Monitoring the effects of mobile phone use on the brain by proton magnetic resonance spectroscopy

PURPOSE

To determine whether extensive use of mobile phones affects brain metabolites detectable by proton magnetic resonance spectroscopy (1H MRS).

MATERIALS AND METHODS

Twenty-one extensive mobile phone users (average use = 5.5 +/- 2.2 years at 2.4 +/- 1.1 hours/day) and 15 control subjects were recruited and submitted to a 1H MRS brain examination at 1.5 Tesla. Data were recorded in the most exposed right temporal and pontobulbar areas as well as in the contralateral left temporal area. The ratios of N-acetylaspartate (NAA), choline (Cho) and myo-inositol (mI) to creatine/phosphocreatine (Cr) were measured.

RESULTS

No statistically significant changes in the NAA/Cr, Cho/Cr and mI/Cr ratios were measured between mobile phone users and control subjects and between the exposed and contralateral temporal areas.

CONCLUSION

These results indicate that extensive exposition to mobile phone radiation does not cause MRS-detectable brain metabolic changes.

Influence of post-treatment delay on the evaluation of the response to focused ultrasound surgery of breast cancer by dynamic contrast enhanced MRI

The assessment of the effectiveness of MRI-guided focused ultrasound surgery (MRIgFUS) of breast carcinomas can be performed by dynamic contrast-enhanced magnetic resonance imaging (DCE-MRI) parameters which monitor the presence of residual tumour. The aim of this study was to evaluate the effect of the post-treatment delay on this assessment. DCE-MRI data were acquired immediately and 3-14 days after MRIgFUS treatment of 26 tumours (<7 days, n = 6; = or > ge;7 days, n = 20). The percentage of residual tumour was determined histologically on the resected mass and correlated with two DCE-MRI parameters: increase in signal intensity (ISI) and positive enhancement integral (PEI). No correlation could be found between DCE-MRI data acquired immediately after treatment and the percentage of residual tumour. Good correlation coefficients were found for data acquired several days after treatment (ISI, r = 0.749; PEI, r = 0.778). However, they were higher when the post-treatment time interval was 7 days or more (ISI, r = 0.962; PEI, r = 0.934). These results suggest that a post-treatment delay of 7 days is necessary for the accurate assessment of the presence of residual tumour by DCE-MRI parameters.

Brain metabolic differences as a function of hemisphere, writing hand preference, and gender

A total of 35 university-educated normal men (24 right handwriters and 11 left handwriters) and 36 age- and education-matched women (25 right handwriters and 11 left handwriters) underwent a proton magnetic resonance spectroscopy examination in seven 8 cm(3) voxels including the right and left frontal lobe tips, the right and left mid-temporal lobes, the right and left thalami, and the hypothalamus. Dependent measures were N-acetylaspartate (NAA), choline-containing compounds (Cho) and creatine/phosphocreatine (Cr) metabolite peak area ratios relative to total H(2)O. As expected, thalamic grey matter contained higher NAA ratios than telencephalic voxels (containing white and grey matter) (p < .001). The thalamic Cr/ H(2)O ratio was higher on the right, but the opposite asymmetry was observed for the temporal lobe (p < .05). Women had a higher left frontal NAA/ H(2)O ratio than men, but men had a higher hypothalamic NAA/ H(2)O ratio than women. Right-handers had a higher temporal lobe NAA/H(2)O ratio than left-handers, particularly in the left hemisphere. In addition, several significant 2- and 3-way interactions between writing hand preference, gender, and hemisphere were observed, but only in the frontal lobe.

Long-term brain metabolic alterations in exogenous Cushing's syndrome as monitored by proton magnetic resonance spectroscopy

The effects of exogenous Cushing's syndrome on the brain metabolism were investigated by proton magnetic resonance spectroscopy (MRS). Thirteen patients having been treated for 2 to 22 years with prednisone were recruited. On the average, none of the metabolites (NAA, Cr, Cho and mI) were significantly different from those of 40 normal subjects in any of the three regions studied: frontal area, thalamus and temporal area. However, the Cho/H(2)O ratios were found to decrease significantly in the thalamic area as a function of treatment period (-1.3%/year). In the frontal and temporal areas, decreases of the Cho/H(2)O ratios were measured with treatment period but they did not reach statistical significance. Effects on Cho levels can be related to those observed for patients with endogenous Cushing's syndrome and suggest an impairment at the membrane level. The Cho/H(2)O reductions were not found to be dose- or age-dependent. Other metabolite ratios did not vary with treatment period, dose or age.

Comparison of metabolite levels and water diffusion between cortical and subcortical strokes as monitored by MRI and MRS

Labelle M, Khiat A, Durocher A, et al. Comparison of metabolite levels and water diffusion between cortical and subcortical strokes as monitored by MRI and MRS. Invest Radiol 2001;36:155-163.

RATIONALE AND OBJECTIVES

Proton magnetic resonance spectroscopy (MRS) and functional imaging techniques are increasingly recognized as useful tools for the characterization of strokes. The aim of this study was to compare cortical and subcortical (lacunar) strokes by MRS and diffusion-weighted imaging (DWI) experiments as a function of time.

METHODS

Single-voxel MRS, DWI, and perfusion-weighted imaging data were recorded on patients with cortical (n = 7) or subcortical (n = 7) strokes in the acute, subacute, and chronic periods. Magnetic resonance spectra were acquired in three regions: hyperintense DWI area, adjacent area with normal DWI intensity, and contralateral area. Neurological deficits were estimated by the National Institutes of Health Stroke Scale.

RESULTS

Decreases in N-acetylaspartate, choline-containing compounds, and creatine/phosphocreatine signal intensity as well as the presence of lactate were observed at all times in the hyperintense DWI area of all lesions. Small decreases were measured in the subacute and chronic phases for the adjacent area of cortical strokes but not for the adjacent area of subcortical strokes. The existence of a surrounding affected area in subcortical strokes is deduced from a combination of MRS and DWI results, possibly corresponding to the ischemic penumbra. Differences were found between the two types of lesion, especially an increased time variability of apparent diffusion coefficients in subcortical strokes.

CONCLUSIONS

Magnetic resonance spectroscopy provides evidence for the existence of affected tissue outside the hyperintense DWI regions in subcortical strokes. Cortical and subcortical strokes display different DWI and MRS characteristics.

