Department of Agricultural and Food Chemistry, The Queen's University of Belfast, Newforge Lane, Belfast, BT9 5PX, and Department of Agriculture, Northern Ireland

(Received 1 July 1977)

Before the onset of egg laying in the domestic fowl, there are increases in the lipid content of the plasma and liver (Chaikoff, Lorenz & Entenman, 1941; Heald & Badman, 1963). These changes are the result of oestrogen secretion by the ovary (Sturkie, 1965). Hepatic lipogenic enzymes also increase in activity during the onset of sexual maturity and their specific activities are greater in the laying hen than in the mature cockerel (Pearce, 1971); similar changes in the activities of hepatic lipogenic enzymes have been observed in oestrogen-treated immature pullets (Balnave & Pearce, 1974). The synthesis of yolk proteins in the liver is also induced by the administration of oestrogen to immature pullets and cockerels (Gruber, 1972). The action of oestrogen is antagonized by

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

ABSTRACT

It has been proposed that changes in steroidogenesis which occur during early development of the corpus luteum may be due to increased availability of lipoproteins. Bovine follicular fluid, however, contains significant amounts of high-density lipoprotein (HDL), and granulosa cells are exposed to this lipoprotein before ovulation. To determine whether bovine granulosa cells can utilize HDL the effects of this lipoprotein on freshly isolated, non-luteinized granulosa cells and on granulosa cells undergoing luteinization in serum-free culture were examined. Cells were isolated from non-atretic, antral follicles and cultured for 12 h in 10% (v/v) lipoprotein-deficient serum to allow cell attachment. After this time cells were cultured in serum-free medium. During culture the cells underwent functional luteinization as assessed by an increase in basal progesterone output (9·6-fold in 7 days) which was associated with a marked increase in activity of cholesterol side-chain cleavage and loss of aromatase activity. Dibutyryl cyclic AMP (dbcAMP) increased basal production of progesterone about twofold but HDL alone had no effect. Addition of HDL plus dbcAMP, in contrast, caused a very marked stimulation (up to ten times) of basal steroidogenesis. This trophic effect of HDL and dbcAMP lasted at least 2 weeks. Activity of cholesterol side-chain cleavage was stimulated (threefold over basal) by dbcAMP during culture but HDL was without effect, alone or with dbcAMP.

Addition of HDL (in the presence or absence of dbcAMP) to freshly isolated granulosa cells had no significant stimulatory effect on progesterone production over 12 h in six experiments, and in two of these experiments a significant inhibitory effect was seen. Incubation with 22R-hydroxycholesterol, in contrast, caused a marked stimulation of progesterone production, indicating that the steroidogenic capacity of the cells was not already saturated. Results presented here suggest that bovine granulosa cells are able to utilize HDL for steroidogenesis only after luteinization. The massive secretion of progesterone by luteinized granulosa cells which occurs in the presence of HDL suggests that this lipoprotein is very important in the development and maintenance of luteal cell function in cattle.

Journal of Endocrinology (1990) 124, 255–260

ABSTRACT

Turnover studies of thyroxine (T₄), 3,5,3′-tri-iodothyronine (T₃) and 3,3′,5′-tri-iodothyronine (rT₃) have been performed in the rabbit. A novel modification of a conventional radioimmunoassay has been used to measure specific ¹²⁵I-labelled iodothyronines in small volumes of plasma in the presence of other ¹²⁵I-labelled metabolites. Kinetic analysis of plasma disappearance of tracer was performed by a new theoretical approach. For T₄ the mean (±s.d.) plasma concentration, clearance and production rates were 34±12 nmol/l, 109±19 ml/kg per day and 3·7±1·4 nmol/kg per day respectively (n = 9). For T₃ the corresponding values were 2·04±0·42 nmol/l, 1·52±0·29 litres/kg per day and 3·07±0·76 nmol/kg per day (n = 8), and for rT₃ 0·12±0·04 nmol/l, 5·7±1·7 litres/kg per day and 0·69±0·23 nmol/kg per day (n = 8). The combination of these two new methodologies affords a simple and convenient means of studying iodothyronine metabolism under normal and abnormal conditions. The techniques employed may be generally applied to turnover studies of other compounds of physiological interest which can be measured by radioimmunoassay.

J. Endocr. (1985) 106, 87–94

ABSTRACT

The development of the reproductive system was studied in juvenile starlings during the acquisition of photosensitivity, the attainment of sexual maturation after photostimulation and the subsequent onset of photorefractoriness, using immunohistochemistry for LHRH and radioimmunoassay measurements of hypothalamic, pituitary and plasma hormone concentrations. The first stage of sexual development induced by exposure of photorefractory immature starlings to short days (8 h light:16 h darkness; 8L:16D) was characterized by a decrease in pituitary prolactin content within 1 week and an increase in hypothalamic LHRH content, in the size of the LHRH perikarya and in the intensity of immunostaining in the median eminence in 4–6 weeks. Sexual maturation occurring after exposure to long days (18L:6D) was associated with further increases in LHRH content and cell size, and increases in LH and prolactin concentrations. During testicular regression, LHRH perikarya were reduced in size and staining intensity but LHRH immunostaining in the median eminence and content in the hypothalamus remained high until gonadal regression was almost complete. Prolactin levels were maximal during testicular regression. These results suggest that gonadal regression is initiated by a reduction in LHRH synthesis and possibly, in addition, an external inhibitory influence on LHRH release. Hypothalamic LHRH content eventually declined and LHRH immunostaining in the median eminence was much reduced in fully photorefractory starlings maintained under long days.

Journal of Endocrinology (1989) 122, 255–268