Plasma Growth Hormone Responses to Intrapituitary Injections of Growth Hormone Releasing Factor (GRF) in the Rat

Abstract

Plasma immunoreactive growth hormone levels were measured in anesthetized rats prior to and at varying times after the intrapituitary injection of purified ovine growth hormone releasing factor (GRF) or control solutions. Twenty-four of 26 animals which were injected with GRF exhibited a rise in plasma growth hormone levels of 5 ng/ml or greater. The response was noted as early as 1.25 min, peaked at 5–10 min and persisted at 20 min. The mean secretory response was 0.78 ± .20 μg growth hormone which represented about 0.2% of total pituitary growth hormone stores. Six of 17 control injected rats exhibited a slight rise in plasma growth hormone which was almost entirely limited to the 1.25-min sample. The results, which represent the first report of a rise in plasma growth hormone levels in response to purified GRF, are discussed in relation to previous negative reports and to the discrepancy between results based on pituitary depletion bioassay and on radioimmunoassay. (