Conformational analysis of endothelin-1 analogs with indolizidinone amino acids incorporated at the C-terminus

In light of the fact that a beta-turn conformation at the C-terminus of endothelin-1 (ET-1) could be responsible for activity at the ET(B) receptor, the incorporation of (3S, 6S, 9S)-2-oxo-3-amino-1-azabicyclo[4.3.0]nonane-9-carboxylic acid (IAA) should provide an elegant method to establish whether a formyl group on Trp21 plays a role in stabilizing a beta-turn. Eight linear ET-1 analogs, four formylated and four nonformylated, ET-1-(Leu17-Asp18-IAA-Trp21); ET-1-(Leu17-IAA-Ile20-Trp21); ET-1-(Leu17-Asp18-Pro19-Ile20-Trp21) and ET-1-(Leu17-Asp 8-Ile19-Pro20-Trp21) have been analyzed by high-resolution nuclear magnetic resonance (NMR) spectroscopy and molecular modeling. Two-dimensional double quantum filtered correlation spectroscopy (DQFCOSY), total correlation spectroscopy (TOCSY) and nuclear Overhausen enhancement spectroscopy (NOESY) were resolved and analyzed for each molecule. Interspatial distance constraints were derived from the intensity of the NOESY connectivities, The formation of hydrogen bonding was monitored from the temperature-dependence of the NH chemical shifts. Molecular models calculated by means of distance geometry, simulated annealing and energy minimization, suggested a global elongated structure for the formylated analogs and a folded arrangement for the nonformylated derivatives, but no hydrogen bonding was detected at the C-terminus of ET-1 analogs.

Role of phospholipase A(2) on the variations of the choline signal intensity observed by 1H magnetic resonance spectroscopy in brain diseases

Phospholipase A(2) catalyzes the hydrolysis of membrane glycerophospholipids leading to the production of metabolites observable by both 1H and 31P magnetic resonance spectroscopy. The signal of choline-containing compounds (Cho) observed by 1H magnetic resonance spectroscopy is constituted of metabolites of phosphatidylcholine, especially phosphocholine (PCho) and glycerophosphocholine (GPCho). The phosphomonoester (PME) and phosphodiester (PDE) signals observed by 31P magnetic resonance spectroscopy are, respectively, precursors and catabolites of phospholipids. A large number of brain diseases have been reported to cause variations in the intensity of the Cho, PME and PDE signals. Changes in the activity of phospholipase A(2) have been measured in many brain diseases. In this review, the relationships between the results of 1H and 31P magnetic resonance spectroscopy and the phospholipase A(2) assays are analyzed. In many brain diseases, the variation in the Cho signal intensity can be correlated with a stimulation or inhibition of the phospholipase A(2) activity.

Recovery of the brain choline level in treated Cushing's patients as monitored by proton magnetic resonance spectroscopy

In a previous study from our group [A. Khiat, C. Bard, A. Lacroix, J. Rousseau, Y. Boulanger, Brain metabolic alterations in Cushing's syndrome as monitored by proton magnetic resonance spectroscopy, NMR Biomed. 12 (1999) 357-363], proton magnetic resonance spectroscopy (1H MRS) was used to evaluate changes in cerebral metabolites in patients with Cushing's syndrome as compared to normal subjects. Data recorded in the frontal, thalamic and temporal areas demonstrated statistically significant decreases of the Cho/Cr ratios in the frontal and thalamic areas but not in the temporal area for Cushing's syndrome patients. No statistically significant changes in the NAA/Cr ratios were measured in any of the areas studied. In this follow-up study, MRS data are reported for ten patients after correction of hypercortisolism which demonstrate a statistically significant recovery of the choline levels in the frontal and thalamic areas. No variation in the NAA, Cr and mI metabolite ratios relative to H(2)O could be measured. Results are interpreted as an inhibition of the phosphatidylcholine degrading phospholipases by glucocorticoids which disappears after correction of hypercortisolism.

Brain metabolic alterations in Cushing's syndrome as monitored by proton magnetic resonance spectroscopy

Proton magnetic resonance spectroscopy ((1)H MRS) was used to evaluate changes in cerebral metabolites in 13 patients with Cushing's syndrome (including seven with pituitary corticotroph adenomas and six with primary adrenal disease) as compared to 40 normal subjects. Data were recorded in the frontal, thalamic and temporal areas; quantification of the MRS signals demonstrated a statistically significant decrease of the Cho/Cr ratio in the frontal and thalamic areas but not in the temporal area for patients with Cushing's syndrome. The largest decrease in Cho/Cr was measured in the thalamic area of patients with a Cushing's syndrome secondary to an adrenal disease. No statistically significant changes in the NAA/Cr ratio were measured in any of the areas studied. These results suggest that the quantification of choline levels could be helpful for monitoring the cerebral metabolite alterations in patients with hypercortisolism.

Conformational analysis of biologically active truncated linear analogs of endothelin-1 using NMR and molecular modeling

Some linear truncated analogs of endothelin-1 display potent agonistic activity at the ET(B) receptor, especially when the side chain of Trp21 is N-formylated. Then, the three-dimensional arrangements of six structurally reduced linear analogs, three formylated and three nonformylated, have been investigated by high resolution NMR spectroscopy and molecular modeling, in order to pinpoint the conformational features related to the biological activity. Two-dimensional double-quantum-filtered correlation spectroscopy (DQFCOSY), total correlation spectroscopy (TOCSY) and nuclear Overhauser enhancement spectroscopy (NOESY) were recorded and analyzed for each molecule. Interspatial distance constraints were derived from the intensity of the NOESY connectivities. The formation of hydrogen bonding was monitored from the temperature dependence of the NH chemical shifts. Molecular models calculated by means of distance geometry, simulated annealing and energy minimization, using the NMR constraints, strongly suggested a global elongated structure for the formylated analogs exhibiting biological activity, and a folded arrangement for the unformylated derivatives. Homology comparisons allowed the identification of a beta-turn-like folding of the C-terminal segment Asp18-Trp21 as a probable key-factor for activity.

