PT  - JOURNAL ARTICLE
AU  - B H Lauterburg
AU  - J Bircher
TI  - Expiratory measurement of maximal amino-pyrine demethylation in vivo: effect of phenobarbital, partial hepatectomy, protacaval shunt and bile duct ligation in the rat.
DP  - 1976 Feb 01
TA  - Journal of Pharmacology and Experimental Therapeutics
PG  - 501--509
VI  - 196
IP  - 2
4099  - http://jpet.aspetjournals.org/content/196/2/501.short
4100  - http://jpet.aspetjournals.org/content/196/2/501.full
SO  - J Pharmacol Exp Ther1976 Feb 01; 196
AB  - Current in vivo methods do not give information about the drug-metabolizing capacity (Vmax) and the affinity of the microsomal enzyme system for the drug i.e., the dose yielding Vmax/2 (Kd). To explore the possibility of estimating these variables from demethylation rates measured by 14CO2 exhalation, various doses of 14C-aminopyrine were injected i.v. to unanesthetized rats. Drug clearances based on 14CO2 exhalation agreed well with those derived from plasma disappearance rates. Evaluation of dose-response curves revealed saturation phenomena. In the normal rat, a Vmax of 173 nmol/min/100 g and Kd of 26 mumol/100 g could be calculated. Enzyme induction with phenobarbital increased Vmax to 745 nmol/min/100 g, whereas in the rat with portacaval shunt, it was reduced to 45 nmol/min/100 g. After 48-hour bile duct ligation, Vmax was not significantly different from the controls, but Kd increased to 36 mumol/100 g compatible with a competitive type of inhibition of aminopyrine demethylation. Two-thirds hepatectomy reduced Vmax by only 50% suggesting that a substantial portion of demethylation occurs extrahepatically. The approach presented appears to be a valid alternative to conventional plasma clearance methods allowing interpretation of some mechanisms affecting in vivo drug metabolism under various conditions.