| Abstract: | The pineal gland in humans is under both alpha- and beta-adrenergic control, although it seems that beta1-adrenoceptors are mainly implicated in melatonin secretion. In the present study, we evaluated the role of beta-adrenergic innervation on melatonin production and its relation with the production of growth hormone (GH). Thirty-four children (15 males and 19 females, mean age 10.5 +/- 0.8 years) from the University of Granada Hospital were studied. The children were included in a protocol for the evaluation of growth delay using the propranolol + exercise test. This standardized test allowed us to study simultaneously the role of an unspecific beta-adrenergic blocker such as propranolol and of an adrenergic stimulus such as exercise on the pineal production of melatonin. Changes in plasma levels of melatonin and GH were determined at basal, 120 and 140 min after the test was applied. Hormonal determinations were carried out by commercial radioimmunoassay kits previously standardized in our laboratory. The results show a significant decrease in plasma melatonin levels at 120 and 140 min after the test (P < 0.05), whereas GH levels increased significantly at 140 min (P < 0.001). The decrease of melatonin levels was a consequence of the test, since in a control group, the circadian decay of melatonin was significantly less pronounced (P < 0.05). These data suggest an inverse relationship between melatonin and GH after the propranolol + exercise test, and the reduction in melatonin may be related to its depletion by exercise-induced oxidative stress. |
