Effects if weightlessness simulated by a 24-hour recumbency on autonomic nervous system in man

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Orthostatic intolerance and increases in heart rate and vasoconstrictive response to orthostatic stress observed after a space flight has been explained by circulating blood volume loss. However, it is also possible that changes in arterial baroreflex sensitivity or target organ sensitivities to sympathetic stimulation account for these abnormalities. To investigate the mechanisms for the abnormal responses to orthostatic stress after a space flight, we studied effects of a 24hour recumbency on the arterial baroreflex sensitivity and catecholamine sensitivities of the heart and vessels in 5 healthy subjects. After a 24-hour recumbency heart rate and forean-n vasoconstrictor responses to lower body negative pressure were increased compared to those before the recumbency. The body weight was decreased by recumbency, suggesting loss of circulating blood volume. The arterial baroreflex sensitivity decreased whereas heart rate and vasoconstrictor responses to exogenous catecholamines were increased. These results suggest that although a d in blood volume primarily accounts for orthostatic intolerance after a space flight, increased hemodynamic responses to orthostatic stress are resulted from an increase in catecholamine sensitivity of target organs in addition to blood volume loss. The decrease in arterial baroreflex sensitivity may counteract the hemodynamic hyperactivities.