Hepatic uptake and excretion of [ 14C]sodium taurocholate by the isolated perfused fetal sheep liver

Abstract

We have developed an in situ isolated perfused fetal sheep liver preparation to study fetal hepatic function free from the confounding influences of the mother and other fetal organs, and we have used the preparation to study the fetal hepatic clearance and biliary excretion of sodium taurocholate (TC). The viability and stability of this model were established by monitoring perfusion pressure, oxygen consumption, perfusate enzymes and electrolytes, the perfusate concentration ratio of lactate to pyruvate, bile flow, and liver histology. Perfusate delivery was 300mL/min with a mean value of 3.94 mL/min/g liver (range: 2.46–6.72 mL/min/g liver). Gadolinium radiolabeled 15μm microspheres were used to quantify the ductus venosus shunt through the liver and to determine relative flow rates between right and left hepatic lobes. TC was added to the reservoir either as a [ 14C]TC tracer bolus dose (2 μCi, N = 5) followed by a constant infusion of unlabeled TC, or as an initial bolus of [ 14C]TC (54μmol) followed by a [ 14C]TC constant infusion (30 μmol/hr, specific activity 30 μCi/mmol; ( N = 3). Perfusate samples were taken from the reservoir every 15 min and bile was collected in 30 min aliquots. Perfusion pressure (7.9 ± 0.30 mmHg), perfusate potassium and oxygen consumption (0.9 ± 0.07 μmol/min/g liver) were constant throughout, and the perfusate lactate/pyruvate concentration ratio was low (<20). Liver histology showed no hypoxic changes. Bile flow fell slightly over the 150 min experiment time from 0.6 to 0.5 μL/min/g liver. These data indicate preparation viability and stability. The extent of the ductus venosus shunt was 16–66% (mean 35 ± 6%) of umbilical vein flow, which correlated inversely with fetal gestational age ( r = 0.94, P < 0.001). Relative flow to right and left lobes of liver was 1:1.4. In bolus dose experiments. TC t 1 2 was 81.6 ± 26 min, clearance ( Cl) was 35.0 ± 22.6 mL/min, shunt corrected extraction (E ∗) was 0.29 ± 0.17 and biliary clearance ( Cl B ) was 35.5 ± 19.5 mL/min. In constant infusion experiments the corresponding results were Cl: 34.7 ± 18.2, E ∗: 0.23 ± 0.16, and Cl B 32.7 ± 17.7 . The cumulative biliary excretion of [ 14C]TC in bolus dose experiments was 86.5 ± 8.7% of the dose, and in constant infusion experiments, concentration of TC in bile was on average over 800 times that in plasma. We conclude that the isolated perfused fetal sheep liver is a suitable model for studying fetal hepatic function, which remains viable for at least 2–3 hr. The fetal liver, while able to concentratively transport TC from perfusate to bile, is only able to clear TC from perfusate relatively slowly, in contrast to the adult liver. This suggests that immaturity of fetal hepatic TC transport reflects a reduced capacity of the transport system which thereby reduces hepatic TC clearance.