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.