal tubule, whereas the fraction of filtered water remaining was only slightly increased. These findings indicate that, after the administration of this drug, inhibition of sodium reabsorption occurred in the water-impermeable segment of the nephron, rather than in the proximal tubule. After furosemide administration, all tubule fluid to plasma potassium concentration ratios in the distal tubule were equal to or greater than one, suggesting inhibition of active potassium reabsorption at or prior to this site.Fluid to plasma bicarbonate concentration ratios from the midportion of the proximal tubule were consistently less than one in normal monkeys. After acetazolamide was administered, the bicarbonate concentration of samples of tubule fluid recollected from these same sites was the same as, or higher than in plasma. This fact demonstrates the inhibition of bicarbonate reabsorption in this portion of the tubule.

A numerical model of the renal distal tubule

A numerical model of the rat distal tubule was developed to simulate water and solute transport in this nephron segment. This model incorporates the following: 1) Na-Cl cotransporter, K-Cl cotransporter, Na channel, K channel, and Cl channel in the luminal membrane; 2) Na-K-ATPase, K channel, and Cl channel in the basolateral membrane; and 3) conductances for Na, K, and Cl in the paracellular pathway. Transport rates were calculated using kinetic equations. Axial heterogeneity was represented by partitioning the model into two subsegments with different sets of model parameters. Model equations derived from the principles of mass conservation and electrical neutrality were solved numerically. Values of the model parameters were adjusted to minimize a penalty function that was devised to quantify the difference between model predictions and experimental results. The developed model could simulate the water and solute transport of the distal tubule in the normal state, as well as in conditions including thiazide or amiloride application and various levels of sodium load and tubular flow rate.

Alternatively spliced isoforms of the putative renal Na-K-Cl cotransporter are differentially distributed within the rabbit kidney

We have used cDNA probes derived from the secretory form of the Na-K-Cl cotransporter to screen both cortical and medullary rabbit kidney cDNA libraries. A sequence of 4750 bases was identified from multiple clones. The DNA encodes a protein containing 1099 amino acids, which is 61% identical over its length to the secretory Na-K-Cl cotransporter from shark rectal gland. From analysis of amino acid hydropathy, we predict that this putative renal Na-K-Cl cotransporter has 12 transmembrane helices and large N- and C-terminal cytoplasmic regions. Two sites for N-linked glycosylation are predicted on an extracellular loop. Three potential sites for modulation by protein kinase A are in the C-terminal cytoplasmic domain. Most of the isolated renal cDNA clones were identical over all regions of overlap; however, there was a 96-bp region for which there were three different but homologous variants (A, B, and F). This region of divergence was identified as an alternatively spliced cassette exon since clones were identified that contained intronic DNA as well as consensus splice acceptor sites that bounded the region. Tissue Northern blot analysis revealed a broad band at approximately 5.1 kb that was unique to the kidney. High-stringency Northern blot analysis of cortical and medullary mRNA using antisense oligonucleotides synthesized over each of the three cassette exons revealed that the isoforms were differentially distributed within the kidney--B almost exclusively in cortex, F almost exclusively in medulla, and A about equally distributed.

Interactions among prostaglandin E2, antidiuretic hormone, and cyclic adenosine monophosphate in modulating Cl- absorption in single mouse medullary thick ascending limbs of Henle

This paper describes the inhibitory effect of prostaglandin E2 (PGE2) on antidiuretic hormone (ADH)-stimulated net Cl- absorption and spontaneous transepithelial voltage (Ve) in single medullary thick ascending limbs of Henle (TALH, thick ascending limb; mTALH, medullary segment; cTALH, cortical segment) obtained from mouse kidney. The experimental data indicate that PGE2 reduced the ADH-dependent values of net Cl- absorption (JnetCl, eq cm-2 s-1) and Ve (mV) in a dose-dependent manner; that increasing concentrations of peritubular ADH reversed the PGE2-mediated reductions in the ADH-dependent moiety of Ve in the mouse mTALH; that PGE2 had no effect on cyclic AMP-stimulated increments in Ve in the mouse mTALH; and that PGE2 had no effect on Ve in the cTALH, where Ve is unaffected either by ADH or by cyclic AMP. These effects might be obtained because of a direct competition between ADH and PGE2 for receptor binding on basolateral membranes. Alternatively, PGE2 might have reduced the affinities between ADH-receptor units and a component(s) of the series of processes leading to adenyl cyclase activation. The latter argument requires that basolateral membranes of the mouse mTALH exhibit receptor reserve, i.e., at the minimum concentration of ADH required to enhance Ve and JnetCl maximally, a fraction of basolateral membrane ADH receptors were unoccupied. According to this view, increasing peritubular ADH concentrations might reverse the PGE2-mediated reduction in ADH-dependent salt transport by increasing the number of basolateral membrane receptors occupied by ADH.

