Luteinizing hormone pulse frequency is increased by arginine infusion in prepubertal sheep.

Puberty in sheep is initiated by a complex neuroendocrine interplay which cascades into an increased LH pulsatility at this time. Blood-borne amino acids have been proposed as metabolic signals for the stimulation of GnRH/LH secretion, a mandatory requirement for pubertal onset. In previous experiments we have demonstrated that a 1 h infusion of L-arginine (ARG) was capable of stimulating the LH secretion in prepubertal ewes. The aim of the present study was to further examine the effect of an intravenous infusion of ARG on LH secretion. Prepubertal ewes were infused for 6 h with 15 (Group ARG15, n = 5) or 30 g (Group ARG30, n = 5) of L-ARG dissolved in 500 mL saline (pH 7.4), while saline was administered as control (Group S, n = 5). Since ARG is metabolized to ornithine (ORN), equimolar doses of L-ORN were additionally tested (Group ORN12 and ORN24, n = 5, respectively). Blood samples were obtained at 15 minute intervals during and after experimental infusions to characterize the LH pulsatile secretion. The resulting hormone data arrays were searched for significant fluctuations by the PULSAR program. The LH pulse frequency was found to be higher in groups of ARG treated than in saline or ORN infused sheep during the 6-h infusion period: 5.6 +/- 1.0 (ARG15) vs. 5.0 +/- 0.5 (ARG30) vs. 2.0 +/- 0.9 (S, p < 0.01) vs. 3.4 +/- 0.9 (ORN12) vs. 3.4 +/- 0.9 pulses/6 h (ORN24, p < 0.05). The total number of pulses was higher in ARG infused lambs than in saline or ORN infused animals: 11.2 +/- 1.2 (ARG15) vs. 10 +/- 1.1 (ARG30) vs. 13.8 +/- 1.4 (S) vs. 5.8 +/- 1.7 (ORN12) vs. 5.8 +/- 2.0 pulses/12h (ORN24), respectively. The LH mean secretion was comparable during both 6-h periods in all groups. Results of our experiments demonstrate increased LH pulse frequencies during ARG infusions, suggesting an action of ARG to stimulate hypothalamic GnRH release. Thus, ARG may be a critical determinant for enhanced LH pulsatility as a prerequirement for the onset of puberty in the sheep.

Effect of arginine and ornithine infusions on luteinizing hormone secretion in prepubertal ewes.

Although activation of the hypothalamic GnRH pulse generator is known to initiate the onset of puberty in immature animals, the metabolic cues by which information about the stage of body growth and development are translated into central nervous activity remain obscure. In the present study, the function of L-arginine (ARG) or L-ornithine (ORN) as promoters of pulsatile LH secretion was investigated in 2-mo-old prepubertal ewes. Suffolk ewe lambs (n = 17) fitted with intrajugular catheters were divided into three groups. Saline (200 mL) was infused for 1 h into all lambs. In Group 1, saline infusion was continued for another hour. In Group 2, 200 mL of an ARG solution (350 mM, pH 7.4) was infused for 1 h, and in Group 3, ORN was infused (200 mL, 350 mM, pH 7.4) for 1 h. Blood samples were collected at 15-min intervals from 60 min before until 285 min after the end of infusions. The Pulsar program was used to identify significant LH episodes during 285 min after infusions. Mean LH concentrations during the 285-min period after infusion were greater (P < .05) in Group 2 than in Groups 1 and 3. The mean LH pulse frequencies and amplitudes did not differ among the groups. However, in lambs infused with ARG, 13 of their 17 LH pulses had amplitudes > 1 ng/mL, whereas in control ewes only 5 of 20 pulses had amplitudes > 1 ng/mL. All of the ARG-infused ewes had two or more LH pulses with amplitudes > 1 ng/mL; only 1 of 6 control lambs had two pulses. Results indicate that ARG stimulates LH secretion in prepubertal ewes.

Disappearance of opioidergic tone on LH secretion in underfed prepubertal sheep.

A study was conducted to determine whether an opioid tonus inhibitory of LH secretion is present in underfed prepubertal sheep. Ten Suffolk ewe lambs were subjected to food restriction during 60 days. During this period they were allowed to pasture only 2 hours per day while control ewe lambs were allowed for 10 hours. Body weight and plasma blood levels of glucose, urea and total proteins were measured weekly. At the end of this period, an intravenous injection of Naloxone (NAL, 1.5 mg/kg BW) was given to control and underfed animals followed 60 min later by an intravenous injection of LHRH to test the pituitary responsiveness. Underfed animals did not show an increase in plasma LH while control animals presented a rise from 0.28 +/- 0.08 to 2.02 +/- 0.6 ng/ml after the NAL stimulus (P less than 0.05). The response to LHRH was similar in both group of animals. Basal plasma levels of insulin were lower in underfed ewe lambs than in control animals (P less than 0.05). Underfed animals were placed on plain feeding with a schedule similar to control lambs for 30 days and the same experiment was repeated. During this occasion, NAL increased plasma LH concentration in both group of lambs. Levels of plasma insulin were not different in both groups. The lack of effect of NAL on LH secretion in food restricted ewe lambs suggests that the opioid modulation of LH secretion is absent by underfeeding in female prepubertal sheep.(ABSTRACT TRUNCATED AT 250 WORDS)