Estimation of the rate of Ca2+ influx in perfused rat liver using the multiple indicator dilution technique

The multiple indicator dilution technique provides a useful mechanism for the determination of the rates of processes involved in the uptake and subsequent release of substances between an organ, such as the liver, and the vasculature. They have used this technique to investigate the uptake of 45Ca2+ into the perfused rat liver at a number of different perfusate Ca2+ concentrations. Each perfused liver is calibrated using reference injections of a mixture of 3H-sucrose and 14C-urea to determine parameters of a microcirculatory model of the liver, such as the characteristic transit times through vascular spaces and the ratio of tissue volume to extracellular volume. Using a single injection of a mixture of 3H-sucrose and 45Ca2+, the profiles of the efflux of the injected tracers are analyzed to estimate the rate constant for uptake of 45Ca2+. The estimated rate constant for uptake decreases with increasing perfusate Ca2+ concentration (kin = 0.155 +/- 0.036, 0.0896 +/- 0.024, 0.0713 +/- 0.009 for [Ca2+] = 0.05, 0.5, 1.25 mM; n=5). From these data, estimates for the maximum velocity of uptake of Ca2+ and the Ca2+ concentration for half maximal uptake are calculated to be 0.1208 mM/sec and 0.7278 mM respectively