Computer simulation of osmotic gradient without active transport in renal inner medulla

Role of three-dimensional architecture in the urine concentrating mechanism of the rat renal inner medulla

Recent studies of three-dimensional architecture of rat renal inner medulla (IM) and expression of membrane proteins associated with fluid and solute transport in nephrons and vasculature have revealed structural and transport properties that likely impact the IM urine concentrating mechanism. These studies have shown that 1) IM descending thin limbs (DTLs) have at least two or three functionally distinct subsegments; 2) most ascending thin limbs (ATLs) and about half the ascending vasa recta (AVR) are arranged among clusters of collecting ducts (CDs), which form the organizing motif through the first 3-3.5 mm of the IM, whereas other ATLs and AVR, along with aquaporin-1-positive DTLs and urea transporter B-positive descending vasa recta (DVR), are external to the CD clusters; 3) ATLs, AVR, CDs, and interstitial cells delimit interstitial microdomains within the CD clusters; and 4) many of the longest loops of Henle form bends that include subsegments that run transversely along CDs that lie in the terminal 500 microm of the papilla tip. Based on a more comprehensive understanding of three-dimensional IM architecture, we distinguish two distinct countercurrent systems in the first 3-3.5 mm of the IM (an intra-CD cluster system and an inter-CD cluster system) and a third countercurrent system in the final 1.5-2 mm. Spatial arrangements of loop of Henle subsegments and multiple countercurrent systems throughout four distinct axial IM zones, as well as our initial mathematical model, are consistent with a solute-separation, solute-mixing mechanism for concentrating urine in the IM.

Kidney structure and function of obligate and facultative hibernators: the white-tailed prairie dog (Cynomys leucurus) and the black-tailed prairie dog (Cynomys ludovicianus)

The white-tailed prairie dog is an obligate hibernator that enters a heterothermic phase when maintained in the cold with low intensity light and ad libitum food and water. The black-tailed prairie dog (a facultative hibernator) will not hibernate under similar conditions. It has been suggested that the black tailed prairie dog remains active during the winter because it can conserve water more effectively due to a more efficient kidney. The present study revealed no significant differences between the species in renal morphology: relative medullary thickness, nephron heterogeneity, renal vasculature, or fornix dimensions, all of which are structures associated with the urinary concentrating mechanism. In addition, there was no difference in number of nephrons between the two species. The black-tailed prairie dog does produce a more concentrated urine when food and water deprived. However, this difference was not observed when the animals were salt loaded. The water-deprivation and salt-loading experiments suggest that the higher urine osmolality produced by the back-tailed prairie dog during fasting is a result of a higher urea load due to a greater protein catabolism and not because of a differential capacity to concentrate urine.

On the solution of equations for renal counterflow models

The results of a comparative study of three discretization techniques and the solution of the resulting algebraic equations by three methods is given. For this study, a four-tube central core model with diffusion in the core was selected and equations were derived for a coherent and efficient implementation. The results of this study show that sparse matrix techniques that take the physiological connectivity of the kidney lead to significant savings in computer storage, running time and overall cost.

Urea and renal concentrating ability in the rabbit

The hypotheses of passive salt accumulation predict an enhancement of renal concentrating ability by urea. We tested this prediction in rabbits, a species whose nephons when studied in vitro show tansport properties that support these hypotheses. We used calm, unanesthetized, hydropenic, vasopressin-treated rabbits with intact kidneys fed a 16% protein diet, and we observed the effect of urea administration at two rates of solute excretion (60 and 190 microOsm/min . kg body wt; N = 10 and 5, respectively). After an i.v. mannitol infusion, when urea was infused, the i.v. solute excretion rate was unchanged, the changes in urine urea concentration were large (a change of 767 and 408 mumoles/ml), but only small and variable changes in urine osmolality occured (a change of 78 +/- 146, and 36 +/- 50 microOsm/g H20). In additional experiments, we removed the kidneys from antidiuretic, or urea- or mannitol-infused rabbits and measured the intrarenal distribution of sodium, potassium, urea, and chloride. When the urine urea level was greater than 400 mmoles, the urine-to-papilla ratios for urea were 1.6 to 3.6. This suggested that a low collecting duct permeability to urea could explain the absence of a marked enhancement of concentrating ability during urea administration. Further analysis, based on a model of inner medullary solute compartments, indicated that sodium chloride was the major (86%) osmotically active solute in the medullary central core of these rabbits and that it was not influenced by changes in urinary urea concentration. The results of tissue analysis were consonant with either active or passive sodium chloride reabsorption from the thin ascending limb of Henle's loop in these rabbits.

