Oxygen delivery in perfused rat kidney: NADH fluorescence and renal functional state

Measurement of [Ca²⁺]i in smooth muscle strips using front-surface fluorimetry

Changes in the cytosolic Ca(2+) concentrations ([Ca(2+)](i)) play a primary role in the regulation of the contraction of smooth muscle cells. However, the relationship between [Ca(2+)](i) and tension exhibits a temporal change during the time course of contraction or relaxation. The extent of the tension development for a given change in [Ca(2+)](i) also varies depending on the type of contraction and relaxation. Therefore, it is essential to measure [Ca(2+)](i) and tension simultaneously in order to determine the molecular and cellular mechanisms in both the regulation of contraction and relaxation of smooth muscle. This chapter provides the basic principles of the technique of front-surface fluorimetry as well as the protocols and tips for the simultaneous measurement of [Ca(2+)](i) and tension in the smooth muscle tissues with use of fura-2 or Fura-PE3 as fluorescent Ca(2+) indicators. The loading of sufficient amount of the Ca(2+) indicators in smooth muscles is essential for the successful measurement of [Ca(2+)](i) with minimum optical artifacts. The protocol gives our practice for the loading of the Ca(2+) indicators in various smooth muscle tissues.

In vivo multiphoton imaging of mitochondrial structure and function during acute kidney injury

Mitochondrial dysfunction has been implicated in the pathogenesis of acute kidney injury due to ischemia and toxic drugs. Methods for imaging mitochondrial function in cells using confocal microscopy are well established; more recently, it was shown that these techniques can be utilized in ex vivo kidney tissue using multiphoton microscopy. We extended this approach in vivo and found that kidney mitochondrial structure and function can be imaged in anesthetized rodents using multiphoton excitation of endogenous and exogenous fluorophores. Mitochondrial nicotinamide adenine dinucleotide increased markedly in rat kidneys in response to ischemia. Following intravenous injection, the mitochondrial membrane potential-dependent dye TMRM was taken up by proximal tubules; in response to ischemia, the membrane potential dissipated rapidly and mitochondria became shortened and fragmented in proximal tubules. In contrast, the mitochondrial membrane potential and structure were better maintained in distal tubules. Changes in mitochondrial structure, nicotinamide adenine dinucleotide, and membrane potential were found in the proximal, but not distal, tubules after gentamicin exposure. These changes were sporadic, highly variable among animals, and were preceded by changes in non-mitochondrial structures. Thus, real-time changes in mitochondrial structure and function can be imaged in rodent kidneys in vivo using multiphoton excitation of endogenous and exogenous fluorophores in response to ischemia-reperfusion injury or drug toxicity.

Methods for studying the physiology of kidney oxygenation

1. An improved understanding of the regulation of kidney oxygenation has the potential to advance preventative, diagnostic and therapeutic strategies for kidney disease. Here, we review the strengths and limitations of available and emerging methods for studying kidney oxygen status. 2. To fully characterize kidney oxygen handling, we must quantify multiple parameters, including renal oxygen delivery (DO2) and consumption (VO2), as well as oxygen tension (Po2). Ideally, these parameters should be quantified both at the whole-organ level and within specific vascular, tubular and interstitial compartments. 3. Much of our current knowledge of kidney oxygen physiology comes from established techniques that allow measurement of global kidney DO2 and VO2, or local tissue Po2. When used in tandem, these techniques can help us understand oxygen mass balance in the kidney. Po2 can be resolved to specific tissue compartments in the superficial cortex, but not deep below the kidney surface. We have limited ability to measure local kidney tissue DO2 and VO2. 4. Mathematical modelling has the potential to provide new insights into the physiology of kidney oxygenation, but is limited by the quality of the information such models are based on. 5. Various imaging techniques and other emerging technologies have the potential to allow Po2 mapping throughout the kidney and/or spatial resolution of Po2 in specific renal tissues, even in humans. All currently available methods have serious limitations, but with further refinement should provide a pathway through which data obtained from experimental animal models can be related to humans in the clinical setting.

A review of attenuation correction techniques for tissue fluorescence

Fluorescence intensity measurements have the potential to facilitate the diagnoses of many pathological conditions. However, accurate interpretation of the measurements is complicated by the distorting effects of tissue scattering and absorption. Consequently, different techniques have been developed to attempt to compensate for these effects. This paper reviews currently available correction techniques with emphasis on clinical application and consideration given to the intrinsic accuracy and limitations of each technique.

