New results on saturation phenomena in BSP metabolism: the role of free and conjugated BSP

The fact that the role of conjugation cannot be neglected in the definition and evaluation of the different maximal transfer rates introduced in the literature (T_m, L_m) is demonstrated. Free and conjugated BSP were measured both in blood and bile during constant infusion of free BSP in man. These data proved very useful to develop a careful model of BSP metabolism able to interpret all major known facts in saturation conditions. The model shows that, in the conditions used for the Wheelter-test, biliary excretion is saturated both for free and conjugated BSP. At higher infusion rates, there is also saturation of the hepatic uptake of conjugated BSP. No other processes are saturated in these conditions. Lastly, free and conjugated BSP are excreted at a maximal rate that is not dependent on their respective amounts in the liver; this suggests the existence of independent transport mechanism in biliary excretion for free and conjugated BSP during the infusion of free BSP.