Long-term effects of human growth hormone-releasing hormone and photoperiod on hormone release and puberty in dairy heifers1

Abstract

Forty-eight Holstein dairy heifers (98.9 kg BW; 3 mo old) were subjected for 246 d to twice-daily s.c. injections of saline (CTL) or human growth hormone-releasing hormone (GRH; 5 micrograms/kg BW) and to photoperiods of 8 h of light (L): 16 h of dark (D) or 16L:8D according to a 2 x 2 factorial arrangement of treatments. Jugular blood samples were collected from 16 heifers at 3, 4, 8, and 11 mo of age to monitor prolactin, growth hormone, and estradiol-17 beta. Plasma progesterone concentrations were monitored weekly in all heifers as an index of puberty (> 1 ng/mL). Growth hormone release was induced by GRH (P < .001) throughout the trial; area under the GH curve (AUC) averaged 1,582 vs 3,643 ng.min-1.mL-1 in CTL vs GRH heifers. However, GRH-induced GH response was less (P < .05) after the second daily injection. There was also an interaction (P = .08) between GRH, photoperiod, and days of treatment on GRH-induced GH response; AUC was greater in GRH-16L:8D than in GRH-8L:16D heifers at 3 mo but less at 8 mo of age. The PRL concentrations were similar for both photoperiods at 3 mo (36.4 vs 41.7 ng/mL) and 8 mo (16.2 vs 12.8 ng/mL) of age but were greater in 16L:8D vs 8L:16D heifers at 4 mo (18.4 vs 39.3 ng/mL) and 11 mo (26.3 vs 44.1 ng/mL) of age (photoperiod x day interaction, P < .001). Photoperiod of 16L:8D vs 8L:16D reduced (P < .01) weight at puberty in CTL heifers (251 vs 303 kg BW) and to a lesser extent in GRH-treated heifers (271 vs 284 kg BW; GRH x photoperiod interaction, P = .10). In conclusion, GH response is maintained throughout 8 mo of GRH treatment, and a 16L:8D photoperiod will reduce age and weight at puberty in heifers. Furthermore, refractoriness to photoperiod-induced PRL changes was detected.

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

“…With a lower dose rate of GRF administration of 10, 25, 50 and 100 mg/352 kg body weight a GRF-induced GH peak was registered within 5 -15 min and GH concentrations declined by 20 min after treatment in Holstein steers (Moseley et al 1984), which is similar to the results of the present investigation. Similar peak values of GH were found to occur within 5 -20 min post-injection in dairy calves and heifers (Gluckman et al 1987, Enright et al 1989, 1993, Lapierre et al 1990, Simpson et al 1992, Hongerholt et al 1992, Ringuet et al 1994, Kazmer & Zinn 1998, Kazmer et al 2000, Wiener et al 2000, which further supports the present investigation.…”

“…AUCs for GH response curves increased in GRF-treated bovines (Moseley et al 1984, Petitclerc et al 1987, Enright et al 1989, 1993, Lapierre et al 1990, Simpson et al 1992, Ringuet et al 1994, Kazmer & Zinn 1998, Kazmer et al 2000, Wiener et al 2000, ovines (Hart et al 1985, Della-Fera et al 1986, Kensinger et al 1987, Wheaton et al 1988, Byrem et al 1989, Beermann et al 1990, Godfredson et al 1990, and swine (Takano et al 1985, Dubreuil et al 1990) over placebo. The present study revealed similar observations in GRF-administered growing Murrah buffaloes.…”

“…The results of the present investigation also agree with those of Hongerholt et al (1992), who found that the time of the GH peak and the height of the peak in response to exogenous GRF was not affected by chronic GRF administration in bovines. In another study (Ringuet et al 1994), twice daily s.c. administration of GRF for 246 days at 5 mg/kg body weight in Holstein dairy heifers resulted in an increase in GH concentrations throughout the trial and all heifers responded to GRF until the last day of the experiment, suggesting that GRF may be used to induce daily GH release without loss of responsiveness over an extended period of time in young dairy heifers. Similarly, exogenous intermittent administration of GRF enhanced secretion and circulating concentrations of GH in ovines without them becoming refractory to GRF (Hart et al 1985, Della-Fera et al 1986, Kensinger et al 1987, Wheaton et al 1988, Byrem et al 1989, Beermann et al 1990, Godfredson et al 1990).…”

“…Puberty is determined by monitoring plasma progesterone concentrations; when plasma progesterone becomes . 1.0 ng/ml for at least two subsequent samples collected at 3 -4 day intervals it is said that the animal has attained puberty (Ringuet et al 1994, Salama et al 1994, Melvin et al 1999. Although the plasma progesterone of GRF-treated animals was found to be always higher than the control group of buffaloes in this study, the level did not reach $ 1.0 ng/ml even in a single sample collected at 3 -4 day intervals for any animal from either group, suggesting that no animal in this experiment had reached puberty by the time of the last injection of GRF.…”

“…A direct administration of a neurohormone, namely GH-releasing factor (GRF), in its synthetic or recombinant form, took forefront as an alternative measure for enhancement of growth because of being active in a wide range of species, and thus GRF treatment could potentially be used to accelerate growth of animals of commercial importance (Gelato & Merriam 1986). Research in this direction has been carried out on a short-term basis by many workers in recent years (Lapierre et al 1992, Enright et al 1993, Ringuet et al 1994, Binelli et al 1995, Kazmer et al 2000.…”