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Abstract

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.