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  • 1
    ISSN: 1432-2013
    Keywords: l-Histidine Transport ; Proximal Tubule ; Transport Kinetics ; Fanconi Syndrome ; Kidney ; Microperfusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The kinetics ofl-histidine reabsorption by the proximal convolution of the rat nephron have been studied by stationary microperfusion with simultaneous perfusion of peritubular capillaries. Steady-state concentrations (C ∞) and transepithelial concentration differences (Δc ∞) were determined over a wide range of peritubular bistidine concentrations. It was found that Δc ∞ increased hyperbolically with increase in luminal and peritubular histidine concentrations suggesting saturation transport kinetics. Furthermore Δc ∞ declined linearly along the convolution suggesting that nett active transport was not constant throughout the tubule. Using an expression to describe the rate of attainment of steady-state concentration in terms of lummal and peritubular histidine concentrations, histidine permeability coefficient (P), the maximum rate of active histidine transport (J max) and the half saturation constant of the transport reaction (K m ), we were able to determine the cause of the tubule inhomogeneity. We find thatP (14.1×10−5 cm/s) andJ max (45×10−10 mol/cm2· s) are constant along the convolution but thatK m increases markedly from about 5.4 mmol/kg 26% of the way along the convolution to 40 mmol/kg at 86%. These findings suggest that the histidine reabsorptive mechanism would be relatively inefficient with histidinuria occurring at all plasma concentrations but it would have enormous reserve capacity so that saturation would not readily occur. This prediction accords with available data on histidine clearance in the rat.
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  • 2
    ISSN: 1432-2013
    Keywords: Amino Acid Transport ; Microperfusion ; Renal Tubule ; l-Glutamine ; l-Histidine ; Fanconi Syndrome
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Stationary microperfusion of the rat proximal convoluted tubule together with simultaneous perfusion of the peritubular blood capillaries has been used to studyl-histidine andl-glutamine transport in the rat kidney. When histidine and glutamine concentrations in the capillary perfusate were 14.1 and 6.9 mmol/kg respectively, the luminal concentrations stabilized at about 5.6 and 2.0 mmol/kg respectively. The transepithelial concentration differences at steady-state were 8 mmol/kg (histidine) and 5 mmol/kg (glutamine). The results indicated that when peritubular capillary concentrations were high enough, nett passive back-flux of amino acids down a concentration gradient can become of considerable importance in determining nett reabsorptive rates. When the steady-state epithelial concentration differences were analysed in relation to perfusion site within the proximal convolution, it was found that the gradient was greatest near the glomerulus and smallest near thepars recta, the rate of decline along the convolution being approximately linear. This inhomogeneity of the proximal tubule seems to be due to a diminution in nett amino acid transport by about 50%. The results correlate well with the observation in Fanconi syndrome (congenital renal aminoaciduria with rickets) that only the first 1/3 of the proximal tubule usually shows marked pathological changes.
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  • 3
    ISSN: 1432-2013
    Keywords: Renal Tubule ; Phosphate Transport ; Sodium Dependence ; Micropuncture ; Microperfusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary The standing droplet method has been used in combination with the peritubular perfusion of blood capillaries to determine the build up of transtubular concentration differences of phosphate (P i ) in the renal proximal convoluted tubule of parathyroidectomized rats. Electron probe analysis was used to estimate P i . At zero time both the intraluminal and the contraluminal P i concentration was 2 mM. The time dependent decrease of the intraluminal P i concentration was approximately 4 times faster in the early than in the late proximal convoluted tubule. After 45 sec an intraluminal steady state concentration of 0.20 mM P i was achieved in the early part. In the late part the intraluminal P i concentration approached a steady state value of 0.54 mM at 120 sec. When sodium free solutions were used the intraluminal P i concentration increased to 2.22 mM in the earlier and to 2.76 mM in the late part. The data indicate that in the proximal convoluted tubule 1. The rate of phosphate reabsorption is greater in the early part than in the later part, and 2. phosphate reabsorption might occur as co-transport with Na+ ions.
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  • 4
    ISSN: 1432-2013
    Keywords: Renal tubule ; Phosphate transport ; pH dependence ; Micropuncture ; Microperfusion
    Source: Springer Online Journal Archives 1860-2000
    Topics: Medicine
    Notes: Summary Early loops of the proximal convoluted tubule of parathyroidectomized rats (PTX-rats) were microperfused with a phosphate (4 mM) containing perfusate. With a perfusion solution of pH around 7.45 as estimated as anion deficit theP i reabsorption was two times greater than with a perfusion solution of pH around 6.85. TheP i reabsorption is reduced in PTX-rats made chronic alkalotic (PTX-cA-rats) but the same pH dependence ofP i reabsorption was found. The data indicate that the divalent phosphate is preferentially reabsorbed.
    Type of Medium: Electronic Resource
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