Comparison of the requirements for hepatic injury with halothane and enflurane in rats

A rat model of enflurane-associated hepatotoxicity was compared with the halothane-hypoxia (HH) model (adult male rats, phenobarbital induction, 1% halothane, 14% O₂, for 2 hr). The enflurane-hypoxia heating (EHH) model involved exposing phenobarbital-pretreated male adults rats to 1.5-1.8% enflurane at 10% O₂ for 2 hr with external heating to help maintain body temperature. Exposure to either anesthetic without temperature support led to a decrease in body temperature of 7-9°C, while heating the animals during anesthesia resulted in only a 0.5-2°C decrease. Reducing the oxygen tension to 10% O₂ combined with heating the animals during exposure produced significant decreases in the oxidative metabolism of both halothane and enflurane as compared to exposures of 14% O₂. The same conditions also caused a significant increase in the reductive metabolism of halothane, indicating that a severe hepatic hypoxia or anoxia occurs during anesthesia at 10% O₂ with external heating. The time course of lesion development in the HH model paralleled results obtained with an oral dose of CCl₄L: gradual progression of necrosis up to 24 hr. EHH resulted in a classic hypoxic/anoxic injury with elevated serum glutamate pyruvate transaminase values and a watery vascuolization of centrilobular hepatocytes immediately after exposure. The HH model required phenobarbital pretreatment of the rats for expression of hepatic injury; EHH did not. Heating of the animals during anesthesia exposure was necessary for enflurane-induced hepatotoxicity but had little effect on the HH model in both requirements for and type of hepatic injury. Thus the HH model appears to act via a bioactivation/chemotoxic mechanism, whereas models of anesthetic-induced hepatotoxicity that require very low oxygen tensions and heating of the animals during exposure result from a severe hypoxia/anoxia of the liver.