Renal calcium and magnesium excretion during vasopressin administration into sheep with acid or alkaline urine

1. The proposition that changes in renal calcium excretion during vasopressin administration are positively correlated with concurrent changes in urine hydrogen ion concentration was tested by administration of vasopressin into twelve conscious diuresing sheep receiving either alkalinizing or acidifying infusions. 2. Vasopressin-induced antidiuresis in sheep with alkaline urine was associated with significant increases in urinary pH and decreases in the rate of calcium excretion whereas antidiuresis in sheep with acid urine was associated with significant decreases in urinary pH and no consistent effect on calcium excretion. 3. Magnesium excretion increased during vasopressin administration in most experiments regardless of urinary pH changes. 4. Vasopressin administration did not significantly alter the rate of excretion of sodium, potassium, chloride and phosphate or the rates of sodium, potassium, chloride, inulin, para-aminohippurate and osmolal clearance in sheep with either acid or alkaline urine. Potassium excretion and clearance in sheep with alkaline ruine was higher than that of sheep with acid urine during vasopressin infusion. 5. The results support the hypothesis that changes in renal tubular hydrogen ion concentration or bicarbonate concentration caused by water reabsorption from the collecting duct and possibly the late distal tubule could be part of the explanation for changes in renal calcium excretion which occur during vasopressin-induced antidiuresis.

Micropuncture study of fluid handling in the cat kidney

Superficial proximal and distal tubules of both kidneys of pentobarbital-anesthetized cats, infused with 2.5% (w/v) polyfructosan in Ringer solution at 0.45 ml/min, were micropunctured. Whole one-kidney GFR average 4.1 ml/min. SNGFR averaged 15 nl/min in 51 determinations and was proportional to whole-kidney GFR in individual cats. Absolute fluid reabsorption along the length of the accessible portion of the proximal tubule was 1.6 nl/min.mm. The length of the whole proximal tubule was 6 mm as measured from the glomerulum to the descending thin limb of Henle's loop. The length of the distal tubule was 3.5 mm from the macula densa to the first branching of the collecting duct. Proximal fluid was isoosmolar with plasma whereas fluid entering the distal tubule was markedly hyposmolar. Late distal tubular fluid samples were also isosmolar. The cat appears to be well suited for micropuncture experiments in terms of freedom from respiratory movements and the presence of surface distal tubules.

Excretion of sodium and water by kidneys in situ and by transplanted kidneys following isotonic, hypotonic, iso-oncotic and hyperoncotic intravenous infusions in sodium-loaded and sodium-deprived dogs

The excretion of sodium and water following isotonic, hypotonic, iso-oncotic and hyperoncotic intravenous infusions has been investigated in the kidneys in situ and in transplanted kidneys of narcotized dogs previously submitted to sodium-enriched or-deprived diets. The fractional excretion of sodium depended basically on the cumulative effect on the kidney of the changes in plasma oncotic pressure, plasma sodium concentration, and haematocrit. The differences in excretory responses of sodium-loaded or-deprived animals did not depend on differences in the distribution of infused fluids between intra- and extravascular compartments, but to the sensitivity of the kidney itself to the direct cumulative effect of these non-specific changes in blood composition.

The effect of high rates of vasopressin administration on renal potassium and sodium excretion during potassium loading in the sheep

1 The influence of potassium loading on the renal excretion of sodium, potassium and solute during high rate vasopressin administration has been investigated in sheep. 2 Adrenalectomized sheep were infused with 0.43 M KCl at 2 ml/min for 2-2.5 hours. Coincident with the rise in plasma potassium concentration, the urinary excretion of sodium, potassium, solute and water was increased as was the reabsorption of solute-free water. The rates of urinary excretion of sodium and potassium, osmolal clearance (COsm) and solute-free water reabsorption (TcH2O) for the first 50 min of potassium infusion were each found to be linearly related to the plasma potassium concentration. 3 After 50 min an infusion of vasopressin at 1 or 4 mu/min was superimposed on the potassium infusion for a period of 30 minutes. The administration of vasopressin was consistently associated with further augmentation of potassium excretion and clearance, of osmolal clearance and of solute-free water reabsorption to values above those anticipated from the pre-vasopressin regression lines for these parameters. Urinary sodium showed a coincident depression in the rate of excretion and clearance during the same period. 4 Thirty to fifty minutes after the cessation of vasopressin infusion the potassium and sodium excretions had returnied to values which approximated the pre-vasopressin relations between plasma potassium and the urinary excretions of these ions. 5 Both rates of vasopressin infusion were equally effective in increasing the potassium clearance. Any differences in clearance between the two rates of vasopressin administration were not statistically significant. 6 The large increments in potassium excretion (averaging greater than 40%) were interpreted as indicating that, when vasopressin is present at high concentrations, the distal tubule is one site of action of the hormone in the nephron of sheep.

