Growth Hormone Size Variants: Changes in the Pituitary During Development of the Chicken

There is considerable evidence for the existence of structural variants of growth hormone (GH). The chicken is a useful model for investigating GH heterogeneity as both size and charge immunoreactive‐(ir) variants have been observed in the pituitary and plasma. The present study examined the size distribution of ir‐GH in the pituitary gland of chicken, from late embryogenesis through adulthood. Pituitaries were homogenized in the presence of protease inhibitor, and the GH size variants were separated by SDS‐PAGE, transferred by Western blotting, immunostained with a specific antiserum to chicken GH, and quantitated by chemiluminescence followed by laser densitometry (chemiluminescent assay). Under nonreducing conditions ir‐GH bands of 15, 22, 25, 44, 50, 66, 80, 98, 105 and >110 kDa were observed. Both the relative proportion of the GH size variants and the total pituitary content varied with developmental stage and age. The proportion of the 15‐kDa fragment was greatest in the embryonic stage, and then it decreased. The proportion of the monomeric 22‐kDa form was lowest at 18 days of embryogenesis (dE) and highest at 20 dE. In contrast, the high MW forms (≥66 kDa) were lowest in embryos, and they increased (P < 0.05) after hatching. The 22‐, 44‐, 66‐, and 80‐kDa forms were assayed for activity by radioreceptor assay following isolation by semipreparative SDS‐PAGE. Only the 22‐kDa GH variant showed radioreceptor activity. Under reducing conditions for SDS‐PAGE, ir‐GH bands of 13, 15, 18, 23, 26, 36, 39, 44, 48, 59 and 72 kDa were oberved, but most of the high MW form disappeared. There was a concomitant increase in the proportion of the monomeric band and of several submonomeric forms. The present data indicate that the expression, processing, and/or release of some if not all size variants are under some differential control during growth and development of the chicken.