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Free keywords:
Amino Acid Transport; Sodium Cotransport; Kidney Tubules; Kidney Mieropuncture
Abstract:
With the technique of stop flow microperfusion with simultaneous capillary microperfusion the zero net flux transtubular concentration differences (Δc) of labelled amino acids which are equivalent to their active transport rates were measured. Alll-amino acids tested (phenylalanine, histidine, aminobicycloheptane-carboxylic acid, aminoisobutyric acid; lysine, ornithine, arginine; aspartic acid; proline and glycine) showed a considerable Δc, i.e. active transport rate. When, however, the ambient sodium was replaced by choline the Δc values dropped to zero. An analysis of the Na+ dependence of the ornithine transport revealed that the sodium-dependence is of the mixed type, i.e. that Km decreased and Vmax increased with increasing Na+ concentration to the same extent.
In contrast to other biological systems no mutual interaction between the Na+-dependent d-glucose and l-histidine transport could be observed.
Incidental to these studies it was observed that the active transport rate of d-histidine was in the range of 40% of that of the l-isomer while for d-phenylalanine it was only in the range of 10% of the active transport of the l-isomer. Furthermore it was found that the l-aspartic acid transport was already saturated at a luminal l-aspartic acid concentration of 0.05 mmol/l while that of l-phenylalanine was not saturated even at a luminal concentration of 9 mmol/l.