The ultrastructural localization of membrane ATPase in rat thin limbs of the loop of Henle

The cytochemical distribution of nonspecific membrane ATPase activity in the epithelial membranes of the thin limbs of the loops of Henle of rat nephrons was studied at the ultrastructural level. Membrane ATPase activity was localized in the luminal, lateral, and (to a lesser extent) basal membranes of only the outer medullary segment of the thin descending limbs of long nephrons (Type II epithelium). The reaction product was lacking in the thin limb of short nephrons (Type I epithelium) as well as in the inner medullary descending (Type III epithelium) and ascending (Type IV epithelium) segments of the thin limbs of long nephrons. These data reinforce the concept of thin limb heterogeneity and may indicate a specialized role for the outer medullary segment of thin descending limbs of long nephrons in the concentrating mechanism.

[Model of the function of the nephron]

After a short review of the functional anatomy of the kidney, we express the usual hypotheses about the phenomena of reabsorption and filtration in the loop of Henle and the collecting duct. Starting from these hypotheses and the laws of biophysics, we formulate the equations of the model. This model accounts for the great increase in concentration of electrolyte and urea in the loop of Henle, the collecting duct and the interstitium, as we "go down" from the outer medullary area towards the inner areas, the active transportation of sodium in the loop of Henle being limited to the thick ascending limb.

The syndromes of primary hormone resistance

The clinical features, genetics, pathophysiology, and management of endocrine diseases in which primary hormone resistance is the fundamental defect have been reviewed. Primary hormone resistance has been documented for nearly all hormones--vasopressin, parathyroid hormone, growth hormone, adrenocroticotropin, thyrotropin, gonadotropins, insulin, androgens, cortisol, aldosterone, progesterone, thyroid hormones, and vitamin D. A striking exception is estradiol, a steroid that may be vital for early embryonic development. Most of the hormone unresponsiveness syndromes represent only partial defects, and it is likely that most such patients go unrecognized. Therefore, hormone resistance should be suspected not only when a patient presents with hypofunction of particular endocrine system combined with high endogenous hormone levels but also whenever apparently normal function of an endocrine system is associated with inappropriately elevated levels of the corresponding hormone. The value of these defects in hormone responsiveness as a natural laboratory for the study of the normal mechanisms of hormone action is discussed.

A functional model of the rat kidney

A mathematical model of the nephron was developed by writing a set of material balance equations for the flow of urea, salt and water along the foregoing study and are taken here as a basis, in particular the model configuration of the collecting duct system. The stimulation of the model equatentration profiles which at the ends of the several tubular sections were consistent with the values observed in experimental investigations.e medullary interstitial solute concentration profiles are taken to increase linearly in outer and inner zone. The several transeptithelial fluxes are driven by diffusion, osmosis, solvent drag and active transport. The development of osmotic gradient in the inner medulla is taken here to be caused by active secretion of salt into the descending LImb of Henle's loop. The parameters in the flux equations for all parts of the nephron and the concentration values at the end of each tubular section are determined by collecting and averaging the values given in literature and by extrapolating the measurement data. The simulation of the model equations with these averaged parameters resulted in concentration profiles which at the ends of the several tubular sections were consistent with the values observed in experimental investigations.

Concentrating engines and the kidney. III. Canonical mass balance equation for multinephron models of the renal medulla

The canonical mass balance relation derived for the central core model of the renal medulla is extended to medullary models in which an arbitrary assemblage of renal tubules and vascular capillaries exchange with each other both directly and via the medullary interstitium and in which not all of the vascular loops or loops of Henle extend to the papilla. It is shown that if descending limbs of Henle and descending vasa recta enter the medulla at approximately plasma osmolality, the concentration ratio is given by: r = 1/[1 - ft(1 - fu)(1 - fw)], where ft is fractional solute transport out of ascending Henle's limb, fu is fractional urine flow, and fw is fractional dissipation; fw is a measure of the solute returned to the systemic circulation without its isotonic complement of water. A modified equation that applies to the diluting as well as the concentrating kidney is also derived. By allowing concentrations in interstitium and vascular capillaries to become identical at a given medullary level, conservation relations are derived for a multinephron central core model of the renal medulla.

A 'bootstrap' model of the renal medulla

The ‘bootstrap’ model of the handling of salt, water and urea by the renal medulla is based on the concepts and data of Stephenson (1972), and Kokko and Rector (1972), and adds support to them. Revision of the traditional counter current hypothesis is necessary since active transport of salt and urea is absent from the inner medulla. This advanced model is structured appropriately and demonstrates how events in the medulla interact and vary with time as change from a diuretic to an antidiuretic state proceeds.

[Electrolyte- and urea concentration profiles of the kidney in experimental asymmetric glomerulonephritis (author's transl)]

Bilateral studies on kidney function concerning the electrolyte and urea concentration-profiles in renal medulla were performed in experimental unilateral acute glomerulonephritis in rabbits.

