Effects of long-term GH-releasing factor administration on patterns of GH and LH secretion in growing female buffaloes (Bubalus bubalis)

Abstract

To investigate the effects of long-term GH-releasing factor (GRF) administration on the patterns of GH and LH secretion in growing female Murrah buffalo (Bubalus bubalis) calves, 12 buffaloes of 6–8 months of age were divided into two groups (treatment and control groups) of six each in such a way that average body weight between the groups did not differ significantly (P > 0.05). Both the groups were administered i.v. with either synthetic bovine GRF (bGRF(1–44)-NH2) at 10 μg/100 kg body weight (treatment group) or an equal volume of normal saline (control group) at intervals of 15 days until 18 injections had been completed (9 months). Blood samples collected prior to and after the first and last injection of GRF at −60, −45, −30, −15, −10, −5 min and +5, +10, +15, +30 min, and thereafter at intervals of 15 min up to 8 h post-injection, were assayed for plasma GH and LH. Plasma progesterone was also estimated in twice-a-week samples to assess whether either group had begun ovarian cyclicity. The body weight of all animals was recorded twice a week. In all animals, a peak of GH was recorded within 5–20 min and 5–30 min after the first and last GRF injections and post-injection mean values for plasma GH were significantly (P < 0.01) higher compared with the control group of animals. Although peak GH values after the first and last GRF injection did not differ (P > 0.05), GH levels were maintained at a higher level for a longer time after the last GRF injection compared with the first (240 vs 150 min). The area under the GH response curve after the last GRF injection was found to be significantly (P < 0.01) higher than after the first injection (9344 ± 99.7 vs 7763 ± 112.4 ng/ml × min). The mean post-injection plasma LH levels of the treatment group were significantly (P < 0.01) higher after both the first and last GRF injections than in the control group of animals. Interestingly, compared with the first GRF injection, the pre-injection plasma LH level was found to be significantly higher (P < 0.01) at the last injection. The plasma LH concentrations around the last injection of GRF were significantly higher (P < 0.01) than those recorded at the time of the first injection in treated buffaloes. Correspondingly, the plasma LH concentrations in controls were also higher (P < 0.01) around the last injection of GRF vis-à-vis the first injection. The hormone concentration exhibited a higher pulsatility with greater amplitude after the last injection as compared with that recorded after the first injection. Although pulses of LH were also recorded in controls following the last injection, these were fewer and of lower magnitude than those seen in treated animals. No animal from either group reached puberty. GRF-treated buffaloes attained higher (P < 0.001) body weight than the controls. In conclusion, long-term administration of GRF induces and even enhances GH release without any sign of refractoriness, and significantly increases plasma LH also. Hence, long-term treatment with GRF may be used to maintain a sustained increased level of plasma GH in buffaloes and it may assist the animals of this species to grow faster.