Identification of the motilide pharmacophores using quantitative structure activity relationships

Erythromycin A and some derivatives have been shown to act as agonists at the motilin receptor site (motilides) and a structural similarity between these molecules and the N-terminal fragment of motilin has been proposed. Conformational analysis and three-dimensional quantitative structure-activity relationship (3D-QSAR) methods have been used to determine the homology between a series of erythromycin A derivatives and motilin 1-10. A total of 18 compounds has been studied to correlate the gastrointestinal motor stimulating (GMS) activity with the structure-related parameters determined by 3D-QSAR. Two models with good predictive power of the GMS activity are presented, leading to the prediction of motilin 1-10 activity. The models are consistent with the majority of the data available. The most significant parameters for GMS activity are a favorable dispersion interaction from the quaternary ammonium group of the desosamine ring. In motilin 1-10, the aromatic side chains of Phe1 and Tyr7 seem to play the same role as the quaternary ammonium group in models 1 and 2, respectively. Some hydroxyl groups of erythromycin A derivatives and hydrophobic groups of the Val2 and lle4 side chains of motilin also contribute to the GMS activity. The experimental GMS activities measured are in good agreement with the predicted values, with correlation coefficient values of 0.98 and 0.94 in models 1 and 2, respectively.

Three-dimensional structure of the Y1 receptor agonist [Leu31, Pro34]NPY as determined by NMR and molecular modeling

The solution structure of the Y1 receptor agonist, porcine [Leu31, Pro34]NPY, has been investigated by two-dimensional NMR and molecular modeling. A complete assignment of the NMR resonances was achieved and 201 inter-residue nuclear Overhauser enhancement spectroscopy (NOESY) connectivities could be identified, comprising several connectivities between the N- and C-terminal segments. A molecular model was calculated by distance geometry, simulated annealing and conjugate gradients energy minimization using the NOE constraints. The results indicate that, like NPY and other peptides of the family, [Leu31, Pro34]NPY adopts a folded hairpin structure with the terminal segments in close proximity. Analysis of the secondary chemical shifts for the CH(alpha)'s and of the temperature dependence of the NH chemical shifts combined with the NOE constraints indicates a tendency toward helix structure for the segment 18-30 and the presence of turn structure for the C-terminal segment (residues 31-36). Native NPY and [Leu31, Pro34]NPY have most of their structures in common but differ slightly in their C-terminal portion. Based on the structures of NPY and of its specific agonists, [Leu31, Pro34]NPY and NPY 13-36, conclusions can be drawn about the structural requirements for binding to the Y1 and Y2 receptor subtypes.

Solution structure of neuropeptide tyrosine 13-36, a Y2 receptor agonist, as determined by NMR

The three-dimensional structure of neuropeptide tyrosine (NPY) 13-36, a specific Y2 receptor agonist, has been investigated by two-dimensional 1H-NMR spectroscopy in solution. Analysis of the double-quantum-filtered correlation spectroscopy (DQFCOSY), total correlation spectroscopy (TOCSY) and nuclear Overhauser enhancement spectroscopy (NOESY) spectra provided a complete assignment of the proton signals. The interproton connectivities observed in the NOESY spectra comprised 166 intraresidue and 95 interresidue distance ranges which were used as constraints for molecular modeling by distance geometry, simulated annealing and energy minimization. The optimal structures are characterized by a helical C-terminal fragment Leu30-Tyr36 and a wide loop from Leu17 to Ser22. The structure of NPY 13-36 is analogous to the structure of NPY under the same solvent conditions. Comparison with other reported Y2 agonists suggests that the helical Leu30-Tyr36 fragment is the most critical for activity.

Tissue-specific regulation of fat cell lipolysis by NPY in 6-OHDA-treated rats

The effects of neuropeptide Y (NPY), peptide YY (PYY), [Leu31, Pro34]NPY, and NPY (13-36) on adipocyte lipolysis have been studied in subcutaneous (inguinal) and visceral (parametrial) rat adipose tissues. A 48-h fasting period and chemical sympathectomy were used to evaluate the regulation of Y1 and Y2 pathways in rat adipocytes. NPY, PYY, and [Leu31, Pro34]NPY significantly inhibited fat cell lipolysis by about 25% in both tissues (p < or = 0.05). This inhibition was achieved mainly through the Y1 pathway. No significant response to NPY (13-36) was observed, suggesting a lack of involvement of the Y2 pathway in the antilipolytic effect of NPY and PYY. The 48-h fasting period led to the loss of the Y1 inhibitory effect previously observed in control rats. On the other hand, the chemical sympathectomy induced a 35% increase of fat cell lipolysis (p < or = 0.05). The latter involved the Y2 pathway as stimulated by NPY (13-36), and was observed in the parametrial tissue exclusively. These results suggest that: a) rat Y receptors reported to exhibit Gi responses can also express Gs-like responses, and b) visceral and subcutaneous adipose tissues exhibit specific regulation of fat cell lipolysis.

Structural differences between the free and bound states of the DNA-bisintercalating peptide YSPTSPSY

The YSPTSPSY peptide is a DNA-bisintercalator that can adopt nonrandom conformations in solution. Strategies based on random conformational search and energy minimizations have been applied to generate populations of conformers characterizing YSPTSPSY. Subsequent analysis based on statistical methods and clustering allowed to determine the existence of four classes of conformers containing beta- and/or gamma-turns. NMR spectra of YSPTSPSY in solution provide evidence for such structures. Employing a Monte Carlo-based docking procedure, the YSPTSPSY peptide was docked in a DNA double-helical fragment with the sequence [d(GACGTC)]2. The peptide binds on the minor groove of DNA stacking the central CG base pairs, in a manner similar to that observed in complexes of triostin A with DNA. Upon binding, the structure of the C-terminal segment is modified into a type I beta-turn. Five intermolecular hydrogen bonds are observed, but the van der Waals interactions constitute the major stabilization factor for the complex. NMR chemical shifts, coupling constants, and NOESY connectivities are in agreement with the molecular model.

Structural comparison of alanine-substituted analogues of the calcitonin gene-related peptide 8-37. Importance of the C-terminal segment for antagonistic activity

Replacement of specific residues of the antagonistic fragment human calcitonin gene-related peptide 8-37 (hCGRP 8-37) by alanine residues produces good antagonists to CGRP1 receptors when the replacement is made at positions 17 and 20 but a poor antagonist when the replacement is made at position 21. The solution structures of hCGRP 8-37 and of the three alanine analogues have been determined by two-dimensional 1H NMR spectroscopy and molecular modeling. Following the complete assignment of the NMR spectra, a comparison of the chemical shifts and of the temperature dependence of the amide chemical shifts showed that these parameters differed for [Ala17]-hCGRP 8-37 and [Ala20]-hCGRP 8-37 relative to hCGRP 8-37 in the N-terminal and central segments but not in the C-terminal segment (residues 31-37). In the case of [Ala21]-hCGRP 8-37, differences were observed all along the chain. Molecular modeling calculations were performed by distance geometry, simulated annealing and energy minimization using NOE distance constraints. Molecular models showed a structural homology between [Ala17]-hCGRP 8-37, [Ala20]-hCGRP 8-37 and hCGRP 8-37 in the C-terminal segment Asn31-Phe37 as well as hydrogen bonding between Val28 and Asn31. These structural similarities are not observed with [Ala21]-hCGRP 8-37. Therefore, the structure of the C-terminal segment of hCGRP 8-37 appears to be critical for antagonistic activity at CGRP1 receptors.