Clonidine-induced increase of renal prostaglandin activity and water diuresis in conscious dogs

I.v. administration of clonidine to conscious dogs induces a water diuresis with hyposmotic urine and minor effect on electrolyte excretion. The diuresis is preceded by an increased urinary PGE excretion, but no change of urinary ADH output is observed. Plasma renin activity decreases. Both ADH infusion and indomethacin pretreatment inhibit the diuretic effect of clonidine. The results support the hypothesis that clonidine-induced water diuresis is mediated via an anti ADH effect due to increased renal prostaglandin activity. Moreover the results suggest that there is no direct stimulation of renin release by PGE.

Functional segmentation of the rabbit distal tubule by microdetermination of hormone-dependent adenylate cyclase activity

Rabbit distal convoluted tubules (DCT) microdissected from collagenase-treated kidneys were observed to contain up to four portions of a different appearance under stereomicroscopic examination: (1) a DCTa portion (generally very short), located right after the macula densa (MD) and resembling the portion of the limb (CAL) located before the MD; (2) a constant, "bright" portion, DCTb; (3) a constant, "granular" DCTg portion which, in most DCT, is connected to a portion of the collecting tubule of a similar "granular" appearance (CCTg); (4) many DCT having contacts with the kidney capsule in the superficial cortex were observed to contain an additional portion of a "light" appearance, DCTl, resembling the portion of the collecting tubule (CCTl) to which these superficial DCT are always branched. The hormone-dependent adenylate cyclase (AC) contained in these different portions was investigated by sectioning microdissected distal structures into successive samples according to the above-mentioned criteria, and by measuring with the help of a previously described micromethod, the enzyme activity contained in each single sample under one of the following conditions: control, parathyroid hormone. (PTH l U/ml), vasopressin, (AVP 10(-6)M), isoproterenol (10(-6)M), fluoride (5 X 10(-3)M). Highly significant and reproducible AC stimulations by these hormones were obtained for the following portions, respectively: DCTa, DCTg and CCTg with PTH; DCTl and CCTl with AVP; DCTg, CCTg and CCTl with isoproterenol. From these data, it is concluded that (a) the distal convoluted tubule can no longer be regarded as a single well-defined functional structure; (b) DCTa is actually a short CAL portion extending beyond MD, (c) DCTg and CCTg are two portions of a same functional segment; (d) similarly, DCTl belongs to the functional segment mainly constituted by CCTl; and, finally, (e) DCTb is the only functional segment which is entirely located in the distal convoluted tubule, i.e., included between the macula densa and the first branching with another tubule.

Evidence against bicarbonate reabsorption in the ascending limb, particularly as disclosed by free-water clearance studies

Bicarbonate reabsorption in the thick ascending limb of Henle's loop was examined by studies of free-water clearance (CH2O) and free-water reabsorption (TcH2O). During maximal water diuresis in the dog, CH2O/GFR was taken as an indes of sodium reabsorption in, and urine flow (V/GFR) as an index of delivery of filtrate to, this scarbonate, infusion of a nonreabsorbable solute (hypotonic mannitol) and administration of an inhibitor of bicarbonate reabsorption (acetaent, but less than that achieved with hypotonic saline infusion. This suggests that sodium that sodium bicarbonate is not reabsorbed in the ascending limb. Rather, it is the sodium chloride, swept out of the proximal tubule by osmotic diuresis due to nonreabsorbed mannitol or sodium bicarbonate, that is reabsorbed in the ascending limb thereby increasing CH2O, whereas the nonreabsorption of mannitol and sodium bicarbonate results in a depressed CH20 per unit V when compared with hypotonic saline. V/GFR is not a satisfactory index of delivery to the ascending limb during osmotic diuresis, since it includes water obligated by nonreabsorbable solutes. When a better index of delivery, the sum of the clearances of chloride (CC1) and free-water (CH2O) is used, hypotonic bicarbonate infusion, hypotonic mannitol infusion and acetazolamide administration increase CH2O/GFR per unit delivery to the same extent as odes hypotonic saline infusion. Studies in dogs and rats on TcH2O also indicate that sodium bicarbonate is an impermeant solute in the ascending limb. Osmotic diuresis due to sodium bicarbonate diuresis, produced either by inhibition of sodium bicarbonate reabsorption (acetazolamide, L-lysine mono-hydrochloride) or infusion of sodium bicarbonate, or mannitol diuresis both produced marked chloruresis and increased TcH2O to the same extent as did hypertonic saline infusion. If chloride excretion was almost eliminated by hemodialysis against a chloride-free dialysate (dogs) or prolonged feeding of a salt-free diet (rats), TcH2O formation was unimpaired if hypertonic saline was infused but virtually obliterated during mannitol or sodium bicarbonate diuresis. Sodium reabsorption in the ascending limb, therefore, appears to be dependent upon chloride as the accompanying anion. At any given rate of bicarbonate excretion, more cloride is delivered out of the proximal tubule (as estimated from CC1 + CH2O) with hypotonic sodium bicarbonate infusion than with acetazolamide administration. This suggests that magnitude of the chlorutesis accompanying bicarbonate diuresis depends, not only on osmotic diuresis due to nonreabsorbed sodium bicarbonate, but also on the extent to which concomitant changes in effective extracellular volume influence overall sodium chloride reabsorption.

