MODULATION OF THYROXINE UPTAKE AND EFFLUX IN-VITRO BY TEMELASTINE ANDPHENOBARBITAL IN CULTURED-HEPATOCYTES FROM DIFFERENT SPECIES, IN RELATION TO TOXICOLOGICAL EFFECTS ON THE THYROID-GLAND

Toxicological studies in the rat with phenobarbital and temelastine (S KandF 93944) are associated with thyroid lesions, characterized by thy roid stimulated hormone-mediated thyroid follicular cell hypertrophy a nd hyperplasia. It has previously been demonstrated that these compoun ds enhance the hepatocellular accumulation and clearance of thyroxine (T-4), in rat but not dog or mouse. In this study these events were fu rther characterized in vitro using cultured hepatocytes from different species. Exposure of rat hepatocytes in vitro to phenobarbital and te melastine produced significant increases (P < 0.05) in hepatocellular [I-125]L-thyroxine accumulation, following 3 hr of exposure to either drug (at a concentration of 10 mu M), in the presence of [I-125]T-4 (0 .107 nM final concentration). At this concentration the accumulation o f [I-125]T-4 after xenobiotic exposure was 132.6 +/- 1.5% phenobarbita l) and 135 +/- 2.0% (temelastine) of control values. There was no appa rent xenobiotic-induced cytotoxicity (as determined by the mitochondri al MTT index) up to 20 mu M temelastine and 50 mu M phenobarbital in r at hepatocytes. Experiments performed at 4 degrees C [or under conditi ons of cellular ATP depletion, induced by 1 mu M carbonyl cyanide p-(t rifluoromethoxy)phenylhydrazone (FCCP) treatment] failed to show any s uch increase in hepatocellular thyroxine accumulation. Pretreatment of hepatocytes with either phenobarbital or temelastine for 3hr before t he addition of radiolabelled thyroxine produced increases in hepatocel lular hormone accumulation similar to those observed when [I-125]T-4 a nd drug were added to the cultures simultaneously. The earliest time a t which any increase in [I-125]T-4 accumulation was observed after dru g exposure was approximately 90 min. Exposure of hepatocytes from guin ea pig or beagle dog; to phenobarbital or temelastine in vitro failed to produce similar increases in hepatocellular [I-125]T-4 accumulation , demonstrating species specificity of the xenobiotic effect in vitro.