Biological and molecular analyses of structurally reduced analogues of endothelin-1

Structurally reduced analogues of endothelin-1 (ET-1) were synthesized through linking with an aliphatic spacer [aminocaproic acid (Aca)], segment 3-11 of ET-1 to carboxyl-terminal fragments of various lengths (16-21, 17-21,...,21). The peptides were prepared in their linear or cyclic form, and a formyl group was or was not introduced on the Trp21 side chain. Pharmacological studies were carried out with the guinea pig lung parenchyma paradigm and the rat thoracic aorta bioassay. In the rat aorta, an ET(A) receptor preparation, all of the analogues were inactive. However, in the lung parenchyma, we observed that among the linear formylated derivatives, [Cys(Acm)3,11,Trp(For)21]-(3-11)-Aca-(17-21)ET was a partial agonist. In this series, the presence of His16, as in [Cys(Acm)3,11,Trp(For)21]-(3-11)-Aca-(16-21)ET, caused a decrease in contractile activity, suggesting that the imidazole group disfavors the proper interaction of the linear molecule with the ETB receptors of the lung parenchyma. The loss of biological activity of the deformylated linear analogues strongly suggested that the formyl group played a stabilizing role in the structure of the linear molecules. Interestingly, molecular modeling studies indicated the adoption of different conformations by the formylated and the nonformylated analogues. In contrast, the stabilizing effect of the formyl group was not observed with the cyclic compounds. Furthermore, the presence of His16 favored the contractile activity of the cyclic peptides. Finally, the results demonstrated that the carboxyl-terminal residues 18-21 are required for the activity in the guinea pig lung parenchyma ETB receptors.

Structural effects of the selective reduction of amide carbonyl groups in motilin 1-12 as determined by nuclear magnetic resonance

Motilin is a 22-residue peptide stimulating stomach and intestinal motility. The motilin 1-12 fragment displays biological effects similar to the native peptide. Selective reduction of the amide carbonyl groups to form CH2NH analogs leads to a significant reduction in activity for the first two N-terminal positions and to a complete loss of activity for all other positions. The structures of motilin 1-12 and ten reduced analogs were investigated using the temperature dependence of the amide NH chemical shifts. In all the analogs, the structure of the N-terminal region (residues 1-5) was different from the structure of motilin 1-12, which is characterized by hydrogen bonding between Phe1 and Ile4. The structure of the C-terminal region of analogs was similar to the structure of motilin 1-12 for the first two reduction positions only (1-2 and 2-3), indicating that the C-terminal portion of motilin 1-12 is more critical for biological activity. Complete structural characterizations of motilin 1-12, [CH2NH]1-2, and [CH2NH]4-5-motilin 1-12 were performed by two-dimensional NMR spectroscopy and molecular modeling. The structural features observed confirm the differences based on the temperature dependence of the amide NH chemical shifts. These results demonstrate that conservation of the amide bond rigidity is essential for the activity of non-hydrolyzable analogs.

Structure of human calcitonin gene-related peptide (hCGRP) and of its antagonist hCGRP 8-37 as determined by NMR and molecular modeling

The solution structures of human calcitonin gene-related peptide (hCGRP, 37 residues) and of its antagonistic fragment hCGRP 8-37 have been determined by two-dimensional 1H nuclear magnetic resonance (NMR) spectroscopy and molecular modeling. Analysis of the double quantum filtered correlation spectroscopy, total correlation spectroscopy and nuclear Overhauser enhancement spectroscopy spectra led to a complete assignment and to the identification of more than 350 intra- and interresidue connectivities for each peptide. Molecular models were calculated by molecular dynamics and energy minimization using distance constraints. The structure of hCGRP is characterized by a rigid N-terminal disulfide-bonded loop followed by helix segments (Val8-Leu16), a gamma-turn (Ser19-Gly21) and several local hydrogen-bonded patterns. The structure of hCGRP 8-37 is less defined than the structure of hCGRP and no helix structure is present. Molecular models of both peptides are consistent with the NH temperature coefficients and secondary chemical shifts of the alpha-protons. Hydrogen bonding with the disulfide-bonded ring appears to be critical for helix formation, both structural elements being essential for agonistic activity.

Could cytoplasmic concentration gradients for sodium and ATP exist in intact renal cells?

In renal cells, the Na+ pump maintains a transmembrane concentration gradient for sodium ensuring the net reabsorption of sodium with or without cotransported species. This process requires a significant fraction of the ATP turnover of proximal tubules and thick ascending limbs. To understand the potential regulatory influences of Na+ and ATP on the activity of the Na+ pump in these nephron segments, the apparent kinetics of the membrane-bound Na+-K+ ATPase and of the cellular Na+ pump were studied in different preparations of dog proximal tubules and thick ascending limbs (tubular suspensions, tissue homogenates, and basolateral membrane vesicles) obtained from dog kidney cortex and red medulla. Two determinant kinetic parameters, i.e., the apparent Michaelis constant (Km) and the saturating concentrations for sodium and ATP, were compared with the intracellular concentrations of Na+ and ATP measured under physiological conditions. In both types of tubules, the apparent Km value for Na+ (5-15 mM) is set well below the measured mean intracellular concentration of sodium (50-60 mM), suggesting that the Na+ pump should be saturated by sodium ions under normal conditions. Nevertheless, a modest increment of the Na concentration in the vicinity of the pump, obtained by equilibrating the intra- and extra-cellular sodium concentrations at various extracellular [Na+] with nystatin, increases the activity of the Na+ pump in intact cortical tubules and thick ascending limbs, even when the extracellular [Na+] is set at the estimated intracellular [Na+], demonstrating that the pump is not saturated by sodium in situ. Similarly, the kinetics of the renal Na+ pump as a function of the ATP concentration suggested that the pump should be saturated by ATP in physiological conditions, since in both tissues the cellular ATP level (3-6 mM) is higher than the concentration required to achieve saturation of this activity (< 2.5 mM). However, in renal cortical tubules, the steady-state intracellular [Na+] is affected by modest changes of ATP concentration, suggesting that the Na+ pump is not functionally saturated by ATP. Our data suggest that concentration gradients for Na+ and ATP may exist in the cytosol of renal cells. These gradients would be related to the polarity of sodium transport and of the ATP-consuming and ATP-regenerating processes in intact cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural characterizations of neuropeptide tyrosine (NPY) and its agonist analog [Ahx5-17]NPY by NMR and molecular modeling

