Involvement of pituitary hormone in the sex-related regulation of hepatic epoxide hydrolase activity in mice.

The involvement of the pituitary gland in the expression of sex-related differences in the activities of epoxide hydrolase (EH) in liver was examined in hypophysectomized and neonatally monosodium L-glutamate (MSG)-treated male and female mice. In hypophysectomized mice, the hepatic microsomal EH activity of males and females increased considerably above those of the respective sham-operated controls, and the female activity exceeded that of the sham-operated males. The hypophysectomy caused a decrease in soluble EH activity in males, while the operation led to an elevation of the activity in females. Neonatal treatment with MSG resulted in marked increases of microsomal EH activity in males and females at adulthood and the activity was about 2 times that of the male control mice. Treatment of hypophysectomized males and females with growth hormone (GH) resulted in a significant decrease in microsomal EH activity below those of the corresponding hypophysectomized controls, while the hormone treatment had no effect on the soluble enzyme activities. These results suggest that murine hepatic microsomal EH activity is principally under the suppressive control of the pituitary and GH is a causal hormone involved in the expression of sexual dimorphism in the enzyme activity.