Effects of age on GH responses to growth hormone-releasing factor in rats

This paper aims to investigate the growth hormone (GH) responses to the growth hormone releasing factor-44 (GRF-44) in young and adult rats in vitro and in vivo. Male Wistar rats aged 28 days and 6 months were used. In pentobarbital anesthetized young rats, an intravenous administration of GRF-44 at a dose of 25 ng/100 g b.w. resulted in an 11.4 fold rise in the serum GH level at 5 min after GRF-44 injection. On the other hand, in adult rats the same dose of GRF-44 did not elicit a significant rise in the serum GH level. However, GH secretion in response to GRF-44 at a dose of 250 ng/100 g b.w. was not attenuated in adult rats. Isolated pituitary cells obtained from young and adult rats were incubated for 3 days. The addition of GRF-44 at a concentration of 5 ng/ml to the cultured pituitary cells from young rats caused a 3.2 fold increase in GH content in the medium. On the other hand, the same dose of GRF-44 added to the cultured pituitary cells from adult rats resulted in only a 2.3 fold increase in GH content. These data indicate that the sensitivity of somatotrophs to GRF may be diminished with age.