Growth hormone secretion in old female rats analyzed by the reverse hemolytic plaque assay.

Growth hormone (GH) release was studied in young (3-4-month-old) and old persistent diestrous (20-21-month-old) female rats using the reverse hemolytic plaque assay. A bimodal distribution of reverse hemolytic plaque area was observed in both young and old female rats. The mean and median of the plaque area of GH cells from old females were smaller than those from young female rats. The percentage of plaque-forming GH cells in old female rats was lower than in young female rats. The percentage of large plaque-forming GH cells (plaque area, more than 8 x 10(3) microns2) was lower in old female rats than in young female rats. GH-releasing hormone (GHRH) increased the mean and median of plaque areas in both young and old female rats. However, responsiveness to GHRH was reduced in old female rats. These results indicate that the amount of GH released from individual GH cells decreases with age in female rats, resulting in diminished GH secretion.