R. Boukhliq

Role of glucose, fatty acids and protein in regulation of testicular growth and secretion of gonadotrophin, prolactin, somatotrophin and insulin in the mature ram

This study tested whether the effects of nutrition on gonadotrophin secretion and testicular growth in mature rams are due to increases in the supply of glucose, fatty acids (FA) or amino acids. Responses to protein (casein) and glucose, alone or in combination, were compared with responses to lupin grain and responses to a combination of protein, glucose and FA (acetate, propionate and vegetable oil). Glucose and casein were infused intra-abomasally whereas lupins and FA were added to the diet. Lupin feeding decreased blood growth hormone (GH) concentrations, but increased pulsatile luteinizing hormone (LH) secretion and increased the concentrations of follicle-stimulating hormone (FSH), prolactin, glucose and insulin. These effects were associated with testicular growth. Glucose or casein increased insulin concentrations and decreased GH concentrations, but did not affect gonadotrophins or testicular growth. There was no synergism between casein and glucose. Responses elicited by adding FA to the glucose+casein treatment were similar to those observed with lupins. In conclusion, the reproductive axis does not seem to be closely linked with dietary intakes of amino acids or with circulating concentrations of glucose, insulin or GH. However, the energetic components of the diet, particularly the fatty acids, appear to play a key role in the reproductive responses to changes in nutrition.

Administration of fatty acids and gonadotrophin secretion in the mature ram

The addition of lupin grain to a maintenance diet increases circulating concentrations of volatile fatty acids and gonadotrophin secretion in mature rams. The experiments reported in this paper tested whether these responses were linked causally. The first experiment used 24 rams, of which 16 had two intra-ruminal cannulae inserted each, one for infusion of buffer and the other for infusion of volatile fatty acids. The treated rams (n = 8) were infused for 10 h each day with 1 litre of a solution of volatile fatty acids containing 1.93 moles acetate, 1.89 moles propionate and 0.68 moles butyrate, and 2 litres of a buffer solution containing 14.78 moles sodium bicarbonate, 6.39 moles potassium bicarbonate and 2.06 moles sodium chloride. The eight negative control rams were fed to maintenance and four of them were infused intra-ruminally each day with 3 litres of buffer containing the same mass of salts as the treated rams were given. The eight positive control rams were fed the maintenance diet plus a daily supplement of 900 g lupin grain and four of them were also infused with buffer salts. Blood was sampled before treatment, on Day 3 and at the end of the treatment period (Day 10) for measurement of the plasma concentrations of gonadotrophins (LH and FSH), glucose, insulin and cortisol. In the second experiment, another 24 mature rams were fed to maintain constant live weight and, for 12 of them, sodium acetate (127 g), sodium propionate (64 g) and vegetable oil (60 g) were added to the diet every day for 11 days. Blood was sampled before, and at the end of the treatment period for measurement of the plasma concentrations of gonadotrophins, inhibin, glucose and insulin. In Experiment 1, intraruminal infusion of buffer alone did not affect any of the endocrine variables measured. The infusion of fatty acids and buffer did not alter gonadotrophin secretion, but depressed food intake and increased the plasma concentrations of glucose, insulin and cortisol. When the fatty acids were added to the diet (Experiment 2), the pulsatile secretion of LH was stimulated and FSH and inhibin concentrations were increased. Lupin feeding increased LH pulse frequency on Days 5 and 10, but did not affect FSH concentrations. There were no changes in the circulating concentrations of glucose or insulin. The effects of intra-ruminal infusion of fatty acids on cortisol concentrations and appetite, which were completely absent when similar amounts of fatty acids were fed, suggest that such infusions have undesirable, perhaps stressful, side-effects that prevent expression of the reproductive endocrine responses. On the other hand, the stimulation of gonadotrophin secretion following addition of fatty acids to the diet possibly suggests that changes in the plane of nutrition or rams, for example with lupin supplements, stimulates the reproductive centres of the brain via pathways associated with the synthesis or utilisation of fatty acids. These pathways do not seem to involve changes in glucose homeostasis.