Zinc-induced cortical neuronal death: contribution of energy failure attributable to loss of NAD(+) and inhibition of glycolysis

Excessive zinc influx may contribute to neuronal death after certain insults, including transient global ischemia. In light of evidence that levels of intracellular free Zn(2+) associated with neurotoxicity may be sufficient to inhibit glyceraldehyde-3-phosphate dehydrogenase (GAPDH), experiments were performed looking for reduced glycolysis and energy failure in cultured mouse cortical neurons subjected to lethal Zn(2+) exposure. As predicted, cultures exposed for 3-22 hr to 40 mixroM Zn(2+) developed an early increase in levels of dihydroxy-acetone phosphate (DHAP) and fructose 1,6-bisphosphate (FBP) and a progressive loss of ATP levels, followed by neuronal cell death; furthermore, addition of the downstream glycolytic substrate pyruvate to the bathing medium attenuated the fall in ATP and neuronal death. However, an alternative to direct Zn(2+) inhibition of GAPDH was raised by the observation that Zn(2+) exposure also induced an early decrease in nicotinamide-adenine dinucleotide (NAD(+)) levels, an event itself capable of inhibiting GAPDH. Favoring this indirect mechanism of GAPDH inhibition, the neuroprotective effects of pyruvate addition were associated with normalization of cellular levels of NAD(+), DHAP, and FBP. Zn(2+)-induced neuronal death was also attenuated by addition of the energy substrate oxaloacetate, the activator of pyruvate dehydrogenase, dichloroacetate, or the inhibitors of NAD(+) catabolism, niacinamide or benzamide. Acetyl carnitine, alpha-keto butyrate, lactate, and beta-hydroxy-butyrate did not attenuate Zn(2+)-induced neurotoxicity, perhaps because they could not regenerate NAD(+) or be used for energy production in the presence of glucose.

Pretransplantation assessment of renal viability with NADH fluorimetry

BACKGROUND

A pathophysiologic feature possibly involved in ischemic injury in transplant kidneys is mitochondrial dysfunction caused by disintegration of oxidative metabolic pathways. Because the ability to synthesize ATP by respiratory activity determines the organ's capacity to recover from ischemic injury, an assessment of respiratory activity may provide information related to graft viability.

METHODS

NADH fluorimetry can be used to monitor kidney cortex metabolism noninvasively. During perfusion with (an)-aerobic perfusate, NADH fluorescence images were recorded. We evaluated the NADH oxidation kinetics of 20 rat kidneys, which were divided over four experimental groups. For six minimally damaged kidneys and six kidneys that had been stored for one hour at 37 degrees C, perfusion was followed by transplantation. We related the kinetic parameters of these kidneys with their post-transplantation function and histology. The transplant function was monitored by serum creatinine and urea levels.

RESULTS

Storage of transplant kidneys for one hour at 37 degrees C significantly reduced the post-transplantation function. Isolated perfusion of grafts, however, was not detrimental for renal function. The rate of NADH oxidation decreased with decreasing graft quality, and a good correlation between NADH oxidation kinetics and post-transplantation function was found.

CONCLUSIONS

A reduction of NADH oxidation rates as a consequence of warm ischemia supports the view that mitochondrial respiratory activity is impaired by ischemic injury. The correlation between NADH oxidation kinetics in perfused grafts and their post-transplantation function indicates that NADH fluorimetry may be useful in predicting the viability of preserved grafts prior to transplantation.

Functional imaging of mitochondria in saponin-permeabilized mice muscle fibers

Confocal laser-scanning and digital fluorescence imaging microscopy were used to quantify the mitochondrial autofluorescence changes of NAD(P)H and flavoproteins in unfixed saponin-permeabilized myofibers from mice quadriceps muscle tissue. Addition of mitochondrial substrates, ADP, or cyanide led to redox state changes of the mitochondrial NAD system. These changes were detected by ratio imaging of the autofluorescence intensities of fluorescent flavoproteins and NAD(P)H, showing inverse fluorescence behavior. The flavoprotein signal was colocalized with the potentiometric mitochondria-specific dye dimethylaminostyryl pyridyl methyl iodide (DASPMI), or with MitoTrackerTM Green FM, a constitutive marker for mitochondria. Within individual myofibers we detected topological mitochondrial subsets with distinct flavoprotein autofluorescence levels, equally responding to induced rate changes of the oxidative phosphorylation. The flavoprotein autofluorescence levels of these subsets differed by a factor of four. This heterogeneity was substantiated by flow-cytometric analysis of flavoprotein and DASPMI fluorescence changes of individual mitochondria isolated from mice skeletal muscle. Our data provide direct evidence that mitochondria in single myofibers are distinct subsets at the level of an intrinsic fluorescent marker of the mitochondrial NAD-redox system. Under the present experimental conditions these subsets show similar functional responses.

