The diminished hepatic D3 activity observed in leptin-treated
chickens could be a secondary response to augmented GH action on
the liver. In line with our presumption, hepatic expression of GHR and
IGF-I was found significantly up-regulated in leptin-treated chickens.
GH secretion is known to be decreased in ob/ob mice (Larson et al.,
1976), suggesting leptin's involvement in the regulation of GH
secretion. Studies in rats have shown that leptin stimulates both
basal and GH-releasing hormone (GHRH)-induced GH secretion
(Carro et al., 2000). Because GH secretion is pulsatile in nature and
a specific antibody against chicken GH is not available, GH was not
measured in the present study, but the up-regulated hepatic GHR and
IGF-I mRNA expression may indicate enhanced GH action in the liver.
Despite the possible role of T3 and GH in mediating the effects of in
ovo leptin on hepatic activity of iodothyronine deiodinases in the
chicken, the direct effect of leptin cannot be excluded, because both
hepatic and serum concentrations of leptin-like immunoreactive
substance are increased in leptin-treated chickens, especially in males
(data not shown). Our in vitro study first confirmed the direct effect of
leptin on hepatic activity of iodothyronine deiodinases in chicken
hepatocytes. In accordance with the in vivo results, leptin caused a
marked increase in D1 and a decrease in D3, at both mRNA expression
and activity levels, in primary cultured CEHs