Testing of neuroendocrine function in astronauts as related to fluid shifts

We addressed the question of optimal conditions for neuroendocrine and cardiovascular testing in astronauts. We tested stress reactions during LBNP of < or = -50 mmHg. There was a mild transient elevation of plasma GH concentration and a nonsignificant rise of plasma ACTH, while PRL, insulin and glucose remained unchanged. Aldosterone was decreased 5 and 10 min after beginning of LBNP, thereafter rose significantly, and displayed further significant concentration increase 5 min post-LBNP. The endocrine and cardiovascular responses to submaximal exercise were tested at 8.00 am and 8.00 pm. Exercise-induced changes of heart rate and blood pressure remained unchanged with daytime whereas plasma concentrations of epinephrine, GH and PRL in response to work load were significantly higher in the evening than in the morning. As expected, basal resting values of plasma cortisol were significantly lower in the evening than in the morning but were similar one hour after cessation of exercise. Our findings demonstrate the importance of frequent sampling in case of transient physiological phenomena, and contribute to existing knowledge on circadian influences upon neuroendocrine stress responses.