Microscopic analysis of NADH fluorescence during aerobic and anaerobic liver preservation conditions: A noninvasive technique for assessment of hepatic metabolism

Gaseous insufflation of oxygen via the venous vascular system is thought to be an useful tool for preventing anoxic tissue injury during extended time periods of ischemic preservation and for allowing for an improved recovery of organ function after transplantation. The present study aimed at the application of a noninvasive technique for monitoring effectiveness and homogeneity of gaseous areation by using an epiillumination microscopic technique for assessment of tissue nicotinamide adenine dinucleotide (NADH) fluorescence. Rat livers were flushed with and stored in University of Wisconsin solution at 4 degrees C for 48 h (n = 20). In half of the experiments (n = 10) gaseous oxygen was applied subsequent to organ harvest. Using ultraviolet-excitation high-resolution microscopy and computer-assisted image analysis liver surfaces were scanned for NADH intensity and spatial heterogeneity at 1, 24, and 48 h preservation time. Livers simply stored without aeration served as controls (n = 10). NADH intensity data were compared with corresponding data of tissue adenosine triphosphate (ATP) concentrations determined enzymatically. NADH fluorescence already differed at 1 h preservation between the two groups with significantly lower values in the aerobically stored livers. NADH fluorescence further decreased between 1 and 24 h preservation and remained low until 48 h, whereas in the anaerobically stored livers NADH fluorescence was found to be constantly high over the entire observation period. Aerobic storage resulted in rather homogeneous tissue oxygenation with an intrahepatic variation of NADH fluorescence <20%. In parallel, oxygen persufflation appropriately restored tissue ATP content within 1 to 24 h of preservation, while the simply stored livers exhibited pronounced depletion of ATP. We demonstrate for the first time that by means of retrograde gaseous oxygenation, ischemic livers can be readily and effectively oxygenated. Our study further indicates that the noninvasive microscopic analysis of tissue NADH fluorescence may be an useful tool for estimating efficiency of strategies in organ preservation.

Measurements of tissue viability in transplantation

Near-infrared spectroscopy has primarily been used in monitoring changes in cerebral haemoglobin oxygenation and haemodynamics. However its use as a method for the assessment of tissue viability following transplantation has recently been explored experimentally in our laboratory. The ability to measure changes in oxygenation and perfusion during harvesting and following transplantation of organs or transfer of free and pedicled flaps potentially important in reconstructive surgery. We have found that near-infrared spectroscopy is extremely useful in detecting vaso-occlusive events and can accurately and reliably distinguish between arterial, venous or total occlusions. Venous congestion indicated by raised levels of deoxygenated haemoglobin with a concomitant increase in blood volume and the presence and magnitude of reactive hyperaemia are both easily recognizable features by near-infrared spectroscopy. We have shown that near-infrared spectroscopy measurements of venous congestion in kidneys (and other tissues) following prolonged storage correlate with medullary vascular congestion confirmed by angiographical and histological analysis of intrarenal perfusion. Clinically we have shown that flap perfusion can be improved by altering fluid replacement regimes and the addition of ionotropes. Cerebral near-infrared spectroscopy measurements in a liver transplant model showed statistically significant differences within minutes after the anhepatic phase in cerebral perfusion and oxygenation, between animals transplanted with ischaemically damaged livers compared to those isografted with minimally stored livers. Similarly we have found that near-infrared spectroscopy can be used as a monitor to assess the adequacy of fluid or blood replacement in haemorrhagic and hypovolaemic models. We believe that near-infrared spectroscopy provides a sensitive and reliable postoperative method for the assessment of tissue viability following the transfer of free and pedicled flaps and organs.

