Studies on the Uncoupling Action of Tolbutamide

Abstract

Tolbutamide has been shown to be an uncoupler of oxidative phosphorylation of liver mitochondria. Previous reports demonstrated that its uncoupling action was greatly accelerated by Ca²⁺ and partially restored by EGTA. The present authors investigated the accelerating effect of Ca²⁺ on the uncoupling action of tolbutamide more precisely, using rat liver mitochondria. The following results were obtained.
(1) The uncoupling action of tolbutamide was accelerated with increase in concentration of Ca²⁺ in the range of 0.1-0.4 mM.
(2) Five mM or 10 mM carnitine, in proportion to its concentration, accelerated the action of tolbutamide.
(3) Decrease in the substrate concentration from 10 mM to 1 mM resulted in an acceleration of the action of tolbutamide.
(4) Under conditions which accelerated the action of tolbutamide, the presence of 10 mM carnitine and 1 mM substrate, further addition of 50μM Ca²⁺ resulted in a strong acceleration of the action of tolbutamide: only 1 mg% tolbutamide caused significant uncoupling and 10 mg% tolbutamide caused complete uncoupling.
(5) Seven mg% tolbutamide demonstrated complete uncoupling in the presence of 50μM Ca²⁺ and 1 mM substrate using aged mitochondria.
The above findings suggest that Ca²⁺ has an intimate relation to the uncouplig action of tolbutamide. Previous results showing that tolbutamide changed the permeability of Ca²⁺ through the mitochondrial membrane seem to be in good accord with the present study.