DOSE-DEPENDENT HEPATIC HANDLING OF L-PROPRANOLOL DETERMINED BY MULTIPLE INDICATOR DILUTION METHOD - INFLUENCE OF TISSUE BINDING OF L-PROPRANOLOL ON ITS HEPATIC ELIMINATION

Concentration-dependency in the hepatic elimination of l-propranolol ( l-PR) was investigated over a wide range of concentrations from 60 to 2200 muM in an isolated rat liver perfusion system. Under the steady-s tate condition produced by unlabeled l-PR at various concentrations, H -3-l-PR and C-14-inulin were bolusly injected into the portal vein, an d the outflow was collected at 0.5 s intervals over 30 s. Up to 300 mu M, the instantaneous hepatic availability of l-PR was approximately 4% , while it abruptly increased when the perfusate concentration exceede d 300 muM. To determine which process (influx or efflux or sequestrati on process) caused the nonlinearity, we calculated the rate constants k1 (influx), k2 (efflux), and k3 (sequestration) based on the ''distri buted'' model. With increasing l-PR concentration in the perfusate, k2 increased approximately two times. whereas k3 decreased to approximat ely one-half. In contrast, k1 was independent of the perfusate concent ration. The concentration-dependency of k2 was explained by saturation of l-PR tissue binding, since the tissue unbound fraction of l-PR obt ained with liver homogenates and isolated hepatocytes increased approx imately two times. The efflux and sequestration clearances were then n ormalized by the unbound fractions in the liver. The efflux clearance for unbound l-PR was constant irrespective of the perfusate concentrat ion, whereas the sequestration clearance for unbound l-PR (CL(int)) sh owed Michaelis-Menten type saturation (K(m) = 28 mum, V(max) = 2.8 mum ol/min/g liver, alpha (nonspecific) = 20 ml/min/g liver). Based on the comparison of the kinetic parameters representing these processes, th e rate-determining process for the hepatic clearance of l-PR is a comb ination of the membrane transport and sequestration processes at low c oncentrations, while at high concentration, the rate-determining proce ss is the ''sequestration'' process. Such concentration dependent shif t of the rate determining process can be attributed to the saturation of its tissue binding as well as to the sequestration process.