The structures of human NPY and of its centrally truncated agonist analog [Ahx5-17]NPY have been investigated in DMSO-d6 by two-dimensional NMR and by molecular modeling. For both peptides, a complete resonance assignment was achieved and a large number (more than 200) of inter-residue NOE connectivities were observed, including long-range connectivities between the N- and C-terminal ends of the chain. Molecular models were calculated using NOE constraints by distance geometry, simulated annealing and conjugate gradient energy minimization. The results indicate that both peptides are folded in the center of their chain, NPY adopting the hairpin shape, whereas the central portion of [Ahx5-17]NPY is characterized by relatively large loops. In contrast to previous models, practically no alpha-helical structure exists for these peptides under our conditions, but two beta-turns are found in NPY and one in [Ahx5-17]NPY. The proximity of the terminal ends could be the determinant factor for their activity.

Heterogeneous metabolism and toxicity of 4-pentenoate along the dog nephron

4-Pentenoate (4P) is a short-chain fatty acid which causes a complete renal Fanconi syndrome. We have examined the mechanism of 4P toxicity along the nephron after a prolonged (30 min) exposition of isolated renal tubular segments to this agent. In proximal tubules, 4P inhibited the activity of alpha-ketoglutarate dehydrogenase, pyruvate dehydrogenase, and beta-oxidation, but not in thick ascending limb or inner medullary collecting duct tubules in suspension. These proximal effects were accompanied by a marked oxidation of the proximal redox state, with a fall in the tissue respiration and a low content of ATP. The acetyl-CoA content of proximal tubules was simultaneously reduced. Butyrate, acetate, hexanoate or octanoate did not exert these effects. In proximal tubules the metabolism of 4P led to the tissue accumulation of 3-keto-4-pentenoyl-CoA, a known unspecific inhibitor of metabolic oxidation. This metabolite was not detectable in thick ascending limbs which metabolized 4P rapidly. No metabolism of 4P was noted in collecting ducts. We conclude that beta-oxidation probably differs in proximal and thick ascending limb tubules, allowing 4P metabolism to exert a specific toxicity in proximal tubules. A selective proximal defect in energy metabolism probably explains the Fanconi syndrome observed with exposition to 4P.

13C and 1H NMR study of the metabolic degradation of 4-pentenoate in different dog nephron segments

The metabolism of 4-pentenoate in isolated kidney tubules has been investigated by 1H and 13C NMR. The 4-pentenoate metabolite, 3-keto-4-pentenoyl-CoA, accumulated in proximal tubules only and its formation could be competitively inhibited by octanoate. 4-Pentenoate was metabolized in thick ascending limbs but not in papillary collecting ducts.

Evolution of aminoacyl-tRNA synthetase quaternary structure and activity: Saccharomyces cerevisiae mitochondrial phenylalanyl-tRNA synthetase

Phenylalanyl-tRNA synthetases [L-phenylalanine:tRNAPhe ligase (AMP-forming), EC 6.1.1.20] from Escherichia coli, yeast cytoplasm, and mammalian cytoplasm have an unusual conserved alpha 2 beta 2 quaternary structure that is shared by only one other aminoacyl-tRNA synthetase. Both subunits are required for activity. We show here that a single mitochondrial polypeptide from Saccharomyces cerevisiae is an active phenylalanyl-tRNA synthetase. This protein (the MSF1 gene product) is active as a monomer. It has all three characteristic sequence motifs of the class II aminoacyl-tRNA synthetases, and its activity may result from the recruitment of additional sequences into an alpha-subunit-like structure.

Basolateral glucose transport in distal segments of the dog nephron

The transport of glucose by canine thick ascending limbs (TAL) and inner medullary collecting ducts (IMCD) was studied using tubule suspensions and membrane vesicles. The uptake of D-[14C(U)]glucose by a suspension of intact TAL tubules was reduced largely by phloretin (Pt), moderately by phlorizin (Pz), and completely suppressed by a combination of both agents. A selective effect of Pz on the transport of [14C]alpha-methyl-D-glucoside, but not on 2-[3H]deoxyglucose, was also observed in TAL tubules. In contrast, glucose transport was unaffected by Pz but entirely suppressed by Pt alone in IMCD tubules. The metabolism of glucose was largely suppressed by Pt but unaffected by Pz in both types of tubules. Membrane vesicles were prepared from the red medulla and the white papilla or from TAL and IMCD tubules isolated from these tissues. Vesicle preparations from both tissues demonstrated a predominant carrier-mediated, sodium-independent, Pt- and cytochalasin B-sensitive glucose transport. Following purification of basolateral membrane on a Percoll gradient, the sodium-insensitive D-[14C(U)]glucose transport activity copurified with the activity of the basolateral marker Na(+)-K+ ATPase in both tissues. However, a small sodium-dependent and Pz-sensitive component of glucose transport was found in membrane vesicles prepared from the red medulla or from thick ascending limb tubules but not from the papilla nor collecting duct tubules. The kinetic analysis of the major sodium-independent processes showed that the affinity of the transporter for glucose was greater in collecting ducts (Km = 2.3 mM) than in thick ascending limbs (Km = 4.9 mM). We conclude that glucose gains access into the cells largely through a basolateral facilitated diffusion process in both segments. However a small sodium-glucose cotransport is also detected in membranes of TAL tubules. The transport of glucose presents an axial differentiation in the affinity of glucose transporters in the renal medulla, ensuring an adequate supply of glucose to the glycolytic inner medullary structures.

