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PÄIVI TAPANAINEN, MIKAEL KNIP, PENTTI LAUTALA, JUHANI LEPPÄLUOTO, Variable Plasma Growth Hormone (GH)-Releasing Hormone and GH Responses to Clonidine, L-Dopa, and Insulin in Normal Men, The Journal of Clinical Endocrinology & Metabolism, Volume 67, Issue 4, 1 October 1988, Pages 845–849, https://doi.org/10.1210/jcem-67-4-845
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Abstract
The effects of synthetic GHRH-(l-44) (1 μg/kg, iv), clonidine (0.15 mg/m2, orally), l-dopa (0.5 g, orally), and insulin (0.1 IU/kg, iv) on plasma immunoreactive (ir) GHRH and GH levels were determined in normal men, aged 31–46 yr (n = 4–8). In addition, plasma ir-GHRH and GH concentrations were determined before and after the administration of clonidine in six younger men, aged 19–25 yr. GHRH was extracted from plasma using Sep-Pak C18 cartridges and measured with a midportion-specific GHRH antiserum. The mean plasma ir-GHRH and GH levels ranged from 9–11 ng/L and 0.5–1.5 μg/L, respectively, in the older men during a 2-h control study. After GHRH administration, the mean plasma ir-GHRH concentration increased to a peak of 512.5 ng/L at 3 min and GH to a peak of 9.2 μg/L at 10 min. Clonidine resulted in a significant increase in mean plasma GH levels (P < 0.05) in the younger men, but not in the older men. Plasma ir-GHRH concentrations did not change after clonidine. l-Dopa increased plasma ir-GHRH at 60 min (P < 0.05) and GH at 60–120 min (P < 0.05). Insulininduced hypoglycemia increased plasma GH levels (to a mean of 23.8 μg/L at 60 min; P < 0.001), whereas plasma ir-GHRH levels did not change. We conclude that the mechanisms of the various GH stimulation tests differ. Some GH responses, including those induced by insulin, do not appear to be mediated by GHRH.
