LUTEINIZING-HORMONE PULSE FREQUENCY IS INCREASED BY ARGININE INFUSIONIN PREPUBERTAL SHEEP

Puberty in sheep is initiated by a complex neuroendocrine interplay wh ich cascades into an increased LH pulsatility at this time. Blood-born e amino acids have been proposed as metabolic signals for the stimulat ion 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 prepuber tal ewes. The aim of the present study was to further examine the effe ct 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 ARG3 0, n = 5) of L-ARG dissolved in 500 mL saline (pH 7.4), while saline w as 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 obtai ned at 15 minute intervals during and after experimental infusions to characterize the LH pulsatile secretion. The resulting hormone data ar rays were searched for significant fluctuations by the PULSAR program. The LH pulse frequency was found to be higher in groups of ARG treate d 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 < 0.0 5). 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 (A RG30) vs. 13.8 +/- 1.4 (S)vs. 5.8 +/- 1.7 (ORN12) vs. 5.8 +/- 2.0 puls es/12h (ORN24), respectively. The LH mean secretion was comparable dur ing both 6-h periods in all groups. Results of our experiments demonst rate increased LH pulse frequencies during ARG infusions, suggesting a n action of ARG to stimulate hypothalamic GnRH release. Thus, ARG may be a critical determinant for enhanced LH pulsatility as a prerequirem ent for the onset of puberty in the sheep.