In vivo determination of cellular uptake in the kidney

This paper describes the application of the pulse injection, multiple indicator dilution method to the study of cellular uptake in the kidney in vivo. By using the uptake of sugars and amino acids as specific examples, a rationale is provided that outlines the use of the technique in distinguishing luminal compared to antiluminal uptake. The site of cellular uptake processes cannot be localized by using a whole-organ approach such as the indicator dilution method; nevertheless, for sugars the correlation between in vivo studies and vesicle uptake measurements carried out with purified brush border and antiluminal membrane fractions confirms that the indicator dilution experiments reflect events that are occurring at the level of the proximal tubule in dog kidney. Because of the heterogeneity of tubular flow and substrate concentration profiles along the length of the nephron, it is difficult to use in vivo methods for carrying out kinetic studies on substrate uptake at the luminal surface. By contrast, a strong argument is made that uptake at the contraluminal surface of the proximal tubule can be optimally studied by using the single-pass indicator dilution method. The particular advantages are that the orientation of the basolateral membrane is known and also that uptake fluxes can be measured over short periods of time, i.e., less than 5 s. As an experimental example, uptake of glutamine in the kidney is discussed, and by a combination of indicator dilution methodology and arteriovenous extraction measurements combined with computer simulation and mathematical modeling, an approach is developed for the purposes of deriving unidirectional substrate fluxes at opposing nephron surfaces.