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An experiment was conducted to investigate the effects of changes in ruminal conditions arising from feeding and sucrose supplementation to the diet on the natural abundance of 15N (delta 15N value) of some biological components and of feces and urine in sheep. Four wethers fitted with ruminal fistulas were fed on alfalfa hay cubes with or without sucrose either 2 or 12 times daily. With twice-daily feeding, the delta 15N values of ruminal ammonia and plasma urea decreased after feeding, whereas only small changes were observed throughout the sampling period with 12 times daily feeding. With 12 times daily feeding with and without sucrose, the delta 15N values of ruminal ammonia were higher (P < .05) than those of the diet and plasma urea. The delta 15N values of ruminal bacteria were lower (P < .05) than those of ruminal ammonia but higher (P < .05) than those of the diet. The delta 15N values for feces were higher (P < .05) than those of the diet, and no differences (P > .25) were observed between the delta 15N values of urine and those of the diet. Dietary sucrose supplementation lowered (P < .05) the delta 15N values of ruminal ammonia but had no effect (P > .10) on values for ruminal bacteria and plasma urea. Nitrogen excretion in urine was decreased (P < .05) by sucrose supplementation to the diet, but the delta 15N value of urine showed no effect (P > .25) of sucrose addition.(ABSTRACT TRUNCATED AT 250 WORDS)

Growth hormone (GH) secretion regularity and the effects of lighting condition and GH-releasing hormone (GHRH) on GH release were determined in steers. First, steers were kept under 12:12 L : D conditions (light: 06.00-18.00 hours). The animals were then subjected to a 1-h advancement in lighting on/off conditions (05.00 and 17.00 hours, respectively). Blood was sampled for 24 h at 1-h interval on the seventh day of each condition. Second, GHRH was injected intravenously (IV) at 12.00 and 00.00 hours under 12:12 L : D and blood was sampled at 15-min interval for 4-h (1 h before and 3 h after the injection). Plasma GH concentrations were measured by a radioimmunoassay. Periodicity of GH secretory profile was calculated by power spectrum analysis using the maximum entropy method. Plasma GH concentrations showed a characteristic pattern consisting of four distinct peaks. Mean periodicity of GH secretory profile was 5.7 h, and it was not altered by any change in lighting conditions. IV injection of GHRH increased GH secretion during the day and night. The increase in GH secretory volume after GHRH injection during the night was equal to that during the day. The present results suggest that GH secreted from the anterior pituitary have regularity in steers.

ABSTRACT. The effects of melatonin (MEL) injection into the third ventricle (3V) on growth hormone (GH) secretion were investigated in conscious Holstein steers. A stainless steel cannula was stereotaxically implanted in the 3V based on the ventriculogram. In Exp. 1, three doses of MEL (100, 300 or 600 µg) were injected into the 3V through the cannula and the GH concentration after the injection was determined. In Exp. 2, intracerebroventricular (icv) and intravenous (iv) injections of MEL (100 µg) and GH-releasing hormone (GHRH; 0.25 µg/kg body weight), respectively, were performed simultaneously to examine the effect of MEL on GHRH-induced GH release. The icv injection of MEL significantly stimulated GH release at 100 µg. The increase in GH concentrations by 100 µg of MEL was persistent. Intravenous injection of GHRH dramatically increased GH release. The injection of MEL did not alter GHRH-induced GH release. These results suggest that MEL stimulates GH secretion possibly through the hypothalamus in cattle. KEY WORDS: cattle, melatonin, somatotropic axis, stereotaxic, third ventricle.J. Vet. Med. Sci. 68(10): 1075-1080, 2006 Growth hormone (GH) is a very important factor for livestock production [6]. GH-releasing hormone (GHRH) and somatostatin (SS) from the hypothalamus are primary factors controlling GH secretion in mammals. In addition to GHRH and SS, many neuromodulators have been considered to be involved in GH release regulation in many species. Although the central regulatory mechanism of GH secretion has been studied in sheep using hypophyseal portal blood sampling technique [4], such information is not well obtained in cattle.The evidences have shown the possibility that melatonin (MEL) from the pineal gland is one of those factors controlling GH secretion; however, the exact effect of MEL on GH secretion is controversial. For example, in humans it was reported that MEL induced an increase in GH levels [27,29]; however, other groups obtained no effects of MEL on GH secretion [13,31]. The intramuscular injection of MEL decreased GH secretion in prepubertal boys and the same dose of MEL increased GH level in some pubertal subjects [17]. In vitro experiments showed that MEL reduced GH secretion from rat pituitary cells [8]. These variations of the effects of MEL on GH secretion could be caused by the differences in the experimental and physiological conditions, and there might be inter-species differences regarding the role of MEL in the regulation of GH secretion.The purpose of the present study was to determine the role of MEL in the regulatory system of GH secretion in cattle. Since MEL has been reported to stimulate GH release via the hypothalamus [23,29], the technique for the direct injection of MEL into the third ventricle (3V) at three doses (100, 300 and 600 µg) was used. We also assessed whether MEL affects GHRH-induced GH release to reveal the involvement of the hypothalamus in GH secretion modulated by MEL. MATERIALS AND METHODS Animals:Eleven Holstein steers (7 to 8 mo old at the time of su...

