Submitted to: Journal of Animal Science Supplement
Publication Type: Abstract Only
Publication Acceptance Date: February 26, 2002
Publication Date: July 21, 2002
Citation: Connor, E.E., Kahl, S., Elsasser, T.H., Rumsey, T.S. 2002. The effects of zeranol implantation on pituitary growth hormone releasing hormone receptor expressionin growing beef steers [abstract]. Journal of Animal Science Supplement. 80 (Suppl. 1):210. Abstract No. 835. Technical Abstract: Estrogenic growth promotants increase rate of gain and feed efficiency in beef cattle in part through stimulated synthesis and release of pituitary growth hormone (GH); however, the mechanism of GH stimulation is not well understood. The purpose of the present study was to examine the effects of an estrogenic anabolic compound, zeranol, on ADG, GH response to thyrotropin-releasing hormone + growth hormone-releasing hormone (TRH+GHRH) challenge, and pituitary GHRH receptor mRNA expression in growing beef steers. Sixteen steers averaging 301 6 kg BW were assigned randomly to a control group or to one of two zeranol treatment groups that were implanted (36 mg Ralgro) for a total of 42 (1 implantation) or 162 d (3 implantations) until slaughter. Five days prior to slaughter, all animals were challenged with TRH + GHRH (1.0 + 0.1 g/kg, respectively, i.v.) and blood samples were collected at 0, 5, 10, 15, 20, 30, 45, 60 and 120 min relative to challenge for plasma GH determination. The GH response to TRH + GHRH challenge was calculated as the area under the GH response curve (AUC) using trapezoid summation. At slaughter, the pituitary glands were collected, weighed, and immediately frozen in liquid nitrogen for subsequent extraction of total RNA. Pituitary GHRH-R mRNA expression was compared using relative quantitative real-time RT-PCR. Zeranol implantation resulted in increased ADG (P < 0.01) and increased AUC in response to TRH+GHRH challenge (P < 0.05). The GH responses to TRH+GHRH challenge were significantly lower on d 157 than on d 37 (P< 0.05). There was no effect (P > 0.05) of time or zeranol implantation on pituitary weight or expression of GHRH receptor mRNA. Our results suggest that increased ADG and GH secretion due to zeranol implantation are not mediated by changes in transcription of the GHRH receptor gene in cattle.