Combined recirculation of the rat liver and kidney: Studies with enalapril and enalaprilat

Combined recirculation of the rat liver (L) and kidney (IPK) at 10 ml min^–1 per organ (LK) was developed to examine the hepatorenal handling of the precursor-metabolite pair: ¹⁴C]-enalapril and ³H]enalaprilat. Loading doses followed by constant infusion of ¹⁴C]enalapril and preformed ³H]enalaprilat to the reservoirs of the IPK or the LK preparation was used to achieve steady stale conditions. In both organs, enalapril was mostly metabolized to its dicarboxylic acid metabolite, enalaprilat, which was excreted unchanged. At steady state, the fractional excretion for ¹⁴C]enalapril (FE=0.45 to 0.48) and preformed ³H]enalaprilat (FE{pmi}=1.1) were constant and similar for both the IPK and LK. The additivity of clearance was demonstrated in the LK preparation, namely, the total clearance of enalapril was the sum of its hepatic and renal clearances. However, the apparent fractional excretion for fanned ¹⁴C]enalaprilat, FE{mi} and the apparent urinary clearance were time-dependent and higher than the corresponding values for preformed ³H]enalaprilat in both the IPK and LK. The FE{mi} and urinary clearance values further differed between the IPK and LK. Biliary clearance of formed vs. preformed enalaprilat displayed the same discrepant trends as observed for FE{mi} vs. FE{pmi} for the LK. These observations on the time-dependent and variable excretory clearance (urinary or biliary) of the formed metabolite vs. the constant, and much reduced, excretory clearance of the preformed metabolite are due to dual contributions to formed metabolite excretion: the nascently formed, intracellular metabolite which immediately underwent excretion and the formed metabolite which reentered the circulation, behaved as a preformed species. When data for the IPK and LK preparations were modeled with a physiological model with parameters previously reported for the L and IPK, all data, including metabolite excretory clearances, were well predicted. Model simulations revealed that the apparent FE{mi} differed between the LK and IPK preparations when the liver was present as an additional metabolite formation organ; the apparent excretory (urinary orGlossary k⁰ infusion rate into the reservoir - C_R reservoir concentration - C_Out,k and C_Out,L venous concentrations for the kidney and liver - C_p,k and c_P,L concentrations in renal and hepatic plasma, respectively - C_k and C_L concentrations in kidney and liver tissue, respectively - C_U and C_Bile concentrations in urine and bile, respectively - CL _b ⁱⁿ andCL _b ^ef influx and efflux clearances, respectively, at the basolateral membrane of the renal tubular cell - C _l ⁱⁿ and CL _l ^ef influx and efflux clearances, respectively, at the luminal membrane of the renal tubular cell - CL _int,K ^m renal metabolic intrinsic clearance of the drug - CL _d ⁱⁿ and CL _d ^ef influx and efflux clearances, respectively, at the sinusoidal membrane - CL _int ^m,L hepatic metabolic intrinsic clearance of the drug - CL _int,L ^b biliary intrinsic clearance - V_R plasma reservoir volume - V_P,K and V_P,L plasma volumes of the kidney and liver, respectively - V_K and V_L tissue volumes of the kidney and liver, respectively - V_U and V_Bile volumes of urine and bile, respectively - Q_K and Q_L total renal and hepatic plasma flow rates, respectively - GFR glomerular filtration rate - Q_U and Q_Bile urine and bile flow rates, respectively - f_P, f_K, and f_L unbound fractions in plasma and kidney and liver tissue, respectively This work was supported by the Medical Research Council of Canada. I. A. M. de Lannoy was a recipient of the Ontario Graduate Scholarship from the Ontario Ministry of Health; K. S. Pang was a recipient of the Faculty Development Award, Medical Research Council.