Hepatic extraction of long- and short-acting narcotics in the isolated perfused rabbit liver

Hepatic extraction of the long-acting narcotic, methadone, was compared to that of the short-acting narcotics, morphine, diacetylmorphine, and meperidine, using an isolated perfused rabbit liver preparation. Methadone was avidly extracted from portal venous blood (86.1 +/- 0.81%) in a single pass through the liver after a bolus injection (1.5 mg) into a nonrecirculating perfusion system. Hepatic extraction of methadone was independent of rate of hepatic blood flow (0.59 to 1.53 ml per g of liver per min) but was altered by increasing the total amount of methadone injected. After a bolus injection of 15.0 and 75.0 mg, the proportions of methadone extracted were reduced to 75 and 56%, respectively. The hepatic extraction of morphine (1.5 mg) was 25%, of diacetylmorphine (1.5 mg) 59%, and of meperidine (1.5 mg) 66% in a single pass, all significantly lower (P less than 0.01) than that of methadone. Subcellular fractionation of whole liver homogenates after a single pass of drug showed that methadone and its metabolites were localized primarily in the fractions containing nuclei, mitochondria, microsomes, and other membranes, whereas morphine was primarily localized in the supernatant cytosol. Unchanged methadone was shown to be slowly released from the liver into hepatic effluent blood along with small amounts of the inactive pyrrolidine and pyrroline metabolites (identified by gas chromatography and mass spectrometry). These findings suggest that the liver may serve not only as a site of biotransformation of methadone, but also as a major reservoir for storage and subsequent release of unchanged compound.