Some reflections on the mechanism of renal tubular potassium transport

Analysis of the driving forces acting on the movement of potassium across individual membranes of tubule cells shows that both active and passive components play an important role in the regulation of potassium transport. Distal and cortical collecting tubule and papillary collecting duct elements are the key nephron sites participating in a complex fashion to translate a wide variety of metabolic challenges into the appropriate excretory response. The latter involves both secretory and reabsorptive activity. The analysis of the factors modulating tubular potassium transfer has shown that the potassium concentration in the cells of the distal nephron is a dey factactors involved in setting the cellular potassium concentration are active potassium uptake at the peritubular and luminal membrane of the cells as well as electrogenic solium extrusion across the peritubular boundary of the cells. Additional factors regulating potassium transport involve the electrical potential difference, sensitive to changes in the sodium concentration in the lumen, the flow rate past the late distal tubular site of potassium secretion, and the activity of a reabsorptive potassium pump in the luminal membranes of the cells. In the cortical collecting tubule, active potassium secretion is also present at the luminal membrane of the cell, but the role of such an additional secretory mechanism in the late distal tubule is presently unknown. Most of these individual transport mechanisms exist along the whole distal nephron, but their relative prominence varies among the late distal tubule, the cortical collecting tubule, and the papilary collecting duct.

Clarification of the site of action of chlorothiazide in the rat nephron

The saluretic effect of the thiazide diuretics has been attributed to inhibition of sodium reabsorption in the distal nephron of the kidney. Recent micropuncture studies have shown, however, that chlorothiazide administration can also inhibit sodium reabsorption in the proximal convolution. To clarify the site of the saluretic effect of chlorothiazide, these micropuncture studies examined the effect of chlorothiazide on chloride transport in the nephron. The effect of chlorothiazide on chloride transport was studied because chlorothiazide's effectiveness as a saluretic is largely due to its ability to enhance sodium chloride excretion; if only changes in sodium transport are examined, it would be then difficult to determine if sodium as bicarbonate or as chloride is affected, since chlorothiazide can inhibit carbonic anhydrase. One group of rats was studied before and after 15 mg/kg per h chlorothiazide. For comparison, another group of rats was studied before and after 2 mg/kg per h benzolamide, a carbonic anhydrase inhibitor. Fractional chloride delivery from the proximal tubule was similarly increased in both groups from 59.4 to 71.0% by chlorothiazide administration, Pless than 0.0001, and from 54.3 to 68.2% by benzolamide administration, P less than 0.001. The increased delivery very of chloride from the proximal tubule was largely reabsorbed before the early distal tubule as fractional chloride delivery to this site increased only from 5.08 to 7.40% after chlorothiazide administration, P less than 0.001, and from 4.50 to 6.29% after benzolamide administration, P less than 0.01. Benzolamide had no effect on chloride reabsorption in the distal convoluted tubule. However, chlorothiazide administration resulted in a marked decrease in distal tubular chloride reabsorption, the fraction of filtered chloride present at the late distal tubule incresing from 1.24 to 6.25%, P less than 0.001. Fractional chloride excretion in the urine increased from 0.29 to 3.44%, P less than 0.001, after chlorothiazide, but did not change after benzolamide. The influence of chlorothiazide on proximal chloride transport presumably is related to its ability to inhibit renal carbonic anhydrase. However, it is not the effect of chlorothiazide in the proximal convolution but rather its effect in the distal convoluted tubule which is primarily responsible for its ability to be an effective saliuretic.

