Growth Hormone Secretory Patterns in Young, Middle-Aged and Old Female Rats

Abstract

Pulsatile growth hormone (GH) secretion was compared in young (5 months), middle-aged (11 months) and old (25–29 months) female Sprague-Dawley rats under nonanesthetized, free-moving conditions. Mean plasma GH levels were 99.1 ± 9.3 ng/ml in young rats, 56.3 ± 5.8 ng/ml in middle-aged rats and 49.7 ± 4.9 ng/ml in old rats (p < 0.01 for young vs. middle-aged and old rats). In young females, 10 out of 17 rats had GH pulses with peak levels greater than 200 ng/ml, in 6 middle-aged females all GH peaks were below 200 ng/ml, and in old females 13 out of 17 rats showed GH peaks of less than 100 ng/ml. The average peak (amplitude) of GH pulses in the old rats (69.3 ± 8.3 ng/ml) was lower than in the young rats (130.4 ± 17.5 ng/ml, p < 0.01) and somewhat lower than in the middle-aged rats (87.0 ± 8.9 ng/ml). There was no change in intervals between GH pulses. Pituitary GH content in middle-aged and old females (1,189 ± 60 and 1,100 ± 89 µg, respectively) was significantly lower (p < 0.05 and p < 0.01, respectively) than in young female rats (1,464 ± 76 µg). Somatostatin content in the median eminence of old rats (22.4 ± 1.9 ng) was significantly lower than in young rats (28.5 ± 1.6 ng, p < 0.05). It is concluded that GH secretion is reduced in aging female rats, but unlike in aging male rats the decrease is seen at an earlier age. This may account for the relatively early body growth stasis observed in female but not in male rats. Among the possible causes for the decline in GH secretion with aging in these rats are an increase in somatostatin release and a decrease in GHRH release, associated with a reduction in hypothalamic catecholamine activity.