Uptake of corticosterone by the perfused rat liver

The mechanism of hepatic uptake of corticosterone from plasma was investigated in the isolated perfused rat liver using an indicator-dilution method. The hepatic influx rate constant for free corticosterone was determined from measurements of the rate of hepatic uptake of corticosterone from protein-free buffer. The rate of hepatic uptake of corticosterone from pooled normal rat serum was then measured. A general model of hormone transport that does not assume that hormone-protein complexes remain at equilibrium during transit through the hepatic sinusoids was used to ask whether this observed rate of uptake could be accounted for by a pool of free corticosterone that turns over very rapidly. Parameter values used in this analysis included the measured concentrations of albumin and corticosteroid-binding globulin in the serum, literature values for the rate constants describing the interactions of corticosterone with these proteins, and the value of the hepatic influx rate constant for free corticosterone determined in the present study. The rate of hepatic uptake of corticosterone from rat serum that we observed was very similar to the rate of uptake predicted by this model to occur via the pool of free corticosterone.