MECHANISMS OF TIME-DEPENDENT KINETICS OF DILTIAZEM IN THE ISOLATED-PERFUSED RAT-LIVER

The time dependent mechanisms of diltiazem (DZ) disposition were studi ed in a single-pass isolated perfused rat liver system. DZ (2-100 mu M ) was infused continuously until a steady state was achieved. The time required to achieve steady state (T-ss) ranged from 15 to 75 min and was inversely related to infusion concentration. Steady-state extracti on (E) of DZ decreased from 0.98 to 0.73 as the outlet concentration i ncreased from 0.03 to 26.34 mu M. This reduction of E is not related t o the saturation of the primary deacetylation and N-demethylation path ways of metabolism. The N-demethylated metabolite went through a chara cteristic maximum prior to reaching a steady state, indicating enzyme inactivation. During a second DZ infusion, spaced with a 30 min washou t period (stop-infusion experiment), the concentration vs. time profil e of DZ was similar to that of the first one. Washout data from stop-i nfusion and [H-3]DZ infusion studies showed that DZ and its metabolite s were tightly bound to liver proteins. This observation is consistent with the unusually long T-ss of DZ. Infusion studies with [H-3]DZ sho wed that 669.5 +/- 156.5 and 974.2 +/- 99.2 nmol of DZ and its metabol ites (calculated as DZ) were bound and/or distributed/g of liver at in let concentrations of 35.5 +/- 3.2 and 67.2 +/- 3.4 mu M, respectively . The amounts of DZ and its metabolites bound irreversibly to the whol e liver, hepatic microsomal and hepatic cytosolic proteins were not di fferent at the two inlet concentrations studied and were 24.5 +/- 6.6, 48.8 +/- 11.8, and 23.7 +/- 5.8 pmol/mg of protein, respectively. Som e irreversible binding of DZ to liver microsomes may be related to the inactivation of enzymes involved in the N-demethylation of DZ. We con clude that reversible tissue binding of DZ is mainly responsible for t he time-dependent kinetics of DZ in rat, and enzyme inactivation plays only a minor role.