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Photoperiodic control of growth hormone secretion and body weight in rams



Photoperiodic control of growth hormone secretion and body weight in rams



Domestic Animal Endocrinology 5(3): 247-256



This study was undertaken to assess the influence of photoperiod on growth hormone (GH) secretion in rams and its possible influence on body weight. Twenty young adult rams were divided into two groups. One was subjected to an annual (AR) and the other to a semestral (SR) light regime during the same 18-month period. In both groups, daylength (DL) varied gradually between 8 to 17 hr. Plasma prolactin (PRL) and GH profiles consisting of 6 hr samples were determined and animals were weighed throughout the course of the experiment. Maximal PRL secretion was observed with largest DL. In contrast, GH secretion increased during increasing DL but it began to decrease before maximal DL was reached in both light regimes. Mean GH secretion was maximal when the DL was about 11 hr in SR and between 8 to 12 hr in AR. Similarly, body weight increased when DL increased and plateaued during decreasing DL in both AR and SR animal groups. Significant (P < 0.05) differences were observed throughout the course of the experiment according to the effects of decreasing or increasing DL in each group. Analysis of variance showed that the effect of DL on plasma PRL and GH levels and weight velocity (WV) was significant (P < 0.05) in both light regimes. This suggests that in SR, plasma PRL and GH levels and WV vary according to a six month period. Estimation of the lag times between DL and GH or WV indicated that maximal plasma GH levels and WV preceded maximal DL by about 45 and 18 days, respectively, and that there was a delay of about 27 days between photoperiod-induced GH and WV changes. In contrast, there was no lag time between DL and PRL. Therefore there was a lag time of 45 days between maximal levels of GH and PRL. These findings indicate that variations in GH secretion and body weight are related to similar photoperiodic stimuli and suggest that GH could be involved in mediating the growth-promoting effect of increasing photoperiod.

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Accession: 001654409

Download citation: RISBibTeXText

PMID: 3224525

DOI: 10.1016/0739-7240(88)90036-7


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