Vasopressin dependent adenylate cyclase in single segments of rabbit kidney tubule

AVP dependent adenylate cyclase activity was measured in single pieces of 8 different tubular segments isolated from collagenase treated rabbit kidneys. High responses were observed in all the tested portions of the collecting tubule, that is its cortical branched part (BCT), its cortical straight part (CCT) and its outer medullary part (MCT). Dose response curves indicated in CCT: 2 fold threshold stimulation at 10(-11) M AVP, 27 fold stimulation at 10(-6) M AVP, half maximal stimulation at about 10(-9) M AVP. Both the medullary (MAL) and, to a lesser extent, the cortical (CAL) portions of the thick ascending limb were also observed to contain AVP sensitive adenylate cyclase (for MAL: 2 fold threshold stimulation at 10(-9) M AVP, 9 fold stimulation at 10(-7) M AVP, half maximal stimulation at 5 X 10(-9) M AVP). In contrast, nearly no responsiveness to AVP was observed in the proximal convoluted tubule, in the thin descending limb of the loop and in the distal convoluted tubule (DCT). The limited response obtained in DCT (which is a structure generally considered as a target site for AVP) as well as the clearcut effect elicited by AVP in MAL (the functioning of which is not known to be controlled by ADH) were expected observations; their possible physiological implications will be discussed.

Current concepts of sodium chloride and water transport by the mammalian nephron

The decision of the editors to solicit a review for the Medical Progress series of this journal devoted to current concepts of the renal handling of salt and water is sound in that this important topic in kidney physiology has recently been the object of a number of new, exciting and, in some instances, quite unexpected insights into the mechanisms governing sodium excretion. These developments have come about largely as a consequence of the fact that segments of nephrons previously inaccessible to direct study are now readily accessible. Many of the findings to be discussed argue for extensive revision of a number of our current widely held views concerning the renal handling of sodium chloride and water. In the opinion of the authors, the strength of this argument rests in the fact that many of these new findings were obtained under circumstances that enabled workers to gain more direct access to the nephron than has been possible heretofore. This is not to say that areas of controversy and disagreement no longer exist. Wherever possible, these have been identified. In attempting to provide a comprehensive review of this topic, it has been necessary at times to overgeneralize and to disregard minor deficiencies in some of the studies cited. Finally, we wish to emphasize that a considerable portion of the information contained herein derives from work still under active investigation. Much of this contemporary work will undoubtedly withstand the rigors of future experimental scrutiny. It is inevitable, however, as William James so aptly noted in the quotation cited below, that some of our present ideas will need to be abandoned or revised in favor of newer, more convincing evidence. Seen in this light, the present effort is intended as nothing more than a timely survey of this active and fertile topic in renal physiology.

Response of the distal tubule and cortical collecting duct to vasopressin in the rat

Renal micropuncture observations in the rat suggest that the entire "distal tubule" (defined by the micropuncturist as that portion of the renal tubule extending between the macula densa and its first junction with another (renal tubule) may be responsive to vasopressin. However, this portion of the renal tubule contains two segments that are morphologically dissimilar. The "early" distal tubule is lined by epithelium characteristic of the distal convoluted tubule, while the "late" distal tubule is lined by epithelium characteristic of the cortical collecting duct. Thus, the present study was initiated to identify the most proximal site of action of vasopressin in the distal renal tubule. A water diuresis was established in rats with hereditary hypothalamic diabetes insipidus. In one-half of the animals the diuresis was interupted by an i.v. infusion of exogenous vasopressin. Morphological preservation of the kidneys was initiated after induction of vasopressin-induced antidiuresis or during maximum water diuresis. Cell swelling and dilatation of intercellular spaces, morphological findings indicative of vasopressin responsiveness, were observed in the cortical collecting duct including the late segment of the distal tubule, a segment that has also been described by morphologists as the initial collecting tubule. Morphological evidence of vasopressin-responsiveness was not observed in the early distal tubule (distal convoluted tubule). Additional morphological studies in Wistar, Long-Evans, and Sprague-Dawley rats demonstrated a marked difference in the random availability of distal convoluted tubules versus initial collecting tubules potentially available for micropuncture just beneath the renal capsule. The results suggest that hypotonic tubular fluid entering the early distal tubule (distal convoluted tubule) remains hypotonic to plasma until it enters the late distal tubule (initial collecting tubule) and that vasopressin-induced osmotic equilibration is a function of the latter segment alone. The findings emphasize the importance of morphological characterization of those segments of the renal tubule that are subjected to physiological investigation.

