Energy metabolism of the renal medulla

Utilizing pathophysiological concepts of ischemia-reperfusion injury to design renoprotective strategies and therapeutic interventions for normothermic ex vivo kidney perfusion

Normothermic machine perfusion (NMP) has emerged as a promising tool for the preservation, viability assessment, and repair of deceased-donor kidneys prior to transplantation. These kidneys inevitably experience a period of ischemia during donation, which leads to ischemia-reperfusion injury when NMP is subsequently commenced. Ischemia-reperfusion injury has a major impact on the renal vasculature, metabolism, oxygenation, electrolyte balance, and acid-base homeostasis. With an increased understanding of the underlying pathophysiological mechanisms, renoprotective strategies and therapeutic interventions can be devised to minimize additional injury during normothermic reperfusion, ensure the safe implementation of NMP, and improve kidney quality. This review discusses the pathophysiological alterations in the vasculature, metabolism, oxygenation, electrolyte balance, and acid-base homeostasis of deceased-donor kidneys and delineates renoprotective strategies and therapeutic interventions to mitigate renal injury and improve kidney quality during NMP.

Inner medullary lactate production and urine-concentrating mechanism: a flat medullary model

We used a mathematical model to explore the possibility that metabolic production of net osmoles in the renal inner medulla (IM) may participate in the urine-concentrating mechanism. Anaerobic glycolysis (AG) is an important source of energy for cells of the IM, because this region of the kidney is hypoxic. AG is also a source of net osmoles, because it splits each glucose into two lactate molecules, which are not metabolized within the IM. Furthermore, these sugars exert their full osmotic effect across the epithelia of the thin descending limb of Henle's loop and the collecting duct, so they are apt to fulfill the external osmole role previously attributed to interstitial urea (whose role is compromised by the high urea permeability of long descending limbs). The present simulations show that physiological levels of IM glycolytic lactate production could suffice to significantly amplify the IM accumulation of NaCl. The model predicts that for this to be effective, IM lactate recycling must be efficient, which requires high lactate permeability of descending vasa recta and reduced IM blood flow during antidiuresis, two conditions that are probably fulfilled under normal circumstances. The simulations also suggest that the resulting IM osmotic gradient is virtually insensitive to the urea permeability of long descending limbs, thus lifting a longstanding paradox, and that this high urea permeability may serve for independent regulation of urea balance.

Inner medullary lactate production and accumulation: a vasa recta model

Since anaerobic glycolysis yields two lactates for each glucose consumed and since it is reported to be a major source of ATP for inner medullary (IM) cell maintenance, it is a likely source of "external" IM osmoles. It has long been known that such an osmole source could theoretically contribute to the "single-effect" of the urine concentrating mechanism, but there was previously no suggestion of a plausible source. I used numerical simulation to estimate axial gradients of lactate and glucose that might be accumulated by countercurrent recycling in IM vasa recta (IMVR). Based on measurements in other tissues, anaerobic glycolysis (assumed to be independent of diuretic state) was estimated to consume approximately 20% of the glucose delivered to the IM. IM tissue mass and axial distribution of loops and vasa recta were according to reported values for rat and other rodents. Lactate (P(LAC)) and glucose (P(GLU)) permeabilities were varied over a range of plausible values. The model results suggest that P(LAC) of 100 x 10(-5) cm/s (similar to measured permeabilities for other small solutes) is sufficiently high to ensure efficient lactate recycling. By contrast, it was necessary in the model to reduce P(GLU) to a small fraction of this value (1/25th) to avoid papillary glucose depletion by countercurrent shunting. The results predict that IM lactate production could suffice to build a significant steady-state axial lactate gradient in the IM interstitium. Other modeling studies (Jen JF and Stephenson JL. Bull Math Biol 56: 491-514, 1994; and Thomas SR and Wexler AS. Am J Physiol Renal Fluid Electrolyte Physiol 269: F159-F171, 1995) have shown that 20-100 mosmol/kgH(2)O of unspecified external, interstitial, osmolytes could greatly improve IM concentrating ability. The present study gives several plausible scenarios consistent with accumulation of metabolically produced lactate osmoles, although only to the lower end of this range. For example, if 20% of entering glucose is consumed, the model predicts that papillary lactate would attain about 15 mM assuming vasa recta outflow is increased 30% by fluid absorbed from the nephrons and collecting ducts and that this lactate gradient would double if IM blood flow were reduced by one-half, as may occur in antidiuresis. Several experimental tests of the hypothesis are indicated.

