Age-related changes in plasma growth hormone response to growth hormone-releasing factor in man.

The response of plasma growth hormone to synthetic growth hormone-releasing factor (hpGRF-44) administered intravenously was examined in normal men of various ages ranging from 20 to 75 years. Most of the subjects who were over forty years old had either no or much lower response of plasma growth hormone to hpGRF-44. In contrast plasma growth hormone increased markedly after hpGRF-44 injection in all men in their twenties and thirties. The mean peak level of plasma GH following hpGRF-44 administration was 29.6 +/- 20.4 (SD) ng/ml in men in their twenties, 30.2 +/- 26.5 ng/ml in their thirties, 9.7 +/- 5.2 ng/ml in their forties, 10.9 +/- 5.4 ng/ml in their fifties, 8.4 +/- 4.8 ng/ml in their sixties and 8.1 +/- 7.5 ng/ml in their seventies. These results suggest that somatotroph cells become less sensitive to growth hormone-releasing factor with aging.