The Nocturnal Increase in Growth Hormone Is not the Cause of the Nocturnal Increase in Serum Osteocalcin*

Osteoblast activity, as measured by the serum osteocalcin (OC) concentration, increases at night in both normal women and women with postmenopausal osteoporosis. Because there is also a physiological increase in GH during deep sleep and the nocturnal increases in serum OC and GH concentrations both become less pronounced with advancing age, we used somatostatin (SMS) infusion to evaluate the effect of suppression of the nocturnal GH increase on the expected increase in serum OC. Serum OC levels were measured during an 8-h iv infusion of SMS and during a similar infusion of isotonic saline (control night) in five young women (mean age +/- SE, 21 +/- 2 yr). SMS significantly (P less than 0.05) decreased secretion of endogenous GH. The mean increases in serum OC concentration were similar during SMS infusion and control nights. Furthermore, the nocturnal pattern of increase in serum OC levels during GH or saline infusions did not differ. From these data, we conclude that the nocturnal increases in serum OC and GH levels are not related.