Diminished Pituitary Responsiveness to Growth Hormone-Releasing Factor in Aging Male Rats*

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Abstract
The pattern of GH secretion undergoes substantial changes in the aging rat, resulting in decreased daily secretion of GH. In this study, the pituitary responsiveness to GH-releasing factor (GRF) was examined in young (2- to 5-month old) and aging (14- to 18-month old) male rats. In vivo studies were performed under sodium pentobarbital anesthesia. After injection of 250 ng GRF/100 g BW, young rats experienced more GH secretion [peak level, 544.5 .+-. 209.5 (.+-. SEM) ng/ml] than did 18-month-old rats (89.3 .+-. 13.7 ng/ml). To investigate the locus of this insensitivity to GRF, anterior pituitary cells from young and aging rats were dispersed and placed in primary culture. While basal GH secretion from the cultured pituitary cells was similar in the two groups (49.7 .+-. 3.5 vs. 47.8 .+-. 2.7 ng/ml .cntdot. 4 h for the 2- and 18-month old rats, respectively), the GH-releasing ability of GRF was partially but significantly impaired in cells derived from both 14- and 18-month old rats; 100 nM GRF stimulated the release of 96.7 .+-. 1.8 ng/ml .cntdot. 4 in the 18-month old rats as opposed to 115.0 .+-. 6.0 (P < 0.05) ng/ml .cntdot. 4 h in the 2-month-old rats. Since GRF stimulates GH release through the activation of adenylate cyclase, intracellular cAMP levels were measured in the cultured pituitary cells. GRF stimulated 65% less intracellular cAMP accumulation in the 18-month-old rats. In 14-month-old rats, the ability of forskolin and (Bu)2 cAMP to release GH was impaired, while phorbol ester-elicited GH secretion was unchanged. In conclusion, the GH response to GRF is blunted in aging rats. While much of the insensitivity to GRF may be mediated by the increased somatostatin tone reported in aging rats, a diminished pituitary cAMP response to GRF may also be an important etiological factor in the hyposomatotropinemia of the aging male rat.