Indicator dilution estimation of capillary endothelial transport.

Mechanisms of transport of substrates and small solutes across the endothelial lining of the capillaries include passive diffusion (through clefts between cells or across the plasmalemma) and transporter-mediated flux across the plasmalemma. Because the transport rates are typically high, the multiple indicator dilution technique is usually the method of choice, as it provides the high temporal resolution required. In the simplest version of this technique, a test solute is injected into the inflow simultaneously with reference solutes that are restricted to intravascular and extracellular space. Interpretation of the resulting data requires models; the most precise approach is to fit the model solutions to the data. When appropriate combinations of indicators and sufficiently complex models (those that account for flow heterogeneity, arteriovenous gradients, passive and saturable transport, reaction, and diffusion in multicomponent systems) are used the transporters can be characterized. Features such as the rapidity of intracellular reaction can also be revealed by this technique.