Non-invasive measurement of respiratory chain dysfunction following hypothermic renal storage and transplantation

Ischemia/reperfusion (IR) damage is a major cause of dysfunction in transplanted organs. The objective of the present study was to correlate in vivo measurements of respiratory chain (RC) function with histological and physiological parameters. Non-invasive in situ (surface fluorescence) measurements of mitochondrial NADH and near infrared spectroscopic measurements of cyt aa3 were made in unstored (Group 1) and 72 hour stored (1 to 2 degrees C) (Group 2) autografted rabbit kidneys. The effect of sodium pentobarbitone on NADH levels was investigated. In Group 1, there was a significant change in the redox state of cyt aa3 in all (N = 6) kidneys on reperfusion which correlated with organ viability and increased NADH oxidation and minimal edema on histological examination. In Group 2 there was no significant change in cyt aa3 compared to baseline, and this correlated with poor long term organ viability, slower NADH oxidation, and severe cortical edema. Pentobarbitone inhibition of the RC resulted in rapid and complete reduction of NAD+ in Group 1, but none or only a slight reduction in Group 2. The results demonstrate that it might be possible in future to predict organ viability and histological changes by non-invasive measurements of RC dysfunction in the clinical transplant situation.

Fiber optic surface fluorometry-reflectometry technique in the renal physiology of rats

Most current knowledge on events in the mitochondria leading to acute renal failure originates from studies in which indirect methods were used. The disadvantage of these methods is that they cannot measure the turnover rate of various metabolites, and only one result per animal can be obtained. Chance et al. /9/ developed a method using optical techniques for continuously monitoring the fluorescence of intramitochondrial NADH, which has been applied mainly to the brain. This optical method has not yet been examined quantitatively in the kidney and no attempt has been made to adapt this method for routine measurement in kidney tissue. The purpose of this study was to adapt the surface fluorometry method for monitoring renal NADH redox state in situ, and to determine whether the hemodynamic artifacts involved in fluorometric studies of the renal surface in situ could be eliminated by using a correction factor. Another purpose was to understand the relationship between the changes in reflectance and blood volume in the rat kidney. This was achieved by measuring the reflectance after: a) blood exchange by FC-43 emulsion; b) intrarenal saline flush; c) occlusion of the renal vein, renal artery and reopening of the renal vein; d) calculation of the correlation between changes in kidney weight after renal artery occlusion, and the reflectance. Our results suggest that in the rat kidney, as opposed to the brain, a correction factor of 1:1 is not always applicable. This factor may vary between animals, and it is therefore necessary to adjust it electronically for each rat kidney. This observation contradicts the view suggesting a constant correction factor of 1:1 in the kidney. The results reported herein indicate that changes in the reflectance in the ischemic rat kidney are due to changes in blood volume. In conclusion, it seems that optical techniques for monitoring fluorescence are suitable for localized, continuous and non-invasive recording of tissue mitochondrial NADH redox states under various conditions in the rat kidney.

Determinants of intrarenal oxygenation: factors in acute renal failure

Oxygen tension within the renal parenchyma is influenced by two factors: metabolic demand and oxygen supply. There are three regions within the kidney in which there is an anatomical basis for limited oxygen availability. The first is the inner stripe where oxygen diffusion between arterial and venous vasa recta reduces PO2. The other two are the outer stripe and medullary rays which are fed by O2-poor blood from venous vasa recta. The balance between oxygen demand and supply is most critical in the inner stripe where the PO2 is most influenced by transport activity. In contrast, altering transport activities in the outer stripe will not change the prevalence of hypoxic S3 injury but will alter its type (i.e., cell fragmentation related to high GFR and increased workload versus cell edema related to low GFR and minimal workload). The effect of transport activity on medullary ray PO2 has not been well defined. Using sensitive oxygen microelectrodes, cortical PO2 (52 +/- 2 mm Hg) in the rat was found to be higher than medullary PO2 (21 +/- 2 mm Hg, p less than 0.001). How are these observations reflected in current models of acute renal failure? The ischemia-reflow model affects proximal tubules with a predilection for S3 (located within the outer stripe of medulla) after short-term ischemia. With hyperfiltration (induced by glycine or renal hypertrophy) and the pursuant increase in transport related O2 demand, hypoxic mTAL inner stripe injury becomes prominent. Renal parenchymal hypertrophy exaggerates injury in the contrast nephropathy model, in which mTAL inner stripe injury is a predominant feature and medullary PO2 is very low.(ABSTRACT TRUNCATED AT 250 WORDS)

A comparison of vasopressin and noradrenaline on oxygen uptake by perfused rat hindlimb, kidney, intestine and mesenteric arcade suggests that it is in part due to contractile work by blood vessels