Relationship between nutritional stimulation of gonadotrophin secretion and the peripheral and cerebrospinal fluid (CSF) concentrations of glucose and insulin in rams

In numerous studies it has been shown that supplementing diets of rams with lupin grain (Lupinus angustifolius), a highly digestible energy and protein source, increases gonadotrophin secretion, perhaps by providing metabolic signals that stimulate gonadotrophin-releasing hormone (GnRH) neurons. In the first experiment, we tested the hypothesis that the increase in gonadotrophin secretion caused by lupin supplementation is accompanied by an increase in cerebrospinal fluid (CSF) concentrations of glucose and insulin. In the second experiment, we tested the hypothesis that supplying peripheral glucose to rams, in quantities approximating those associated with digestion of a lupin supplement, would increase gonadotrophin secretion. After 10 days of dietary treatment in the first study, rams (n = 6) given a lupin supplement had increased luteinising hormone (LH) pulse frequencies (P < 0.05) and mean follicle stimulating hormone (FSH) concentrations (P < 0.05) compared with rams (n = 6) not given a lupin supplement. Compared with control values, glucose concentrations in CSF, but not plasma, were higher (P < 0.05) in the lupin-supplemented group by Day 10, and insulin concentrations in both plasma and CSF were higher (P < 0.05) in the lupin supplemented group on Days 3 and 10. In the second study, rams (n = 6) infused intravenously for 10 days with glucose (1.6 mol in l l of saline infused over 4 h) had increased plasma concentrations of glucose and insulin (P < 0.05) on Days 2 and 10, compared with values in saline-infused controls (n = 6). The secretion of LH and FSH was not affected by infusion with either saline or glucose. These experiments demonstrate that nutritional supplementation increases gonadotrophin secretion and peripheral and CSF concentrations of glucose and insulin, but short-term glucose infusion, which increases peripheral glucose and insulin concentrations, does not increase gonadotrophin concentrations. The absence of an increase in gonadotrophin secretion in glucose-infused rams suggests that glucose and insulin in the peripheral circulation do not influence gonadotrophin secretion. However, the glucose and insulin in CSF are not necessarily derived from the peripheral circulation, so further studies are needed to determine whether central glucose or insulin are involved directly in the control of gonadotrophin secretion.

Effect of nutrition on the balance of production of ovarian and pituitary hormones in ewes

Nutrition-induced changes in liveweight can induce changes in ovarian function and ovulation rate. This study was designed to test the hypothesis that heavy ewes are able to maintain similar concentrations of follicle-stimulating hormone (FSH) to light ewes through a loss of responsiveness to ovarian negative feedback. This hypothesis leads to the prediction that administration of ovarian hormones on the basis of metabolic body weight would result in the same gonadotrophin (FSH) concentrations in-light and heavy ovariectomised ewes. We therefore examined the ovarian content of oestradiol and inhibin, and the FSH response to administration of these ovarian hormones in acutely ovariectomised heavy and light Merino ewes produced by differential feeding. At ovariectomy, follicular fluid was aspirated from the 6 largest follicles in each ewe and assayed for oestradiol and inhibin. Six days later, blood was sampled every 6 h for 2 days. Ewes were then given two subcutaneous progesterone implants and injected subcutaneously every 6 h for 5 days with oestradiol (37.4 ng kg-0.75) and charcoal-stripped bovine follicular fluid (0.04 ml kg-0.75). Blood was sampled before each injection and the plasma was assayed for FSH and inhibin. Oestradiol and its receptors were assayed in uterine tissue. Nutrition-induced increases in liveweight led to increases in the number of oestrogenic, potentially ovulatory ovarian follicles and the total ovarian content of oestradiol and inhibin. In the uterus, more oestradiol receptors and a higher oestradiol uptake were observed in light than in heavy ewes. After ovariectomy, FSH concentrations were inversely related to liveweight. However, when the ewes were treated with doses of ovarian hormones that were proportional to liveweight, the slope of the decline in FSH concentrations was independent of liveweight. We conclude that nutrition alters the balance between pituitary FSH secretion and gonadal feedback by changing the responsiveness to the inhibitory effects of oestradiol and inhibin.