Summary
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.
Key words
Arginine - luteinizing hormone - pulsatility - sheep