Sodium chloride and water transport in the medullary thick ascending limb of Henle. Evidence for active chloride transport

Transport of NaCl and water was examined in the rabbit medullary thick ascending limb of Henle (ALH) by perfusing isolated segments of these nephrons in vitro. Osmotic water permeability was evaluated by perfusing tubules against imposed osmotic gradients. In these experiments the net transport of fluid remained at zero when segments of thick ALH were perfused with isotonic ultrafiltrate in a bath of rabbit serum in which the serum osmolality was increased by the addition of either 239+/-8 mosmol/liter of raffinose or 232+/-17 mosmol of NaCl indicating that the thick ascending limb of Henle is impermeant to osmotic flow of water. When these tubules were perfused at slow rates with isosmolal ultrafiltrate of same rabbit serum as used for the bath, the effluent osmolality was consistently lowered to concentrations less than the perfusate and the bath. That this decrease in collected fluid osmolality represented salt transport was demonstrated in a separate set of experiments in which it was shown that the sodium and chloride concentrations decreased to 0.79+/-0.02 and 0.77+/-0.02 respectively when compared with the perfusion fluid concentrations. In each instance the simultaneously determined transtubular potential difference (PD) revealed the lumen to be positive with the magnitude dependent on the perfusion rate. At flow rates above 2 nl.min(-1), the mean transtubular PD was stable and equal to 6.70+/-0.34 mv. At stop-flow conditions this PD became more positive. Ouabain and cooling reversibly decreased the magnitude of this PD. The transtubular PD remained positive, 3.3+/-0.2 mV, when complete substitution of Na by choline was carried out in both the perfusion fluid and the bathing media. These results are interpreted to indicate that the active transport process is primarily an electrogenic chloride mechanism. The isotopic permeability coefficient for Na was 6.27+/-0.38 x 10(-5) cm.s(-1) indicating that the thick ALH is approximately as permeable to Na as the proximal convoluted tubule. The chloride permeability coefficient for the thick ALH was 1.06+/-0.12 x 10(-5) cm.s(-1) which is significantly less than the chloride permeability of the proximal tubule. These data demonstrate that the medullary thick ascending limb of Henle is water impermeable while having the capacity for active outward solute transport as a consequence of an electrogenic chloride pump. The combination of these characteristics allows this segment to generate a dilute tubular fluid and participate as the principal energy source for the overall operation of the countercurrent multiplication system.

Time course of changes in renal tissue and urinary composition after cessation of constant infusion of lysine vasopressin in the conscious, hydrated rat

1. The changes in urinary and renal tissue composition in conscious rats were determined for up to 2 hr following the cessation of intravenous infusion of lysine vasopressin, LVP (at 60 muu./min. 100 g body wt. for 4(1/2) hr). A constant water load (4% body wt.) was maintained during and after lysine vasopressin infusion, by quantitative replacement of excreted water. In these circumstances, any changes in urinary and renal tissue composition are presumed to represent direct consequences of the rapid plasma and tissue clearance of lysine vasopressin.2. Urinary flow increased and osmolality decreased, rapidly, reaching stable values characteristic of sustained water diuresis after about 60 min.3. The steepness of the corticomedullary solute concentration gradients also decreased rapidly. Papillary Na and urea concentrations fell to values characteristic of sustained water diuresis in about 45 min.4. The changes in medullary composition were compounded of a moderate significant increase in water content, a moderate, significant decrease in Na content, and a profound decrease in urea content.5. In the eventual steady-state water diuresis, urinary outputs of Na and K were significantly lower, and of NH(4) significantly higher, than those observed in control experiments where LVP infusion was continued for the corresponding 2 hr.6. It is concluded that the diuresis following the cessation of LVP infusion is due not merely to reduced nephron permeability to water but also to a rapid reduction in the osmotic force responsible for water reabsorption from the collecting duct.

A micropuncture study of collecting tubule function in rats with hereditary diabetes insipidus

The reabsorption of water and solute by the papillary collecting duct was studied during water diuresis and vasopressin-induced antidiuresis in young rats with hereditary hypothalamic diabetes insipidus. The tip of the left renal papilla was exposed and fluid was obtained by micropuncture from loops of Henle and from collecting ducts at the papillary tip, and at an average of 1 mm proximal to the tip. In water diuresis the ratio of tubule fluid to plasma (TF/P) osmolality (osm) of loop fluid was 1.73 +/-0.058 (SE); of fluid from the proximal collecting duct, 0.63 +/-0.027; and from the tip, 0.55 +/-0.024; indicating a substantial osmotic pressure difference across the collecting duct epithelium. The fraction of filtered water reabsorbed (x 100) by the terminal collecting duct was 1.58% +/-0.32. In antidiuresis the TF/P osm of loop fluid was 2.65 +/-0.109; of fluid from the proximal collecting duct, 2.20 +/-0.093; and from the tip, 2.71 +/-0.111; indicating a marked decrease in the driving force for water reabsorption. The fraction of filtered water reabsorbed (x 100) by the terminal collecting duct was reduced to 0.58% +/-0.08, while the delivery of solute to the same segment was unchanged from that in water diuresis. The glomerular filtration rate (GFR) of the right kidney declined from 327 +/-24.4 mul/min in water diuresis to 274 +/-24.4 mul/min in antidiuresis (P < 0.005); similar results were obtained in a study comparing right and left GFRs in five additional rats. Thus, fractional reabsorption (and very likely the absolute volume) of water reabsorbed by the terminal collecting duct was less in antidiuresis than in water diuresis (mean difference, 1.01% +/-0.29, P < 0.005).

