Modulation of thyroxine uptake and efflux in vitro by temelastine and phenobarbital in cultured hepatocytes from different species, in relation to toxicological effects on the thyroid gland.
Toxicological studies in the rat with phenobarbital and temelastine (SK&F 93944) are associated with thyroid lesions, characterized by thyroid stimulated hormone-mediated thyroid follicular cell hypertrophy and hyperplasia. It has previously been demonstrated that these compounds enhance the hepatocellular accumulation and clearance of thyroxine (T(4)), in rat but not dog or mouse. In this study these events were further characterized in vitro using cultured hepatocytes from different species. Exposure of rat hepatocytes in vitro to phenobarbital and temelastine produced significant increases (P < 0.05) in hepatocellular [(125)I]l-thyroxine accumulation, following 3 hr of exposure to either drug (at a concentration of 10 mum), in the presence of [(125)I]T(4) (0.107 nm final concentration). At this concentration the accumulation of [(125)I]T(4) after xenobiotic exposure was 132.6 +/- 1.5% (phenobarbital) and 135 +/- 2.0% (temelastine) of control values. There was no apparent xenobiotic-induced cytotoxicity (as determined by the mitochondrial MTT index) up to 20 mum temelastine and 50 mum phenobarbital in rat hepatocytes. Experiments performed at 4 degrees C [or under conditions of cellular ATP depletion, induced by 1 mum carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone (FCCP) treatment] failed to show any such increase in hepatocellular thyroxine accumulation. Pretreatment of hepatocytes with either phenobarbital or temelastine for 3 hr before the addition of radiolabelled thyroxine produced increases in hepatocellular hormone accumulation similar to those observed when [(125)I]T(4) and drug were added to the cultures simultaneously. The earliest time at which any increase in [(125)I]T(4) accumulation was observed after drug exposure was approximately 90 min. Exposure of hepatocytes from guinea pig or beagle dog to phenobarbital or temelastine in vitro failed to produce similar increases in hepatocellular [(125)I]T(4) accumulation, demonstrating species specificity of the xenobiotic effect in vitro.
Aylward
S P
SP
The Celltox Centre, Division of Biosciences, University of Hertfordshire, College Lane, Hatfield, Herts. AL10 9AB, UK.
Walker
T M
TM
Atterwill
C K
CK