Effects of thyroid dysfunction on the metabolism of halothane, enflurane and methoxyflurane in rats

Effect of hyperthyroidism on the in vitro metabolism and covalent binding of 1,1-dichloroethylene in rat liver microsomes

Hyperthyroidism potentiates the in vivo hepatotoxicity of 1,1-dicholoroethylene (DCE) in rats, with a concomitant increase in [14C]-DCE covalent binding. The enhanced injury produced in hyperthyroid livers by DCE could be due to alterations in either the bioactivation or detoxication phases of DCE metabolism. Previous in vitro studies suggested that hyperthyroidism did not potentiate DCE hepatotoxicity by increasing DCE oxidation to intermediates which were able to covalently bind. Several factors, however, that could contribute to the magnitude of DCE bioactivation or covalent binding were not examined. Our objectives were to characterize the effects of hyperthyroidism in male Sprague-Dawley rats on: (1) covalent binding of [14C]-DCE to microsomes and other subcellular fractions, (2) microsomal mixed-function oxidase (MFO) and glutathione S-transferase (GST) activities, and (3) inactivation of microsomal enzyme activities by presumptive DCE reactive intermediates. Hyperthyroid (HYPERT) and euthyroid (EUT) rats received 3 s.c. injections of thyroxine (100 micrograms/100 g) or vehicle, respectively, at 48-h intervals; microsomes and other subcellular fractions were isolated from HYPERT and EUT livers 24 h after the last injection. [14C]-DCE-derived covalent binding was consistently greater in EUT than HYPERT microsomes. The absence of NADH, and the addition of low concentrations (0.1 and 0.5 mM), but not higher concentrations (> 1 mM), of glutathione (GSH) diminished covalent binding to a greater extent in HYPERT than EUT microsomes. Covalent binding in mitochondrial, nuclear, and cytosolic fractions of EUT and HYPERT livers was equivalent. Regression analysis of covalent binding to liver cell fractions from both EUT and HYPERT rats showed a significant correlation with P-450 content. Hyperthyroidism decreased microsomal, but not mitochondrial, cytochrome P-450 content, and MFO activities for 7-ethoxycoumarin and benzphermine were similarly decreased. Hyperthyroidism also diminished microsomal GST activity, and altered GST kinetics for both GSH and 1-chloro-2,4-dinitrobenzene (CDNB). The magnitude of inactivation of MFO and GST activities in the presence of DCE (presumably by DCE reactive intermediates) was comparable between EUT and HYPERT microsomes. When covalent binding was standardized to cytochrome P-450 concentrations in microsomes and mitochondria, HYPERT fractions exhibited slightly greater covalent binding than EUT fractions, suggesting that hyperthyroidism does not reduce the capacity of P-450 hemoproteins to bioactive DCE. Thus, potentiation of DCE hepatotoxicity by hyperthyroidism may be predominantly due to diminished Phase II constituents, and major increases in reactive intermediate/conjugates that covalently bind to and impair critical cellular molecules.

1,1-Dichloroethylene hepatotoxicity: hypothyroidism decreases metabolism and covalent binding but not injury in the rat

Our objective was to determine if the previously reported protective effect of hypothyroidism against 1,1-dichloroethylene hepatotoxicity was associated with a change in distribution and covalent binding. Sprague-Dawley male rats were made hypothyroid (HypoT) by surgical thyroidectomy 2 weeks prior to studies and compared to euthyroid (EuT) rats. Hypothyroidism decreased body weights and liver to body weight ratios while mitochondrial non-protein sulfhydryl groups and cytosolic alcohol dehydrogenase activities were increased by 50%. Rats received a single oral dose of 100 mg [14C]1,1-dichloroethylene (DCE)/kg in mineral oil and were killed at 2, 4, 12 or 24 h; controls received mineral oil only. More rapid liver injury, as measured by serum alanine aminotransferase activity and histology, was present at 2 and 4 h after DCE in HypoT than EuT rats, but a similar magnitude of injury was evident at 12 and 24 h. DCE decreased liver non-protein sulfhydryl groups to a comparable extent in HypoT and EuT rats. Cytosolic glutathione S-transferase and alcohol dehydrogenase activities were decreased only in HypoT rats after DCE. HypoT rats excreted approximately 30% less total [14C]DCE-derived label in urine and their livers, kidneys and lungs consistently contained slightly less covalently bound [14C]DCE-derived label. In contrast, between 1 and 4 h after DCE, greater amounts of acid-soluble and acid-precipitable [14C]DCE-derived label were recovered in red blood cells of HypoT rats. Our results indicate that hypothyroidism did not protect against oral DCE hepatotoxicity but was associated with a more rapid injury at early times. Concurrently, hypothyroidism was found to change the fate of [14C]DCE with higher amounts of 14C-label recovered at early times in red blood cells while less 14C-label was excreted in urine and bound to liver.

Halothane reductive metabolism in an adult surgical population

The reductive metabolism of halothane was studied in 34 adult patients undergoing routine surgery. Reductive biotransformation of halothane was more extensive in females than males and was also enhanced in two patients treated preoperatively with phenytoin, an enzyme-inducing drug. Tobacco, ethanol and the patient's age, body weight and previous exposure to halothane did not influence reductive metabolism of halothane.

Halothane hepatotoxicity in hyperthyroid rats as compared to the phenobarbital-hypoxia model

Halothane hepatotoxicity was observed after exposing hyperthyroid rats to 0.625% halothane for 4 hr under hypoxic conditions (10% O2). In this model, increases in serum enzyme activities of the alanine aminotransferase (GPT) and the sorbitol dehydrogenase (SDH) were evident immediately following exposure and were six-fold higher than in the phenobarbital-hypoxic model. Plasma free-fluoride levels estimated immediately after exposure to halothane were increased twofold in halothane-exposed hyperthyroid rats under hypoxic conditions as were increased twofold in halothane-exposed hyperthyroid rats under hypoxic conditions as compared to a sixfold increase in the phenobarbital-hypoxic model. The concentration of glutathione in liver was more markedly decreased in hyperthyroid rats than in phenobarbital-induced rats. The fact that no clear-cut correlation was found between defluorination and hepatotoxicity in both models may favor the hypothesis that a non-defluorinated metabolite of halothane, e.g., 2-chloro-1,1,1-trifluoroethyl radical, is the reactive intermediate responsible for the liver lesions. On the other hand, intracellular hypoxia due to hypermetabolism during the hyperthyroid state may be the reason for the higher sensitivity of hyperthyroid rats.