The postobstructive kidney. Observations on nephron function after the relief of 24 hr of ureteral ligation in the dog

After the relief of 24 hr of complete unilateral ureteral obstruction in the dog, the experimental kidney is characterized by a decrease in filtration rate and an increase in fractional and often absolute excretion of sodium before and after the administration of mannitol. In the hydrated state, the failure to conserve sodium is associated with increases in fractional free water clearance and fractional sodium supply to water-freeing sites signifying that the augmented sodium excretion is derived from a proximal source. In the hydropenic state there is decreased fractional free water reabsorption, and sometimes free water excretion, in the postobstructive kidney. An early plateau in free water reabsorption is associated with an increased fractional excretion of sodium. These findings are attributed to the early development of distal nephron impermeability to water as a result of enhanced distal tubular supply and transport of sodium. There is a decrease in maximal tubular reabsorptive capacity (Tm) of glucose in the post-obstructive kidney which is, however, less marked than the decrease in filtration rate. The fall in filtration rate is to some extent likely due to a dropping out of nephrons from the circulation while the remaining nephrons are hypoperfused. The magnitude of the sodium reabsorptive defect is markedly exaggerated as the concentration of nonreabsorbable solute (mannitol) in the glomerular perfusate is increased. It is concluded that the postobstructive increase in sodium excretion during mannitol administration is in part due to a limit in the capacity to reabsorb sodium against a concentration gradient in the proximal tubule.

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.

Urine concentration during solute diuresis in potassium-depleted rabbits. Evidence for a defect in tubular sodium transport

1. The relationship between osmolal clearance (C(osm)) and the reabsorption of solute-free water by the kidney (T(H2O) (c)) was examined during 10% mannitol and 2.3% saline diuresis in normal and potassium-depleted rabbits.2. In normal rabbits at osmolal clearances close to 3.0 ml./min, T(H2O) (c) during mannitol diuresis was 0.87 +/- 0.06 ml./min and during saline diuresis 1.19 +/- 0.07 ml./min. The mean difference in T(H2O) (c) of 0.32 +/- 0.05 ml./min was highly significant (P < 0.001).3. In one group of potassium-depleted rabbits with a reduction in maximal urinary concentration, T(H2O) (c) during both mannitol and saline diureses was reduced significantly below normal and the increment in T(H2O) (c) normally seen during saline diuresis was abolished.4. In a second group of potassium-depleted rabbits maximal urinary concentration (1253 +/- 88 m-osmole/kg H(2)O) was not significantly different from that in normal rabbits (1272 +/- 116 m-osmole/kg H(2)O). In these animals, T(H2O) (c) at osmolal clearances close to 3.0 ml./min was not significantly different during mannitol diuresis (0.83 +/- 0.07 ml./min) from that in normal animals, whereas it was reduced significantly during saline diuresis (0.89 +/- 0.07 ml./min, P < 0.001) and the difference in T(H2O) (c) normally seen between mannitol and saline diuresis was abolished.5. The inability to increase T(H2O) (c) during saline diuresis above that achieved during mannitol appears to be the earliest manifestation of the concentrating defect associated with potassium depletion. It probably results from an impairment of sodium transport by the ascending limb of the loop of Henle. This is supported by the fact that potassium-depleted rabbits excreted a greater percentage of the filtered load of sodium than did normal controls.

Micropuncture study of hypertonic mannitol diuresis in the proximal and distal tubule of the dog kidney

Fractional reabsorption of water, sodium, and potassium at proximal and distal tubular sites within the nephron was studied by recollection-micropuncture experiments on dogs undergoing hypertonic mannitol diuresis. After an initial control hydropenic phase, 16% mannitol in modified Ringer's solution was administered intravenously, resulting in marked increases in fractional excretion of water (28.7%), sodium (12.6%), and potassium (63.9%). Inulin clearance decreased significantly from 35.1 to 25.2 ml/min. Analysis of paired micropuncture data revealed a significant decrease in tubule fluid to plasma (TF:P) inulin ratios in both the proximal tubule (1.63-1.45) and distal tubule (5.38-1.94). There was also a significant decrease in proximal TF:P sodium ratios (0.99-0.93) and potassium ratios (1.05-0.98). Distal TF:P sodium ratios, in contrast, rose significantly (0.38-0.59), while TF:P potassium ratios tended towards unity whether initially greater or less than one. Fractional reabsorption of sodium and water decreased by 5% and 10% respectively in the proximal tubule, but to a lesser extent than the resulting increases in fractional urinary excretion. The nonreabsorbed fraction, however, had increased sharply at the point of distal puncture for water (32%), sodium (26%), and potassium (26%), indicating a large inhibitory effect within the loop of Henle in addition to the smaller proximal effects.