Effects of isoflurane, halothane, and enflurane on myocardial flow and energy stores in the perfused rat heart

The effect of three volatile anesthetics (halothane, enflurane, and isoflurane) on coronary flow and metabolic state of isolated rat hearts was studied. These anesthetics are coronary dilators and their effects are dose dependent. At 2 MAC (minimum alveolar concentration), isoflurane, enflurane, and halothane increase coronary flow by 114 +/- 5.9, 93 +/- 6.1, and 77 +/- 6.4%, respectively (p less than 0.001). At these concentrations, they also have a modest but significant metabolic effect causing a 30% reduction in myocardial ATP and phosphocreatine levels, with no significant modification in ADP and AMP concentrations. Energy charge and lactate/pyruvate ratio were also unaffected by these anesthetics. The vascular and metabolic effects were reversible within 2 and 30 min, respectively. Perfusion of the hearts with a Krebs-Henseleit solution without Pi did not interfere with the vascular and the metabolic effect of the anesthetics; however, in this case, ATP and phosphocreatine concentration did not return to control levels after their discontinuation despite full recovery of the vascular effect. These data suggest that the volatile anesthetics have direct coronary vascular and myocardial metabolic effects and that these effects occur independently.

Identification of structurally and functionally important histidine residues in cytoplasmic aspartyl-tRNA synthetase from Saccharomyces cerevisiae

Cytoplasmic aspartyl-tRNA synthetase from Saccharomyces cerevisiae is an alpha 2 dimer (alpha, Mr 63,000), each alpha containing 12 histidines. The covalent incorporation of 6-7 mol of diethyl pyrocarbonate per monomer corresponded to complete enzyme inactivation. This inactivation was reversed by hydroxylamine hydrolysis which regenerates free histidine (and tyrosine) while leaving the carbethoxy group still attached to the epsilon-amino group of lysine. Three histidines, one tyrosine, and four lysines were the main targets of the reagent. Site-directed mutagenesis was also tried to replace each of these modified residues. Given the unstability of the carbethoxy-imidazole bond, the nine histidines that were not modified by diethyl pyrocarbonate were mutated too. For these experiments, the enzyme was expressed in Escherichia coli by using a vector bearing the structural gene in which the first 13 codons were replaced by the first 14 of the CII lambda gene. This substitution had no effect on the kinetic parameters. The combined results of chemical modification and site-directed mutagenesis show that one histidine seems to be part of the active site while two others play an important structural role. On the other hand, labeled lysines and tyrosine are nonessential residues. These results are discussed in light of two recent articles establishing the existence of a second family of aminoacyl-tRNA synthetases devoid of the HIGH and KMSKS consensus sequences and containing no Rossmann's domain in their three-dimensional structures.

The complete amino acid sequence of bovine antithrombin (ATIII)

Bovine antithrombin (ATIII) is a glycoprotein of Mr 56,600. Its primary structure was established using peptide sequences from five different digests. Bovine ATIII exhibits four glcosylation sites as well as human ATIII. The primary structures of bovine and human ATIII were compared: all the residues required for the integrity of the heparin-binding domain are strictly conserved. However, there are differences in the secondary structures of both proteins, bovine and human ATIII.

Interaction of the tRNA(Phe) acceptor end with the synthetase involves a sequence common to yeast and Escherichia coli phenylalanyl-tRNA synthetases

Modified lysines resulting from the cross-linking of the 3' end of tRNA(Phe) to yeast phenylalanyl-tRNA synthetase (an enzyme with an alpha 2 beta 2 structure) have been characterized by sequencing the labeled chymotryptic peptides that were isolated by means of gel filtration and reversed-phase chromatography. The analysis showed that Lys131 and Lys436 in the alpha subunit are the target sites of periodate-oxidized tRNA(Phe). Mutant protein with a Lys----Asn substitution established that each lysine contributes to the binding of the tRNA but is not essential for catalysis. The major labeled lysine (K131) belongs to the sequence IALQDKL (residues 126-132), which shares three identities with the peptide sequence ADKL found around the tRNAox-labeled Lys61 in the large subunit of Escherichia coli phenylalanyl-tRNA synthetase [Hountondji, C., Schmitter, J. M., Beauvallet, C., & Blanquet, S. (1987) Biochemistry 26, 5433-5439].

Structural determination of the vasoactive intestinal peptide by two-dimensional H-NMR spectroscopy

The structure of the vasoactive intestinal peptide 1-28 in 40% 2,2,2-trifluoroethanol was investigated by two-dimensional 1H-nmr spectroscopy. All 1H resonances, except the gamma, delta, and epsilon protons of the lysine residues, could be sequentially assigned. Numerous intraresidual as well as short-range interresidual nuclear Overhauser effect spectroscopy connectivities were observed. Using a variable-target function minimization, a molecular model consisting of two helical stretches involving residues 7-15 and 19-27 connected by a region of undefined structure was calculated. The existence of an undefined structure between residues 16 and 18 confers mobility to the peptide molecule.

Metabolism of lactate by a suspension of dog thick ascending limbs: relations with transport

The addition of substrate in the form of lactate (L), but not glucose (G), increases the respiration of canine thick ascending limb (TAL) segments in a saturable (above 2 mM) fashion. More than 60% of this stimulation is ouabain-sensitive (1 mM ouabain) even if L and G transport are both sodium-insensitive processes in TAL. Thus L, but not G, specifically stimulates Na+ entry in TAL cells and its subsequent transport by the Na+,K(+)-ATPase. If chloride is substituted for by gluconate, no significant substrate-induced stimulation of ouabain-sensitive respiration is observed. SITS (4-acetamino-4'-isothiocyanostilbene-2,2'-disulfonic acid) also interferes with the L-induced stimulation of respiration. Thus L entry in TAL appears to be directly or indirectly coupled to the transepithelial flux of Cl-. Furosemide (F), but not amiloride, also inhibits this stimulation suggesting that the accelerated Na+ entry triggered by the application of L occurs through the F-sensitive carrier or that lactate transport is F-sensitive in TAL cells. In accord, F specifically impairs the metabolism of L (as compared to G). These data suggest that in intact TAL tubules both lactate uptake and oxidation are directly or indirectly influenced by the transcellular flux of NaCl. This organization may participate to maintain a stoichiometry between the transport of NaCl and the availability of L to support the energetic needs of TAL cells.

Variable NMR visibility of intracellular sodium induced by Na(+)-substrate cotransport in dog cortical tubules

The intracellular sodium concentration [( Na+]i) of dog kidney cortical tubules was monitored by flame photometry and 23Na NMR using dysprosium tripolyphosphate as shift reagent. Upon addition of substrates cotransported with sodium, flame photometry showed an increase in [Na+]i while no change (glutamine, glucose) or even a decrease (lactate) in the Na+i NMR signal was observed. This discrepancy could not be explained by a lack of ATP prior to the addition of substrates or by a decrease of NMR visibility of Nai+ induced by binding of substrate to membrane transporters (and pump). We propose that a variation of the "apparent visibility" of Nai+ may occur, arising from either a compartmentation of Nai+ in dog cortical tubules or an inhomogeneous extracellular distribution of the shift reagent.