Renal oxygen consumption, thermogenesis, and amino acid utilization during i.v. infusion of amino acids in man

The renal contribution to the amino acid-induced whole body thermogenesis was examined. Using indirect calorimetry and catheter techniques, pulmonary and renal oxygen uptake and blood flow, blood temperatures, and net renal exchange of amino acids, glucose and lactate were measured in eight healthy men before and during 3 h of intravenous infusion of 720 kJ of an amino acid solution. During the infusion, the pulmonary oxygen uptake increased from 252 +/- 12 to 310 +/- 8 ml/min, cardiac output increased from 5.9 +/- 0.3 to 6.8 +/- 0.3 l/min, and the arterial blood temperature increased from 36.34 +/- 0.04 to 36.68 +/- 0.07 degrees C. Renal oxygen consumption, heat production, blood flow, and net glucose exchange remained unchanged during the infusion. The net renal uptake of amino acid energy from the blood rose from 2 +/- 2 to 11 +/- 4 W. The total renal energy expenditure was 9-10 W throughout the study period. It is concluded that intravenous amino acid infusion greatly augments the uptake and utilization of amino acids in the kidneys but does not stimulate the renal oxygen consumption, heat production, blood flow, or glucose release.

Chemically induced renal papillary necrosis and upper urothelial carcinoma. Part 1

In the past, renal papillary necrosis (RPN) has been commonly associated with long-term abusive analgesic intake, but over recent years a wide variety of industrially and therapeutically used chemicals have been shown to induce this lesion experimentally or in man. Destruction of the renal papilla may result in: (1) secondary degenerative cortical changes which precede chronic renal failure or (2) a rapidly metastasizing upper urothelial carcinoma, which has a very poor prognosis. This article will briefly review the published data on the morphology, function, and biochemistry of the normal renal medulla and the pathology associated with RPN, together with the secondary changes which give rise to cortical degeneration or epithelial carcinoma. It will then examine in detail those chemicals which have been reported to cause RPN in an attempt to delineate structure-activity relationships. Finally, the many different theories that have been proposed to explain the pathophysiology of RPN will be examined and an hypothesis will be put forward to explain the primary pathogenesis of the lesion and its secondary consequences.

Chemically induced renal papillary necrosis and upper urothelial carcinoma. Part 2

In the past, renal papillary necrosis (RPN) has been commonly associated with long-term abusive analgesic intake, but over recent years a wide variety of industrially and therapeutically used chemicals have been shown to induce this lesion experimentally or in man. Destruction of the renal papilla may result in: (1) secondary degenerative cortical changes which precede chronic renal failure or (2) a rapidly metastasizing upper urothelial carcinoma, which has a very poor prognosis. This article will briefly review the published data on the morphology, function, and biochemistry of the normal renal medulla and the pathology associated with RPN, together with the secondary changes which give rise to cortical degeneration or epithelial carcinoma. It will then examine in detail those chemicals which have been reported to cause RPN in an attempt to delineate structure-activity relationships. Finally, the many different theories that have been proposed to explain the pathophysiology of RPN will be examined and an hypothesis will be put forward to explain the primary pathogenesis of the lesion and its secondary consequences.

In vitro and in vivo lipogenesis of the rat renal papillae from glucose

The rate of [14C]glucose incorportion into rat renal papillary glycerolipids has been studied. Glucose is incorported mainly into the glycerol moiety of these lipids. 74 and 93% of the activity incorporated into triacylglycerols in vitro and in vivo, respectively, was present in the glycerol backbone. In phospholipids it was 82 and 96%, respectively. The fatty acid synthesis from [14C]glucose was similar to that from [14C]acetate. In accordance with previous experiments with [14C]acetate the papillary tissue was found to have a high capacity for chain elongation of preexisting fatty acids with the major product being cis-7,10,13,16-docosatetraenoic acid (chain elongated arachidonic acid). The in vitro and in vivo turnover rates of papillary glycerolipids can be calculated. A half-life of 25 h can be estimated for membrane phospholipids and less than 11 h for lipid droplet triacylglycerols. These appreciably high rates of turnover are discussed in relation to membrane turnover in renal papillae from rats in different physiological states and to variation in number of lipid droplets of papillary interstitial cells reported by others.

