Relationship between hepatic malate dehydrogenase and thyroid hormones in the chicken during embryogenesis and the neonatal period.

The synthesis of hepatic cytosol malate dehydrogenase (MDH) can be stimulated by thyroid hormones under various conditions, and consequently, this enzyme has been considered as a marker of the expression of thyroid hormone action. Since the concentrations of serum thyroid hormones increase considerably during the late phase of chick embryogenesis, we have studied the relationship between this phenomenon and liver MDH activity. A delay of 5 days was observed between the increase in circulating T4 and T3 concentrations and the enhancement of MDH activity. However, treatment with pharmacological doses of T4 (60 microgram/day) resulted in increased MDH activity 48 h after administration of the hormone, while hypothyroidism induced by methimazole decreased the level of the enzyme. Sex-linked differences in MDH activity were found in newborn chickens between 5-9 days of age. No such differences were observed for serum T3 and T4. These findings suggest that under physiological conditions MDH activity is not directly modulated by thyroid hormones in the chick embryo. However, these hormones may well play a permissive role with this enzyme.