NMR visibility of 39K and 35Cl in erythrocytes and kidney tubules

The NMR visibility of 39K and 35Cl has been investigated in erythrocytes and in dog renal tubules. In erythrocytes, the 39K NMR visibility was determined by comparing the signal intensities before and after hemolysis with water and by comparing the NMR and flame photometry results. Both procedures showed a NMR visibility of 100% for intracellular potassium. The visibility of intracellular chloride in erythrocytes was estimated at 40% by monitoring the intensity of the 35Cl signal as a function of the hematocrit value. In the case of kidney proximal tubules, the 39K visibility appeared to be very low but could not be accurately determined due to the low sensitivity of the nucleus. The 35Cl signals for intracellular chloride in renal tubules were too broad to be detected.

The three cysteine residues of cytoplasmic aspartyl-tRNA synthetase from Saccharomyces cerevisiae are not essential for its activity

Cytoplasmic aspartyl-tRNA synthetase from Saccharomyces cerevisiae is a dimer made up of identical subunits (Mr 63,000) each of these containing three cysteines (residues 255, 512 and 519 in the amino acid sequence). Thiol-specific probes were used to label these cysteines and study the resulting effect of the modification on the kinetic parameters of both the ATP/PPi exchange and tRNA aminoacylation reactions. Using the classical techniques of protein chemistry it was shown that none of the three cysteines was labelled with iodoacetic acid, whilst N-ethylmaleimide and 5,5'-dithiobis(2-nitrobenzoate) reacted with Cys512 and Cys255, respectively. Only the latter modification was accompanied by a decrease in the rates of both enzyme activities whilst the Km values for the various substrates remained unaffected. Site-directed mutagenesis was also used to replace each of the three cysteines by other residues, either individually or simultaneously. For these experiments the enzyme was expressed in Escherichia coli using an expression vector bearing the structural gene in which the first 13 codons were replaced by the first 14 of the CII lambda gene. The resulting substitution in the amino-terminal part of the expressed enzyme had no effect on the kinetic parameters, compared to those of the enzyme purified from S. cerevisiae. Taking into account the consequences of such substitutions, as well as those of chemical modifications on the two reactions catalysed by the enzyme. ATP/PPi exchange and tRNA aminoacylation, it could be concluded that none of these three cysteines plays any essential role in either substrate binding or catalysis.

Relationship between intracellular ATP and the sodium pump activity in dog renal tubules

To examine the potential effect of the cellular ATP concentration and of the phosphate potential on the function of the sodium pump in intact renal cells, the ATP content of dog cortical tubules was first modified by a 30-min preincubation with one of the following effectors: 5 or 10 mM fructose, 2.5 mM adenosine 5'-monophosphate (AMP), or 2.5 mM adenosine in the presence of substrates (10 mM glutamine + 1 mM glutamate with either 10 mM lactate (low ATP) or 10 mM pyruvate (high ATP)). The tubules were then incubated in Krebs-Henseleit saline using two different phosphate concentrations and the same substrate mixture. The ATP content in tubular cells was modified by these treatments, ranging from 2.2 to 5.7 mM. The oxygen uptake by the tubules was measured before and after application of a small amount of nystatin (0.05 mM, 6 mumol/g wet wt.), added to impose an identical and submaximal increment of work to the Na(+)-K+ ATPase in tubules, irrespective of their ATP condition. This manoeuvre was followed by the addition of 1 mM ouabain to inhibit the sodium pump and quantify the respiration related to the activity of the Na+ pump. No significant effect of the ATP content on the respiratory cost of the Na(+)-K+ ATPase activity was noted when the [ATP] was above the normal concentration of approximately 3.0 mM before or after introduction of nystatin. In a second group of experiments, tubules were treated with 0.1 mM digitonin (13 mumol/g wet wt.) and resuspended in intracellular-like and sodium-free medium. The respiration was measured before and after the addition of increasing Mg-ATP concentrations (0-12 mM). A fixed quantity of Na+ (20 mM) was then introduced before ouabain was applied. The oxygen uptake was measured in these three conditions. We observed a fixed increment of ouabain-sensitive respiration upon stimulation of the pump activity by sodium at ATP concentrations ranging from 2 to 7 mM. The same observation applied when the free energy released from ATP hydrolysis ranged from -50 to -56 kJ.mol-1 and when the [ATP]/[ADP].[Pi] ratio ranged from 1.5 to 7.5 mM-1. These results suggest that the Na+:ATP stoichiometry of the Na(+)-K+ ATPase is not modified by [ATP] in dog cortical tubules when the ATP content is at or above the physiological value. Furthermore, the stoichiometry of the pump does not appear to change when the phosphate potential and (or) the free energy released from ATP hydrolysis are altered.

An incubation system for the NMR study of kidney tubules

Isolated kidney cortical tubules require a very rapid oxygen supply and mechanical agitation to be optimally functional. A sample chamber in which a tubule suspension is oxygenated by recirculating oxygen gas inside a coil of dialysis fibers to avoid cell loss through bubbling and in which the tubules are agitated by a gas-driven turbine has been designed. In such a system, dog cortical tubules (35-45 mg/ml) were found to be metabolically stable for more than 3 h as indicated by linear lactate consumption and glucose production. Small pH variations resulting from carbon dioxide and bicarbonate productions were measured. Good-quality 23Na NMR spectra of dog kidney cortical tubules were recorded with such a system, allowing a 1-min time resolution.

Metabolic effects of acetate on the heart

The effects of various substrates (15 mM glucose, 5 mM glucose, 20 mM acetate, or a combination of these substrates) on the coronary blood flow and on the energetic status of myocytes were studied in isolated perfused rat hearts. We demonstrate that low level glucose (5 mM) or high concentration of acetate (20 mM) leads to a simultaneous fall in tissue ATP, rise in tissue adenosine, and significant increment in coronary blood flow. The latter effect is especially marked with 20 mM acetate. Dipyridamole (10(-6) M) does not enhance the vasodilatation induced by acetate. The provision of 15 mM glucose together with 20 mM acetate fully prevents these changes, indicating that the vasodilatation induced by acetate is probably mediated by metabolic changes. The evidence supports the concept that a redistribution of blood flow together with a fall in tissue ATP may explain some of the adverse effects of acetate dialysis in man, and suggests that the provision of glucose may alleviate these changes.