1. The rat hindlimb, kidney and intestine were each perfused in a nonrecirculating mode at 25 degrees C using an artificial perfusate (initial pressure 85 +/- 5 mmHg) and the effects of vasopressin and noradrenaline on oxygen uptake and perfusion pressure determined. 2. Both vasopressin (K0.5 = 0.1 nM) and noradrenaline (K0.5 = 2 nM) increased oxygen uptake as well as perfusion pressure by the perfused hindlimb; changes in oxygen uptake were closely matched by changes in pressure. The maximum increase in oxygen uptake was approx. 9 mumol/hr per g wet wt of hindlimb. 3. The perfused kidney also responded to vasopressin and noradrenaline with parallel increases in oxygen uptake and perfusion pressure for each agent. The largest increase in oxygen uptake was approx. 30 mumol/hr per g wet wt but this was not maximal. 4. Vasopressin increased oxygen uptake and pressure by the perfused intestine over the range 0.01-2 nM, but the changes in pressure only became significant at doses greater than 0.1 nM. 5. Noradrenaline inhibited oxygen uptake and increased perfusion pressure in a dose-dependent manner at pharmacological concentrations (greater than 30 nM) when shunting of perfusate may have contributed to unperfused regions. 6. A network of mesenteric blood vessels estimated to contain approx. 6% vascular tissue by weight, with the remainder white fat cells, lymphatics and connective tissue, was also perfused. 7. Vasopressin (K0.5 = 0.3 nM) and noradrenaline (K0.5 = 30 nM) each increased oxygen uptake and perfusion pressure in a dose-dependent manner.(ABSTRACT TRUNCATED AT 250 WORDS)

[Kidney medullary hypoxia: a key to understanding acute renal failure?]

The ability to produce a concentrated urine is imposed by a uniquely low ambient oxygen pressure in the renal medulla due to shunt diffusion within the vascular bundles. As the thick ascending limb of Henle's loop (TAL-segment) is able to glycolyse anaerobically, a phase of oxygen deficiency may be bridgespanned. It allows an exceptionally high oxygen extraction of 80% in this area. If oxygen capacity is reduced systematically, which can be effected in the isolated kidney model by using cell free perfusate, a typical pattern of lesions occur in TAL-segments. Segments near vascular bundles remain intact, as they take advantage from a radial oxygen diffusion originating from vascular bundles. The extent of lesions is increasing directed to the inner medulla due to the reduction of oxygen pressure, whereas lesions are not present in the inner medulla itself. Cells of TAL-segments are swelling during oxygen deficiency, when transport work surpasses the available energy necessary due to the luminal fluid inflow. Lesions could be prevented, when oxygen capacity was enhanced by adding erythrocytes or when transport was blocked by furosemide. Swollen cells in TAL-segments however are able to aggravate medullary hypoxia by an outflow block in vivo. Secondly, it can be demonstrated, that oxygen shunt diffusion is not only present in renal medulla but also within renal cortex especially as a preglomerular diffusion shunt for blood gases. Thus PCO2 has been measured to be 65 mmHg in the outermost cortical zone and thereby some 20 mmHg higher than renal venous blood.(ABSTRACT TRUNCATED AT 250 WORDS)

Mitochondrial function in normal and hypoxic states of the myocardium

The relationships among isometric tension development, the oxidation-reduction states of pyridine nucleotides and cytochrome c, and the oxygenation state of myoglobin have been assessed using the arterially perfused rabbit interventricular septum under different conditions of contraction rate, perfusate [Ca2+] and pH, catecholamine stress, and hypoxia. Hypoxia was produced either by decreasing oxygen availability with maintained flow (high-flow hypoxia) or by decreasing the flow rate (ischemia). Under normoxic conditions, increased work caused a fall of the cytosolic adenine nucleotide phosphorylation potential, delta G(ATP)c, an oxidation of the pyridine nucleotides, and a reduction of cytochrome c; the opposite occurred with decreased work. Thus, the redox potential span from NADH to cytochrome c, delta Eh, varied with the energy demand such that delta Eh and delta G(ATP)c changed in the same direction. Under hypoxic conditions, all respiratory components became more reduced, and myoglobin was partially deoxygenated. The percentage change of developed tension under hypoxic conditions was approximately proportional to the percentage change of oxidized cytochrome c. When high-flow hypoxia and ischemia were compared at the same rates of oxygen delivery, the developed tension at any level of cytochrome c reduction was always lower with ischemia than with high-flow hypoxia. This difference was attributed to the low intracellular pH of ischemic tissue. Myoglobin deoxygenation was linearly related to cytochrome c reduction under all conditions of hypoxia, indicating steep oxygen gradients. The results support the concept of heterogeneous oxygenation of the tissue with mixed populations of aerobic and anaerobic mitochondria in the hypoxic state. In the full aerobic state, the control of mitochondrial respiration in situ appears similar to that of isolated mitochondria.

