The Effect of Changes in Osmolality on Amino Acid Transport in the Proximal Convoluted Tubule of the Rat Kidney

Abstract

Summary. In order to differentiate between the efTect of increased amino acid (substrate) concentration and the accompanying rise in fluid osmolality on renai tubular amino acid transport properties. Ihe transport of cycloleucine has been examined under conditions where ihe osmolality only was altered with raffinose. In stationary micro perfusion experiments it has been demonstrated that an increase in osmolality to 400 mosmol/kg on both sides of the epithelium at once does not alter the active transporting capacity achieved. Although no experimental evidence is yet available, theoretical considerations suggest that neither component of this active transporting capacity, viz. active transepithelial transport rate or the permeability coefTicient, would have changed, INTRODUCTIONEarlier stationary microperfusion studies of amino acid transport in the proximal convoluted tubule of the rat kidney had shown that the capacity for reabsorption decreased at sites further removed from the glomerulus (Lingard, Rumrich and Young, 1973a). In an attempt to define the cause of this decreased transporting capacity, the kinetics of the transport system were studied by changing the amino acid concentration of both the luminal and peritubular capillary perfusates over a wide range (Lingard, Rumrich and Young, 1973b; Lingard, Gyory and Young, 1975). In these experiments, atnino acid was added to both the luminal and peritubular capillary perfusates whose composition otherwise remained fixed, in order to keep the concentrations of the electrolyte components constant. This meant that the total osmolality of both the luminal and peritubuiar capillary perfusates increased with increasing amino acid concentration. The question thus arose as to whether the amino acid transport rates at the higher concentrations (and thus also the kinetic analysis) could have been affected by the fact that the perfusates were hypertonic.Evidence is available that changes in the osmotic pressure of the bathing fluid can affect transport properties of an epithelium, and, not surprisingly, the