Measurement of parameters of cholic acid kinetics in plasma using a microscale stable isotope dilution technique: application to rodents and humans

A stable isotope dilution method is described that allows measurement of ch olic acid (CA) kinetics, that is, pool size, fractional turnover rate (FTR) , and synthesis rate in trace, rats, and humans. Decay of administered [2,2 ,4,4-H-2(4)]CA enrichment was measured in time in 50-mul plasma samples by gas-liquid chromatography/electron capture negative chemical ionization-mas s spectrometry, applying the pentafluorobenzyl-trimethylsilyl derivative. T he kinetic data expressed species-dependent differences. The CA pool sizes were 16.8 +/- 2.1, 10.6 +/- 1.2, and 2.4 +/- 0.7 mu mol/ 100 g body weight for mice, rats, and humans, respectively. The FTR values were 0.44 +/- 0.03 , 0.88 +/- 0.10, and 0.46 +/- 0.14 per day for mice, rats, and humans. The corresponding synthesis rates were 7.3 +/- 1.6, 9.3 +/- 0.1, and 1.0 +/- 0. 2 mu mol/100 g body weight per day. The human data agreed well with literat ure data obtained by conventional isotope dilution techniques. For rats and mice these are the first reported isotope dilution data. The method was va lidated by confirmation of isotopic equilibrium between biliary CA and plas ma CA in the rat. Its applicability was demonstrated by the observation of increased CA FTR and CA synthesis rate in rats fed cholestyramine, which is known to increase fecal bile acid excretion. The presented stable isotope dilution method enables the determination of CA kinetic parameters in small plasma samples. The method can be applied in unanesthetized rodents with a n intact enterohepatic circulation and may also be valuable when studying t he development of human neonatal bile acid kinetics.