Intra- and transepithelial analytical techniques

Epithelia transport a variety of solutes and water. Study of such transport requires a determination of the driving forces responsible for transport, of the pathways through which transport occurs, and of the factors controlling such transport. Transepithelial driving forces are readily determined where the composition of the bathing media can be altered and electrical forces negated. Where substances move only through a paracellular pathway such manipulations may be adequate to define the permeability and selectivity of the pathways. For substances utilizing a cellular pathway, driving forces and permeabilities across the two dissimilar apical and basolateral cellular membranes must be determined. Where a substance can be shown to move across a membrane against its electrochemical potential gradient, the source of the energy for such movement must be assessed. This review focuses on the applicability and validity of a variety of techniques utilized for the study of epithelial transport to answer these questions. These include microelectrode techniques, chemical analyses, microprobe analysis, microscopy, and techniques for assessing the coupling of metabolism to transport.

Coupling of aerobic metabolism to active ion transport in the kidney

1. The redox state of mitochondrial NAD was monitored fluorometrically as a function of active ion transport work in the isolated doubly perfused bullfrog kidney. 2. Initial experiments to measure the O2 consumption (QO2) of small pieces from the bullfrog kidney gave a basal QO2 - 3.0 (+/- 0.43) nmoles O2/mg dry wt. min. Addition of 50 microM-ouabain inhibited QO2 by 72.7%. Subsequent addition of the mitochondrial uncoupler 1799 stimulated QO2 by 226%, while cyanide totally inhibited respiration. 3. Ion transport functional parameters and NADH fluorescence were simultaneously monitored during systematic reductions in perfusate PO2 to test the sufficiency of O2 delivery to the isolated perfused frog kidney. No significant changes in transport functions or fluorescence were observed until the PO2 dropped to 184 mm Hg or below. O2 tensions of 184 mm Hg or below caused decreases in G.F.R. and transport functions which were accompanied by an increase in NADH fluorescence. The lack of changes in kidney function in the PO2 range 550-340 mmHg suggested that the tissue is adequately oxygenated at the normal perfusate PO2 of 550 mmHg. 4. The relationship between active transport rate and NAD redox levels was studied by increasing transport work (via increased G.F.R. or ADH) or by decreasing transport work (via decreased G.F.R. or ouabain) while simultaneously monitoring the NAD redox state of the intact tissue fluorometrically. In all cases, an increase in work caused a net oxidation of NAD while a decrease in work caused a reduction of NAD. 5. It is concluded that the NADH fluorescence responses are indicative of mitochondrial active to passive transitions in response to changes in active transport work. The aerobic production of ATP and the normally functioning Na-K-ATPase appear to be essential to maintain active transport and to elicit the appropriate state transitions. Thus, ATP (and, possibly, ADP and Pi) may be part of the coupling mechanism linking active ion transport and aerobic metabolic rate in the kidney.

Coupling of active ion transport and aerobic respiratory rate in isolated renal tubules

We report the results of studies in which the cytoplasmic coupling between Na+,K+-ATPase activity (presumably a measure of active transport) and the mitochondrial respiratory rate was investigated in a tubule suspension from the rabbit kidney cortex. Simultaneous measurements of the redox state of mitochondrial nicotinamide adenine dinucleotide (NAD) (performed fluorometrically), the cellular ATP and ADP concentrations, and the oxygen consumption rate (QO2) were made under conditions known to alter the Na+,K+-ATPase turnover. Ouabain (25 microM) caused: (i) a 54% inhibition of QO2, (ii) a net reduction of NAD, and (iii) a 30% increase in the ATP/ADP ratio. The addition of K+ (5 ?M) to K+-depleted tubules caused: (i) an initial 127% stimulation of QO2 followed by a new steady-state QO2 50% above control, (ii) an initial large oxidation of NAD followed by a new steady state more oxidized than the control level, and (iii) a 47% decrease in the cellular ATP/ADP ratio. These data indicate that the cellular ATP and ADP concentrations or the ATP/ADP ratio may be part of the coupling mechanism linking Na+,K+-ATPase turnover and the aerobic metabolic rate in kidney.