The influence of urea on lipogenesis in renal papillae of rats

The osmolality has been determined for the papillary interstitial fluids obtained from the isolated papillae of rats in different states of water balance. Hypertonic buffers for in vitro experiments were prepared by addition of urea to regular Krebs-Ringer phosphate buffers (340 mosmol/kg H2O) since urea is the most changeable solute of the renal papillary tissue in response to external influences. The effect of such hypertonic buffers on papillary lipogenesis was very pronounced. The fatty acid synthesis from acetate decreased by a factorr of 7-9 in response to increased osmolality from 340 to 1370 mosmol/kg H2O. In the physiological range of osmolalities (500-1800 mosmol/kg H2O), the de novo synthesis of papillary glycerolipids from glucose decreased by a factor of ca. 5. A possible specific inhibitory effect of hypertonic buffers on the pentose phosphate cycle was studied with a negative result. It is concluded that the addition of urea causes a decrease of total energy metabolism in the tissue.

Effect of acidosis, alkalosis and monofluoroacetate administration on citrate and ATP content of rat renal medulla and papilla

1. --Renal distribution of citrate showed that there is an increase in citrate content from cortex to medulla and a decrease from medulla to papilla. Alkalosis produced an increase in citrate content and acidosis a decrease in renal citrate content, in each of the studied renal area. Monofluoroacetate produced no significant change in citrate content of medulla or papilla; it did not interfere with the acido-basic related changes in cortex citrate content, but its effect was additive. 2. --Renal distribution of ATP significantly decreased from cortex to medulla and from medulla to papilla. Acid or basic diet had no influence on intratissular ATP content. Fluoroacetate decreased renal ATP content.

Localization of pyruvate kinase isozymes in bovine kidney and comparison of these patterns with those of lactate dehydrogenases and aldolases

Electrophoretic and immunofluorescence analysis were used to study the distribution of pyruvate kinase isozymes in the bovine kidney. Electrophoretic analysis demonstrated the presence of large amounts of K4 plus small amounts of K-M hybrids in cortical, medullary, and papillary sections cut from the kidney. Nearly all of the K-L hybrids seen in whole kidney extracts were found in cortical sections. Immunofluorescence of frozen sections revealed the presence of type L subunits in the tubules but the complete absence of this subunit type in flomeruli. Glomeruli do contain large quantities of pyruvate kinase isozymes, probably K4 and K-M hybrids, that cross-react with antibodies produced against type M pyruvate kinase. Type L-containing forms of pyruvate kinase and aldolase type B both appear to be found in cell types thought to be capable of catalyzing of gluconeogenesis, while type K pyruvate kinase and type A aldolase are found in predominantly glycolytic cell types of the kidney. Lactate dehydrogenase isozymic patterns appear to be less closely correlated with glycolytic versus gluconeogenic functions of the kidney but may be determined more directly by other metabolic functions.

Substrate metabolism and renal function

Glucose metabolism is related to two major renal functions. Glycolysis may be related to an energy requiring function--fine adjustment of renal sodium absorption. Gluconeogenesis may be related to controlling intracellular pH and salvation of energy otherwise wasted in acidosis. Ammoniagenesis is related to gluconeogenesis in the metabolic acidosis, but the individual phenomena are not necessarily dependent on each other.

Effects of acute anoxia on renal transport processes

Acute disruption of the renal blood supply has been shown by several workers to alter renal cortical metabolic functions and some transport processes. The present study was designed to examine acute ischemic effects on transport functions for both organic and inorganic substances. Acute clamping of the renal artery, renal vein, and ureter for 45 min produces a reversible disruption of tissue electrolyte and water balance. Longer occlusion appeared to produce irreversible effects. Alpha-Aminoisobutyrate (AIB) and lactate-stimulated rho-aminohippurate (PAH) transport were altered selectively by the 45-min occlusion. A longer occlusion period also depressed base-line PAH and tetraethylammonium (TEA) transport. Some of the depression of the organic compound transport functions are reversible. Renal cortical oxygen consumption measured in vitro was affected minimally.

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

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

Volume adjustment by renal medullary cells in hypo- and hyperosmolal solutions containing permeant and impermeant solutes