Nuclear magnetic resonance monitoring of sodium in biological tissues

In recent year, the 23Na nuclear magnetic resonance (NMR) technique has been applied to the study of biological tissues. The advantages of this method are noninvasiveness and good sensitivity. The resonances of the intra- and extra-cellular sodium can be separated by the addition of shift reagents to the extracellular compartment. The method has been mostly applied to cell suspensions, kidney tubules, glands, and small organs. Owing to line-broadening effects, the NMR visibility of the intracellular sodium is reduced to 40% in most cases but can be lower or higher. Time-dependent measurements are possible with adequate life-supporting equipment, allowing the determination of transport parameters. 23Na relaxation times are short in tissues (below 50 ms) and highly dependent on the medium composition. The application of the 23Na NMR technique to intact organs can be hampered by the difficulty of getting a good distribution of the shift reagent in the extracellular milieu. A summary of the studies performed is presented with specific examples to illustrate typical applications.

Identification of the major tRNA(Phe) binding domain in the tetrameric structure of cytoplasmic phenylalanyl-tRNA synthetase from baker's yeast

Native cytoplasmic phenylalanyl-tRNA synthetase from baker's yeast is a tetramer of the alpha 2 beta 2 type. On mild tryptic cleavage it gives rise to a modified alpha 2 beta 2 form that has lost the tRNA(Phe) binding capacity but is still able to activate phenylalanine. In this paper are presented data concerning peptides released by this limited proteolytic conversion as well as those arising from exhaustive tryptic digestion of the truncated beta subunit. Each purified peptide was unambiguously assigned to a unique stretch of the beta subunit amino acid sequence that was recently determined via gene cloning and DNA sequencing. Together with earlier results from affinity labelling studies the present data show that the Lys 172-Ile 173 bond is the unique target of trypsin under mild conditions and that the N-terminal domain of each beta subunit (residues 1-172) contains the major tRNA(Phe) binding sites.

Structure and expression of the genes encoding the alpha and beta subunits of yeast phenylalanyl-tRNA synthetase

The two genes FRS1 and FRS2 encoding, respectively, the large (alpha) and small (beta) subunits of cytoplasmic phenylalanyl-tRNA synthetase from bakers' yeast have been cloned and sequenced. The derived protein primary structures are confirmed by peptide sequences evenly distributed along the reading frames. These predict a subunit Mr of 67,347 for alpha and 57,433 for beta, in good agreement with earlier determinations carried out on the purified protein. These subunit sequences have been compared to those of Escherichia coli phenylalanyl-tRNA synthetase as well as to the small beta subunit of the corresponding yeast mitochondrial enzyme; limited but significant homology was found between the two alpha subunits on the one hand and between the three beta subunits on the other hand. The results suggest that these three enzymes, from E. coli, yeast cytoplasm, and yeast mitochondria, have strongly diverged from one another. The initiation sites of transcription have been determined for both yeast genes. Their 5'-upstream regions show no sequence similarities that would have indicated a coordinate control of gene expression at the transcriptional level. Measurements of steady-state levels of FRS-mRNAs in overproducing strains indicate that there is no restriction in mRNA synthesis. Therefore the control of gene expression, leading to a balanced synthesis of alpha and beta subunits, is likely to occur at the translational level.

Biochemical characterization and osmolytes in papillary collecting ducts from pig and dog kidneys

Papillary collecting duct tubules were prepared in gram quantities from the papillae of dog and pig kidneys. Measurements of substrate and oxygen utilizations by these tubules under both aerobic and anaerobic conditions showed the potential for both glycolysis and oxidative phosphorylation. Oxygen is not necessary to maintain a normal adenosine 5'-triphosphate concentration, but oxidative phosphorylation contributes to more than 65% of the metabolism under aerobic conditions in the two species. Both phosphorus-31 and proton nuclear magnetic resonance spectra recorded from extracts of dog cortex, red medulla, and papilla showed a clear gradient from cortex to papilla for osmolytes, such as glycerophosphorylcholine, sorbitol, inositol, betaine, and sugar phosphates. Other molecules identified in the spectra included glucose, sorbitol, mannitol, lactate, glutamine, alanine, threonine, and adenosine 5'-triphosphate. Conventional biochemical measurements supported these findings. An increase in osmolality from 300 to 600 mosmol/kg H2O for 120 min did not increase the glycerophosphorylcholine and sorbitol concentrations of dog papillary collecting ducts in vitro, but a small effect of a 24-h dehydration was detected in vivo.

Isolation and structure of a pseudopeptide gamma-L-glutamyl-L-aspartic acid from Datura stramonium that impairs learning retention in mice

Datura stramonium contains a compound that impairs learning retention in mice. It has been purified to homogeneity and its structure has been established as that of a gamma-L-glutamyl-L-aspartate. The biological activity of this pseudodipeptide has been found to be identical with that of the corresponding synthetic one. It has also been compared to those of various synthetic di- and tripeptides containing L- and/or D-enantiomers of the constitutive amino acids. The results show that the activity is associated with a peptidic structure containing only one type of enantiomer.

Characterization and metabolism of canine proximal tubules, thick ascending limbs, and collecting ducts in suspension

Preparations of distinct nephron segments were obtained from dog kidneys by collagenase treatment. Four morphologically different tissues were isolated: glomeruli, proximal tubules, thick ascending limbs, and papillary collecting ducts. Each segment possessed a characteristic assay of membrane-bound and cytoplasmic enzymes. Specific metabolic characteristics also were found: gluconeogenesis and ammoniagenesis in proximal tubules, glycolytic aerobic metabolism in thick ascending limbs, and glycolytic anaerobic metabolism in papillary collecting ducts. The assay of Na+ -K+ ATPase, H+ -ATPase, and Ca2+ -ATPase activities in these nephron segments demonstrated a specific enrichment of Na+ -K+ ATPase in thick ascending limbs, and of H+ -ATPase in proximal tubules and papillary collecting ducts. Tubular respiration in the absence or presence of ouabain, 1,3-dicyclohexylcarbodiimide, or furosemide demonstrated that the respiration of each segment could be correlated to the activity of specific ion motive ATPases. Furthermore, a tight coupling between ion transport, ATP turnover, and substrate oxidation was demonstrated. These isolated tubular structures are thus viable and capable of transepithelial transport. Our preparation provides large amounts of defined population of tubules and are thus useful for the study of biochemical and functional heterogeneity along the nephron.