GROWTH HORMONE RELEASING FACTOR AND THE BIOASSAY-RADIOIMMUNOASSAY PARADOX REVISITED

Abstract

Relationships between depletion of growth hormone (GH) from the rat adenohypophysis and its release into the circulation were re-evaluated using bioassay (BA: rat tibia test) and radioimmunoassay (RIA). Anesthetized male rats were injected with a putative growth hormone releasing factor (GRF) and killed 15 and 30 min later. For the assays, the plasma and adenohypophyses were pooled; the latter were separated into granules and cytosol fractions prior to assay. Fifteen minutes after GRF injection the BA recorded a 75% depletion of GH from the granular fraction but the RIA measured a loss of only 13%. By 30 min both assays recorded restoration of pituitary GH. Only slight changes occurred in the levels of BA- or RIA-detectable GH in the cytosol fractions. The massive depletion of bioassayable GH from the granule fraction was accompanied by a large increase (3.2 mU[milliunits]/ml) in the plasma concentration of BA-detectable hormone by 15 min. The slight depletion of RIA-detectable GH that occurred during this time was associated with a meager increase (0.05 mU/ml) in the plasma level of immunoreactive GH. The plasma concentration of BA- and RIA-detectable GH did not change significantly between 15 and 30 min. Analysis of the relationship between depletion of GH from the pituitary and its release into the extrapituitary compartment disclosed that the BA recordings of these events were in excellent quantitative agreement but RIA measurements were not. The results confirm that biologically active GH can be depleted from and released by the adenohypophysis in vivo independently of RIA-detectable GH.