ONTOGENY OF PITUITARY AND SERUM GROWTH-HORMONE IN GROWING TURKEYS AS MEASURED BY RADIOIMMUNOASSAY AND RADIORECEPTOR ASSAY

Some mammalian studies have revealed a wide discrepancy in pituitary a nd circulating growth hormone (GH) measurements determined by immunolo gical 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 biological ly 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 meas ured by avian GH RIA and radioreceptor assay (RRA). The highest mean s erum GH concentration occurred at 3 wk, and the ontogeny of serum GH c ontent from 1 to 8 wk was similar whether measured by RIA or RRA. Pitu itary GH content was highest at 6 wk, but RIA and RRA estimates differ ed markedly throughout the study. Pituitary content of biologically ac tive GH, as estimated by RRA, exceeded that of immunologically active GH from 2 to 10 wk, whereas the reverse was true at 14, 18, and 20 wk. We conclude that this avian GH RIA accurately measures bioactive circ ulating turkey GH, but that the pituitary of the young turkey may cont ain bioactive GH isoforms that have poor immunological activity in our RIA.