1. The changes in the volumes of cells in slices (thickness 0-3-0-4 mm) of rat renal outer and inner medulla have been investigated during aerobic incubation for 20 min at 37 degrees C in Krebs phosphate-bicarbonate Ringer modified by the addition of urea or sucrose in order to produce a range of media hypo- and hyperosmolal with respect to the calculated tissue fluid osmolalities in these regions. 2. On the assumption that under these conditions the measured inulin space approximates to the true extracellular space (ECS), it was found that osmotic swelling or shrinkage of cells was not accompanied by any significant variation in the absolute size of the ECS. 3. Calculated cell volume changes in both regions were minimal when slices were incubated in urea-containing media iso-osmolal with tissue fluids in that region. In sucrose-containing media minimal cell volume changes occurred when media were hypo-osmolal in relation to tissue fluids by a factor of approximately 0-68. 4. In all except the most hypo-osmolal media studied, calculated cell volume changes (as percentage of initial volume) were linearly related to the reciprocal of the incubation media osmolalities. The points of interception of the regression lines on the cell volume axis were dependent upon both the region studied and the composition of the incubation medium (urea or sucrose). 5. These changes were accompanied by variations in slice solute concentrations. Slice [Na] was greatest, and slice [K] least, following incubation in those media producing the greatest percentage changes in cell volume. 6. The volume of distribution [14-C]sucrose within the inner medulla was 61-7 plus or minus 2-5 mul./100 mg wet weight of tissue (mean plus or minus S.E., n equals 6) after 10 min incubation. The increase to 70-8 plus or minus 4-2 mul./100 mg (n equals 6) after 100 min was not significant (0-1 greater than P greater than 0-05). The volume of distribution within the outer medulla rose markedly during this period, from 38-1 to 58-2 mul./100 mg.

Effects of glucose on bicarbonate reabsorption in the dog kidney

The effects of glucose on renal bicarbonate reabsorption were investigated in the dog. The infusion of small amounts of glucose calculated to slightly exceed the renal threshold for glucose absorption increased bicarbonate reabsorption in bicarbonate loaded dogs. Galactose in similar doses also increased the reabsorption of filtered bicarbonate. This effect is not due to insulin secretion since insulin alone did not alter bicarbonate reabsorption and the infusion of glucose into alloxan-diabetic dogs given a steady infusion of insulin also enhanced bicarbonate reabsorption. It is more likely that the increased tubular reabsorption of glucose, secondary to an increased filtered load, resulted in the increase in bicarbonate reabsorption since phlorizin reversibly inhibits the effect of glucose.

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.

Uphill transport of urea in the dog kidney: effects of certain inhibitors

To study the renal medullary transport and accumulation of urea in dogs independent of water transport, we obliterated the medullary electrolyte gradient by a sustained ethacrynic acid diuresis. Infusions of urea were also given at various rates to vary urinary urea concentration. In the steady state, the kidneys were removed, and slices were analyzed for water, urea, and electrolytes. In every experiment in 15 dogs over a range of urinary urea concentration from 19 to 230 mmoles per L and urine flow from 0.5 to 9.7 ml per minute per kidney, an intrarenal urea gradient persisted, and urinary urea concentration was always lower than papillary water urea concentration. The magnitude of this uphill urinary-papillary gradient (mean +/- SE = - 21 +/- 2.9 mmoles per L) was not affected by hemorrhagic hypotension or a nonprotein diet. In 12 additional experiments begun similarly, inhibitors were infused into one renal artery. Both iodoacetate, an inhibitor of anaerobic glycolysis, and acetamide, an analogue of urea, markedly and significantly reduced both the intrarenal urea gradient and the uphill urinary-papillary gradient. In contrast, cyanide, an inhibitor of oxidative metabolism, had no observable effect on the urea gradients. The data are best explained by postulating an active transport system for urea in the medullary collecting duct deriving its energy from anaerobic glycolysis.

Isoenzymes in clinical diagnosis

The recent literature attests to the increasing number of enzymes for which isoenzymes or subunit structure or both are known. Several symposia and reviews have been published within the past few years. 46-50 Some enzyme activities for which isoenzymes have been described are listed in table 4. Only those found in mammals are included. They are ranked in rough order of apparent usefulness in clinical diagnosis, with the enzymes displaying tissue localization and catalytic differences listed first. Also, the number of molecular forms and the presence or absence of known subunit structure is indicated.

Clinical studies have been made using several of the isoenzymes listed in table 4, and references denoting these studies are marked with an asterisk. Data presented in the table indicate that valuable clinical information might derive from study of other isoenzymes, notably phosphofructokinase, for which tissue differences and adaptable catalytic differences exist. The guidelines set forth in this review can be used for investigating still other isoenzymes, such as those studied in lower forms. 79-85 In brief, the study of isoenzymes, like the study of enzymes and other organ-specific chemicals, presents an opportunity for great specificity and accuracy in localizing and following disease processes.

Accumulation of Potassium Anaerobically by Renal Medullary Slices

Potassium accumulation occurred in leached slices of the inner medulla of the dog kidney incubated under anaerobic conditions at 37 degrees C with glucose as substrate, but was blocked by inhibitors of glycolysis. Aerobically, only slight accumulation occurred with succinate as substrate at 37 degrees C, and this was blocked by dinitrophenol. These findings were in contrast to those obtained with kidney cortex slices under the same conditions.