The effect of acute changes in haematocrit in the anaesthetized dog on the volume and character of the urine

1. Acute changes in haematocrit were produced by exchange transfusion of dextran-in-saline or packed red cells.2. There were no significant changes in glomerular filtration rate, blood pressure, central venous pressure, heart rate, respiratory rate, blood volume or extracellular fluid volume following the exchange transfusions.3. Urine volume increased after haemodilution but decreased after haemoconcentration.4. The diuresis after haemodilution occurred despite an infusion of ADH or alcohol. Thus it could not be attributed to a change in circulating ADH level.5. There were two types of diuresis. The ;water diuretic' response was characterized by an increase in free water clearance with a reduction in urinary sodium concentration; the ;sodium diuretic' response by an increase in urinary sodium concentration but no change, or a fall, in free water clearance.6. The results were related to changes in medullary osmotic gradient found by other workers to occur when medullary blood flow rate is altered.

The importance of aerobic metabolism in the renal concentrating process

The extent to which the concentrating function of the kidney depends on oxidative processes was investigated by infusing cyanide into one renal artery of dogs undergoing mild mannitol diuresis while receiving an infusion of vasopressin. This produced an abrupt fall in concentrating capacity (T(c) (H2O)) that was reversed when the cyanide infusion was stopped. The change could not be accounted for by the accompanying solute diuresis, since it was not reproduced by increasing the rate of mannitol infusion. The reduction in T(c) (H2O) induced by cyanide did not result from increased delivery of dilute urine to the collecting ducts, since free water clearance (C(H2O)), studied in other dogs during water diuresis, was unchanged or decreased by cyanide. Cyanide produced renal vasodilatation, as did intraarterial acetylcholine, but in contrast to the striking reduction in concentrating capacity evoked by cyanide, T(c) (H2O) was not significantly changed by acetylcholine. The data indicate that concentrating ability is closely tied to oxidative metabolism in the kidney, and it is suggested that the region where this is critically important is the red medulla and the thick ascending limb of Henle's loop.

Mechanism of exaggerated natriuresis in hypertensive man: impaired sodium transport in the loop of Henle

To evaluate the effects of saline loading on distal sodium reabsorption in hypertensive man, studies were performed during both water deprivation and water diuresis in eight hypertensive subjects, and the results were compared to data obtained from similar studies in normal subjects. All hypertensive patients exhibited an enhanced excretion of filtered sodium (C(Na)/C(In)) at any level of distal delivery of sodium compared to normal controls. Free water reabsorption (T(c) (H2O)) during hypertonic saline loading was quantitatively abnormal in the hypertensives at high levels of osmolar clearance (C(Osm)), and also the curve of T(c) (H2O) vs. C(Osm) leveled off above a C(Osm) of 18 ml/min per 1.73 m(2) in the hypertensive group in contrast to the normal controls in whom T(c) (H2O) showed no evidence of achieving an upper limit. Sodium depletion exaggerated the abnormality in T(c) (H2O) in hypertensives, and resulted in a positive free water clearance (C(H2O)) during hydropenia. During hypotonic saline loading in water diuresis, changes in free water clearance per 100 ml of glomerular filtrate (C(H2O)/C(In)) were less at any given increment in urine flow per 100 ml of glomerular filtrate (V/C(In)) in the hypertensives compared to normal controls (P < 0.001). This abnormality in C(H2O)/C(In) in the hypertensives in conjunction with the defect in T(c) (H2O) observed during hydropenia indicates that sodium reabsorption in the loop of Henle was abnormal at any given rate of distal delivery of sodium in hypertension. Furthermore, these abnormalities in T(c) (H2O) and C(H2O) coincided temporally with the development of the exaggerated natriuresis. Although the distal defect in sodium transport, in large part, accounted for the augmented natriuresis in hypertension, evidence was present also for enhanced rejection of sodium in the proximal tubule during saline loading in the hypertensives. Additional studies utilizing acetazolamide which increases distal delivery of sodium without extracellular fluid volume expansion showed only minimal abnormalities in C(H2O) in the hypertensive group, indicating that the defect in sodium transport in the loop of Henle in hypertensives is mainly an abnormal response to extracellular fluid expansion rather than an intrinsic defect in the loop to handle increased tubular loads of sodium. It is possible that the abnormality in sodium reabsorption in the loop of Henle is due to the transmission of the abnormally elevated blood pressure of the hypertensives to the medullary vasa recta during saline loading.