In vivo biliary excretion and in vitro cellular accumulation of thyroxine by rats or cultured rat hepatocytes treated with a novel histamine H₁-receptor antagonist

Abstract

Rats treated with temelastine (SK&F 93 944), a novel histamine H₁-receptor antagonist, develop thyroid lesions characterized by hypertrophy and colloid depletion. To investigate the mechanism underlying the lesion the biliary clearance and hepatocellular accumulation of radio-labelled iodothyronines was measured in rats or cultured rat hepatocytes. Treatment with temelastine increased both the biliary clearance (approximately 300% of control) and hepatocellular accumulation (approximately 200%) of thyroxine (T₄) but had little or no effect on tri-iodothyronine (T₃). Chromatographic analysis of bile samples from temelastine — treated rats showed that the majority (approximately 78%) of T₄ was present in the unconjugated form. This contrasted with data from phenobarbitone — treated rats which showed that approximately 80% of T₄ in the bile was present as the glucuronide conjugate. Studies with cultured hepatocytes showed that the hepatocellular accumulation of T₄ was energy dependent. At 4° C the treatment — related increases in accumulation of T₄ were abolished, suggesting that temelastine is specifically affecting the high affinity, energy dependent system which preferentially transports thyroxine into hepatocytes. Because temelastine is metabolized extensively, investigations were undertaken to discover if the hepatic effects were caused by the parent compound or an oxidative metabolite. The results showed that the hepatocellular accumulation of T₄ remained increased in hepatocytes co-incubated with temelastine and 1-aminobenzotriazole (a suicide inhibitor of cytochrome P450), even though no measurable P450 could be found in the cells. Also, in studies with two major “rat” metabolites of temelastine, i.e. 93944-Met I or 93944-Met VIII, treatments failed to reproduce the responses seen with the parent compound. These data suggest that it is the parent compound and not some oxidative metabolite which is responsible for the hepatic effect. It is concluded that temelastine produces the observed thyroid changes by increasing the hepatic clearance of T4, leading to increases in TSH release resulting in hypertrophy of the gland. The increased hepatic clearance of thyroxine appears to be mediated via a novel mechanism not associated with the induction of thyroxine metabolizing enzyme systems.