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Preserved Growth Hormone (GH) Secretion in Aged and Very Old Subjects after Testing with the Combined Stimulus GH-Releasing Hormone plus GH-Releasing Hexapeptide-6¹

Institute of Endocrinology, University Clinical Centre (D.M., V.P., M.D., D.M.), Belgrade Yugoslavia Y-11000; and Department of Physiology (C.D.) and Medicine (F.F.C.), School of Medicine, Santiago de Compostela University, Santiago de Compostela, Spain E-15780

Address all correspondence and requests for reprints to: F. F. Casanueva, P.O. Box 563, Santiago de Compostela, Spain E-15780. E-mail: meffcasa{at}uscmail.usc.es

Either spontaneous or pharmacological stimulated GH secretion is reduced with advanced age. This observation is an added difficulty for the biochemical diagnosis of GH deficiency in adults. Furthermore, the combined administration of saturating doses of GH-releasing hormone (GHRH) plus GH-releasing hexapeptide (GHRP)-6 is nowadays the most effective GH-releasing stimulus tested in a variety of settings related to altered somatotroph function. To understand whether the GH discharge elicited by the combined stimulus declines with age, 26 normal subjects of both sexes, divided into 3 age groups [adults 19–40 yr; aged 46–65 yr; and very old (75–96 yr) subjects] were studied. They were administered iv, as bolus and in combination, 90 µg GHRH plus 90 µg GHRP-6.

In the three groups, the combined administration of GHRH plus GHRP-6 elicited a GH area under the curve (µg/L per 120 min) of 3,127 ± 262, 3,409 ± 573, and 4,655 ± 737 for adults, aged, and very old subjects, respectively (nonsignificant differences). The mean GH peak was 47.5 ± 4.5 µg/L for adults, 52.9 ± 8.4 µg/L for aged subjects, and 76.0 ± 11.7 for very old subjects (nonsignificant differences). Individually examined, there were no nonresponders to the combined stimulus, and all subjects (independently of age) showed a GH peak over 25 µg/L (the lowest peak was 27.3 µg/L, and the highest peak was 119.2 µg/L).

In conclusion, the GHRH plus GHRP-6-induced GH release is well preserved in aged and very old subjects, which suggests that the GH secretory capability of the combined test is not reduced by age. This combined test may be useful for the diagnosis of GH-deficient states in adults.

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