Endocrine response to acute changes of brain blood flow due to lower body negative pressure in man

The decrease of brain flow due to lower body negative pressure (LBNP) was used to study the role of cerebral glucopenia in the activation of anterior pituitary function in 10 young healthy men. During LBNP with a maximum negative pressure of--50 mm Hg the changes in heart rate, blood pressure cerebral blood flow (with the aid of rheoencephalography) and electrocardiogram were recorded and the levels of hGH, ACTH, hPRL, insulin and cortisol were measured with the aid of radioimmunoassay. During the first investigation an infusion of 20% glucose (1 g per min for first 30 min and 0.5 g per min for next 30 min) was used, while during the second investigation (one week later) the infusion of saline was applied. It was found that the infusion of glucose prevented the increase of hGH which was observed only during the infusion of saline. No differences in the level of ACTH, cortisol and hPRL were observed between the investigations with glucose and saline infusion. The increased level of insulin was related to the level of glucose during the infusion, while no changes were found during saline infusion. It was suggested that the increase of hGH level during LBNP resulted from glucopenia in cerebral tissue.