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Title: ONTOGENY OF PITUITARY AND PLASMA GROWTH HORMONE IN GROWING TURKEYS AS MEASURED BY RADIOIMMUNOASSAY AND RADIORECEPTOR ASSAY

Author
item Proudman, John
item KRISHNAN K A - PENNSYLVANIA STATE UNIV.
item Maruyama, Kimiaki - Kim

Submitted to: Journal Of Poultry Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/8/1995
Publication Date: N/A
Citation: N/A

Interpretive Summary: Growth hormone (GH), produced by the pituitary gland, plays a major role in regulating growth and body composition in mammals. In poultry, the role of GH is less certain. It is known that GH exists in numerous isoforms- molecules which are slightly different in structure. Different isoforms may have different functions in regulating growth and metabolism. Radioimmunoassays are commonly used to measure GH in pituitary and blood, and our knowledge of GH physiology is largely based on radioimmunoassay (RIA) results. However, RIAs employ antibodies which may not equally recognize hormone isoforms that vary in chemical structure. The present study was conducted to determine if pituitary and serum GH measurements made by RIA are in agreement with GH measurements made on the same samples using a GH radioreceptor assay (RRA), which provides a measure of biological activity. The results showed good agreement between the RIA and the RRA in measuring serum GH levels of young, growing turkeys. This suggests that RIA results provide an accurate assessment of serum levels of biologically-active GH. However, the RRA detected significantly higher GH levels in pituitaries from young poults than did the RIA, suggesting that biologically-active isoforms of GH present in the pituitary of young poults may not be accurately measured by RIA. This work demonstrates to researchers the need to use care in interpreting pituitary GH measurements made by RIA since the concentrations determined may not accurately reflect the concentration of biologically-active hormone at some ages.

Technical Abstract: Some mammalian studies have revealed a wide discrepancy in pituitary and circulating growth hormone (GH) measurements determined by immunological and biological assay methods. Recent studies demonstrating that avian GH exists in numerous isoforms raise concerns that immunological measurement of GH may not accurately reflect the amount of biologically-active hormone present. We sampled eight different male turkeys of a commercial strain weekly until 6 wk of age, and then biweekly until 20 wk. Total pituitary GH content and serum GH concentration were measured by avian GH radioimmunoassay (RIA) and radioreceptor assay (RRA). The highest mean serum GH concentration occurred at 3 wk, and the ontogeny of plasma GH levels from 1 to 8 wk was similar whether measured by RIA or RRA. Pituitary GH content was highest at 6 wk, but RIA and RRA estimates differed markedly throughout the study. Pituitary content of biologically- -active GH, as estimated by RRA, exceeded that of immunologically-active GH from 2 to 10 wk, while the reverse was true at 14, 18, and 20 wk. We conclude that this avian GH RIA accurately measures bioactive circulating turkey GH, but that the pituitary of the young turkey may contain bioactive GH isoforms which have poor immunological activity.