Contribution of growth hormone-releasing hormone and somatostatin to decreased growth hormone secretion in elderly men

Objective. The pathophysiology of the decline in circulating growth hormone (CH) concentrations that may occur with ageing remains elusive. We have in vestigated the potential contributions of decreased endogenous GH-releasing hormone (GHRH) and increased somatostatin secretion to this phenomenon. Design and methods. The strategy used was to stimulate GH secretion in 8 yo ung (20 - 24 years old, body mass index (BMI) 22.8 +/- 2.8 kg/m(2)) and 8 e lderly (68 - 82 years old, BMI 23.4 +/- 1.6 kg/m2) male subjects on separat e occasions by means of: (i) intravenous bolus 0.5 mug/kg D-Ala(2) GHRH(1-2 9)-NH2 alone; (ii) 0.5 mug/kg GHRH after pre-treatment with two oral doses of 50 mg atenolol (to inhibit somatostatin secretion); (iii) 1.25 mg oral b romocriptine alone (to increase endogenous GHRH and/or inhibit somatostatin ); (iv) 50 mg oral atenolol plus 1.25 mg oral bromocriptine; and (v) 0.5 mu g/kg GHRH after pre-treatment with 1.25 mg oral bromocriptine. Results. The elderly men had a significantly lower peak and area under curv e (AUC) GH response to intravenous GHRH when compared with 8 young men (pea k 3.1 +/- 1.0 ng/ml v. 21.6 +/- 5.0 ng/ml, AUC 205 +/- 56 ng/ml/min v. 1 31 5 +/- 295 ng/ml/min, P < 0.05). Pre-treatment with atenolol before GHRH adm inistration produced no significant increase in peak and AUC GH response in both groups, which remained lower in the elderly men than in their young c ounterparts (peak 5.5 <plus/minus> 1.8 ng/ml v. 29.3 +/- 7.0 ng/ml, AUC 327 +/- 90 ng/ml/min v. 2 017 +/- 590 ng/ml/min, P < 0.05). Bromocriptine alon e did not cause a significant rise in GH concentration in either elderly or young subjects (peak 3.1 <plus/minus> 1.1 v. 8.8 +/- 3.2 ng/ml, P > 0.05). When atenolol was administered before bromocriptine, both groups responded but the elderly subjects had a significantly greater peak and AUC response (peak 3.6 +/- 0.7 v. 10.7 +/- 2.1 ng/ml; AUC 191 +/- 39 v. 533 +/- 125 ng/ ml/min, P < 0.05). Bromocriptine given before GHRH failed to potentiate GHR H action on GH release in either group. Of 5 elderly men who underwent furt her evaluation of GH secretory ability, 2 subjects had GH levels > 10 ng/ml , either basally or after intravenous GHRH. The remaining 3 had an initiall y impaired GH response to bolus intravenous GHRH. After 100 mug GHRH subcut aneously twice daily for up to 2 weeks the GH responses to intravenous bolu s GHRH (0.5 mug/kg) were reassessed. One exhibited a normal response (> 10 ng/ml) after 1 week of daily GHRH treatment, another had a near-normal resp onse after 2 weeks (9.7 ng/ml), while the third still had an impaired respo nse by the end of the 2-week treatment period (3.2 ng/ml). Conclusions. The restoration of endogenous GH secretion in these elderly su bjects by means of GHRH priming, and the failure of manipulation of somatos tatinergic tone to restore a normal GH response to GHRH suggests that somat otroph atrophy due to a reduction in endogenous GHRH secretion is the princ ipal cause of the diminished GH secretion with ageing.