SUMMARY

The effect of gonadal hormone administration on hepatic lipogenic enzyme activity, and some physiological parameters was investigated in immature pullets.

Pullets (aged 4 weeks) were allocated to treatment groups and received intramuscular injections of oestradiol, testosterone, progesterone or oestradiol + testosterone (all in 0·3 ml corn oil) or corn oil alone (control group). There was no evidence of any hormone-induced changes in the specific activity of hepatic ATP-citrate lyase 1, 3, 6 and 12 h after hormone administration. NADP-malate dehydrogenase exhibited significant variations in specific activity over this period of time and it is probable that these changes reflected an increased requirement for NADPH for synthetic purposes in hormone-treated birds.

The effect of 1, 2, 4 and 9 days of hormone administration was also investigated. In testosterone-treated birds there were significant increases in the specific activities of both lipogenic enzymes after 1 day of hormone treatment whereas for birds receiving oestradiol the maximum specific activities were found on the second day. Similarly, the liver lipid content of oestradioltreated birds showed a substantial increase on day 2. After 9 days of hormone administration no significant differences in the specific activity of ATP-citrate lyase were observed between treatments but the specific activity of NADP-malate dehydrogenase was significantly reduced in oestradiol- or mixed hormone-treated birds; it is possible that the reduced enzyme activity is associated with a reduced requirement for NADPH and in this connexion there were no further increases in liver lipid content or liver weight after 4 days of hormone administration. The liver RNA:DNA ratio tended to be greatest in birds receiving oestradiol or oestradiol + testosterone.

Studies utilizing inhibitors of RNA and protein synthesis showed that such compounds abolished the increases in lipogenic enzyme activity following hormone administration suggesting that these increases were hormone-induced effects.

These results are discussed in relation to the effects of the various hormones on liver lipid metabolism and also in relation to the situation in the mature laying hen.

The increases in liver and blood lipid contents which occur at the onset of lay in the fowl can be simulated in the immature pullet by oestrogen administration (Lorenz, 1954). The liver is the major site of lipogenesis (Goodridge, 1968) and also of oestrogen-induced lipaemia (Ranney & Chaikoff, 1951). Androgens and progestagens are also involved in the physiological changes encountered at point-of-lay (see Balnave & Pearce, 1974) but neither affects the total blood or liver lipid content. Balnave (1968, 1969) suggested that testosterone and progesterone can influence hepatic lipid metabolism and gonadal hormones other than oestrogens can affect hepatic lipogenic enzyme activities (Pearce & Balnave, 1973; Balnave & Pearce, 1974). The present experiments investigated the hypothesis (Balnave, 1968) that gonadal hormones may also affect lipid degradation.

Four-week-old pullets, given food and water ad libitum, received i.m. injections, in 0·2 ml corn oil, of either 2 mg oestradiol dipropionate, 2 mg