Transcriptional regulation of pituitary synthesis and secretion of growth hormone in growing wethers and the influence of zeranol on these mechanisms.
This experiment evaluated relationships between pituitary messenger RNA levels of the transcription factor Pit-1, the growth hormone releasing-hormone receptor (GHRHR), and synthesis and secretion of GH in growing wethers. The experiment also evaluated the influence of the estrogenic compound, zeranol, on these relationships. Seventy wethers that were 9.5 +/- 1 day of age were randomly assigned to a control group or to one of three zeranol treatment groups that were implanted (12 mg, Ralgro) at 0, 45, and (or) 90 days of age. Twenty-eight days after implantation (i.e., Days 28, 73, 118) and on Day 135, sera were collected serially from wethers (n > or = 5) from each group and then their pituitary was collected. As wethers gained weight with age, the pituitary increased in size and so did the relative message levels of Pit-1 and GH (effect of time, P < 0.01). However, as wethers reached 135 days of age, serum concentrations of GH had declined while concentrations of IGF-I had increased (linear contrast, P < 0.01). Additionally, zeranol increased serum concentrations of GH and IGF-I and this effect on GH appeared to be a consequence of increased pulse amplitude, particularly at 73 and 118 days of age (treatment x time, P </= 0.074). This result could have been due to increased pituitary expression of GH (treatment, P < 0.08). Zeranol appeared to modulate pituitary expression of transcripts containing exon 3 of Pit-1 (treatment, P < 0.01) and GHRHR (treatment x time, P = 0.056), however, pituitary message levels of Pit-1 and GHRHR were not positively correlated to pituitary expression of GH mRNA or serum concentrations of GH.
Thomas
M G
MG
Department of Animal and Range Sciences, MSC 3I, New Mexico State University, Las Cruces, NM 88003, USA. milthoma@nmsu.edu
Carroll
J A
JA
Raymond
S R
SR
Matteri
R L
RL
Keisler
D H
DH
D000818:Animals; D019540:Area Under Curve; D001835:Body Weight; D016000:Cluster Analysis; D015342:DNA Probes / Q000737:chemistry; D004268:DNA-Binding Proteins / Q000235:genetics; D004968:Estrogens, Non-Steroidal / Q000494:pharmacology*; D005260:Female; D005786:Gene Expression Regulation; D013006:Growth Hormone / Q000096:biosynthesis* / Q000097:blood / Q000235:genetics / Q000378:metabolism; D007334:Insulin-Like Growth Factor I / Q000032:analysis; D016014:Linear Models; D008297:Male; D009693:Nucleic Acid Hybridization; D010902:Pituitary Gland / Q000378:metabolism*; D016133:Polymerase Chain Reaction / Q000662:veterinary; D012333:RNA, Messenger / Q000737:chemistry / Q000302:isolation & purification; D011863:Radioimmunoassay / Q000662:veterinary; D011897:Random Allocation; D018013:Receptors, Neuropeptide / Q000235:genetics; D018017:Receptors, Pituitary Hormone-Regulating Hormone / Q000235:genetics; D012756:Sheep / Q000502:physiology*; D018709:Statistics, Nonparametric; D050817:Transcription Factor Pit-1; D014157:Transcription Factors / Q000235:genetics; D015029:Zeranol / Q000494:pharmacology*
D015342:DNA Probes; D004268:DNA-Binding Proteins; D004968:Estrogens, Non-Steroidal; D012333:RNA, Messenger; D018013:Receptors, Neuropeptide; D018017:Receptors, Pituitary Hormone-Regulating Hormone; D050817:Transcription Factor Pit-1; D014157:Transcription Factors; D007334:Insulin-Like Growth Factor I; D015029:Zeranol; D013006:Growth Hormone; C048250:somatotropin releasing hormone receptor