S U M M A R YThe effects of dietary sucrose on the metabolic rate of plasma glucose and ruminal propionate as well as the change in nitrogen kinetics were examined in four mature wethers fitted with rumen fistulas in Tsukuba, Japan in 1990. Wethers were fed at 12 equal intervals daily on crushed lucerne hay cubes (1233 g DM/day), with or without 204 g/day of sucrose. Plasma urea and glucose kinetics were determined following a single intravenous injection of [ I5 N]urea and [U-13 C]glucose respectively; and the kinetics of ruminal ammonia and propionate were determined following a single intraruminal injection of [ 15 N]ammonium chloride and [2-13 C]sodium propionate respectively. Following supplementation of sucrose to the diet, nitrogen retention was increased (P < 005) with a decrease in plasma urea concentration (P < 005) and urinary urea excretion (P < 005). Sucrose supplementation decreased (P < 005) the concentration and irreversible loss rate of ruminal ammonia. Urinary allantoin excretion did not change with sucrose treatment, but the flow rate of non-ammonia-nitrogen from the rumen was increased (P < 0-05). The transfer rate of ruminal ammonia to plasma urea was also decreased (P < 001), whilst the transfer rate of plasma urea to ruminal ammonia was increased (P < 005) by dietary sucrose. Sucrose supplementation resulted in a higher concentration of propionate and butyrate (P < 005) in the rumen with no significant change in acetate or pH. The concentration of plasma glucose did not change with sucrose treatment, but the concentration of insulin, pool size (P < 0-05) and the irreversible loss rate of glucose (P < 001) were increased, reflecting the increase in the production rate of ruminal propionate (P < 005). It was concluded that the supplementation of sucrose affected the metabolism of urea and glucose in plasma via a change in ruminal production rate of ammonia and propionate, respectively.

To clarify the role of serotonin (5-HT) in the regulatory mechanism of L-tryptophan (TRP)--induced growth hormone (GH) secretion in cattle, changes in 5-HT concentrations in the cerebrospinal fluid (CSF) in the third ventricle (3V) and GH in plasma before and after the peripheral infusion of TRP were determined simultaneously. The direct effect of TRP on GH release from the dispersed anterior pituitary cells was also assessed. A chronic cannula was placed in 3V by stereotaxic surgery, then CSF and blood were withdrawn under physiological conditions. TRP (38.5 mg/kg BW) was infused through an intravenous catheter from 12.00 to 14.00 hours and CSF and blood sampling were performed from 11.00 to 18.00 hours at 1-h intervals. The concentration of 5-HT in CSF was determined by high-performance liquid chromatography with electrochemical detection. GH, melatonin (MEL), and cortisol (CORT) concentrations were measured by radio-immunoassay and enzyme-immunoassay. Concentrations of 5-HT were increased by TRP infusion. The TRP infusion significantly increased GH release. On the other hand, TRP did not stimulate GH release from the bovine pituitary cells. MEL and CORT concentrations were not altered by TRP infusion. These results suggest that TRP induced GH release via the activation of serotonergic neurons in cattle.

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