Spontaneous and GnRH-induced pulsatile LH and testosterone release in pubertal, adult and aging male beagles

Baseline concentrations of LH and testosterone (T) in blood, their pulses, and LH and T response to GnRH (5mug/kg) treatment were compared in 19 sexually sound male beagles and in 2 sexually dysfunctional dogs. The intact beagles were allocated to 4 groups according to age, which ranged from pubertal 7-mo-old animals to 11-yr-old adults. Baseline concentrations of LH and T were measured every 15 min for a period of 6 h and for a further 3 h following challenge with GnRH. Both LH and T were released in a pulsatile fashion with a wide range of pulse frequency and amplitude. The time intervals between the LH and T pulses ranged from 30 to 60 min, with no significant difference between groups. However, LH concentrations were significantly higher (P<0.01) and T values were markedly lower in the 7-mo-old pubertal dogs than in the other age groups. Following GnRH administration, LH peaked within 15 to 30 min in all the animals, with a significantly higher increase occurring in the pubertal group (P < 0.05). Peak T values occurred 15 to 105 min after the LH peaks, with no clear increases occurring in the pubertal dogs. In the 2 sexually dysfunctional animals, LH levels increased following GnRH treatment; however, T values remained extremely low both before and after treatment, indicating loss of Leydig cell function.