RESULTS

1. Both in the intact and in the glomerulonephritic kidneys sodium is concentrated mainly in the outer medulla and urea in the inner medulla. 2. In the glomerulonephritic kidneys the reduction of the filtered load seems to impair the regions of mainly passive concentrating mechanisms more than those of mainly active transport mechanisms. 3. Potassium gradients are not involved, the tissue concentrations of the glomerulonephritic kidneys however are diminished by 20 %. 4. According to the decreasing interstital electrolyte and urea gradients the osmolarity of the collecting ducts of the glomerulonephritic kidneys are diminished. These results support the conclusion, that the urinary concentrating defect in Masuginephritis is due mainly to the unability to concentrate urea.

The ultrastructure of the thin loop limbs of the mouse kidney

The thin limbs of the loops of Henle in the mouse kidney have been investigated by conventional electron microscopy. Resulting from light microscopic investigations, a distinction in the epithelia of short and long loops can be demonstrated. Ultrastructurally, the thin limbs (descending) of short loops are composed of a uniformly thin and simple epithelium. In contrast, long loops (thin descending and ascending) are composed of three different epithelial types which are representative of a distinctly more complex epithelial system. Two epithelial types were observed in the thin descending limbs of long loops and the third type was observed in the ascending thin limbs. Based upon these findings it is suggested that the thin descending limbs of short and long loops of Henle in the mouse kidney cannot perform the same functions in the renal concentrating mechanism.

Urea handling by the renal countercurrent system: insights from computer simulation

The renal handling of urea has been investigated with the aid of a computer model of the countercurrent system in which active electrolyte reabsorption occurs along the entire ascending limb of Henle's loop. In this model, summarized in Fig.9, the buildup of a corticopapillary gradient for urea is optimized if there is net addition of urea to loops of Henle only in the outer medulla. This added urea remains within the tubular system until it is reabsorbed from collecting ducts in the inner medulla. Thus, a net transfer of urea from outer to inner medulla is accomplished (via distal tubule and cortical collecting duct). There is no net addition of urea to loops of Henle within the inner medulla; in this region, the loops act simply as countercurrent exchangers for urea. Computer simulation of systematic variation in the urea permeabilities of each nephron segment shows that interference with any element of the above schema results in impairment of the medullary accumulation of urea relative to plasma. Simulation of varying rates of urinary urea excretion demonstrates that this model can account for the ability of the kidney to excrete substantial amounts of urea without an accompanying osmotic loss of water. The major insight gained from this study is that net addition of urea to loops of Henle in the outer medulla greatly enhances the medullary accumulation of urea, whereas, net addition of urea to loops within the inner medulla tends to defeat such accumulation and hence the urinary concentrating process. This general principle applies also to an alternate model of the countercurrent system, in which electrolyte reabsorption from thin ascending limbs of Henle is passive.

Cellular action of antidiuretic hormone in mice with inherited vasopressin-resistant urinary concentrating defects

Previous work has suggested that resistance to vasopressin in two strains of mice with nephrogenic deficiency of urinary concentration may entail a defect in the action of vasopressin at the cellular level. Several components involved in this action were therefore examined in vitro in renal medullary tissues from control mice (genotype VII +/+) and two genotypes with mild diabetes insipidus (DI +/+ nonsevere) and marked (DI +/+ severe) vasopressin-resistant concentrating defects. No significant differences were found in the affinity of adenylate cyclase for [8-arginine]-vasopressin (AVP), tested over a range of hormone concentration from 10(-10) to 10(-5) M. However, maximal stimulation of adenylate cyclase by saturating concentrations of AVP (intrinsic activity) was markedly decreased from control values in DI +/+ severe mice, and decreased to a lesser extent in DI +/+ nonsevere animals. A significant correlation was found between the activity of adenylate cyclase maximally stimulated by AVP in a given genotype, and the urine osmolality in the same animals. There were no significant differences in maximal stimulation of renal medullary adenylate cyclase in control experiments: not when stimulated nonspecifically by sodium fluoride, nor when stimulated by AVP in tissues from rats with induced water diuresis as compared to antidiuretic rats. Nor were there significant differences between VII +/+ and DI +/+ severe mice in the activity of renal cortical adenylate cyclase, either basal or when stimulated by parathyroid hormone. Furthermore, the abnormal genotypes did not differ significantly from control mice in the renal medullary activities of cyclic AMP phosphodiesterase or cyclic AMP-dependent protein kinase, nor in the content of microtubular subunits (assessed as colchicinebinding protein). The results are compatible with the view that impaired stimulation of renal medullary adenylate cyclase by vasopressin might be the sole or contributing cause of the vasopressin-resistant concentrating defect in the diseased mice; however, a causal relationship has not yet been proved.