Age-related changes in growth hormone and prolactin messenger RNA levels in the rat

Growth hormone (GH) secretion declines with age, while prolactin (PRL) secretion increases in rats. The age-related changes in the pituitary GH and PRL mRNA levels in the Wistar Tw rat were studied at 6, 12 and 18 months of age by cytoplasmic dot hybridization. Hormone and mRNA concentrations were expressed as values per micrograms GH-cell or PRL-cell DNA. Concurrent decrease in GH concentrations and GH mRNA concentrations with age was observed in female rats, but not in male rats. GH mRNA concentrations (per microgram GH-cell DNA) in male rats at 12 and 18 months of age were 88 and 66% of those at 6 months of age, but there was no difference. GH mRNA concentrations in female rats at 12 and 18 months of age were 51 and 53% of those at 6 months of age. A concurrent decrease in PRL concentrations and PRL mRNA concentrations was observed in both male and female rats. PRL mRNA concentrations (per microgram PRL-cell DNA) in male rats at 12 and 18 months of age were 37 and 18% of those at 6 months of age. PRL mRNA concentrations in female rats at 12 and 18 months of age were 51 and 31% of those at 6 months of age. Pituitaries in 12- and 18-month-old females contained more PRL and PRL mRNA than those at 6 months of age. These increases in PRL and PRL mRNA contents may result from the increase in the number of PRL cells, although each PRL cell had less PRL and PRL mRNA. These results suggest that the age-related changes in pituitary GH and PRL levels occurred at the transcription level of GH and PRL syntheses, and that the age-related changes in GH-cell and PRL-cell populations are responsible for the changes in GH and PRL secretions.