Abstract
The effect of sustained Gz acceleration ("high G") upon endocrine function has been investigated in man using a human centrifuge. Five male volunteers were exposed to accelerations of up to 6Gz for one minute without G protection. No effect of acceleration was observed on the peripheral blood levels of growth hormone, prolactin, thyrotropin and the gonadotrophins. In contrast, cortisol levels increased in a "dose-related" manner following acceleration i.e. the higher the acceleration, the greater the response. The unusual specificity of these endocrine changes is discussed. In a second experiment on the same subjects exposed to 6Gz for one minute, the modification of this cortisol response by two methods of G protection, the anti-G suit and the reclined seat, was investigated. Noradrenaline, adrenaline and arginine vasopressin were also measured and shown to increase rapidly after acceleration. These endocrine changes were all reduced by the anti-G suit but reductions were not clearly observed with seat reclination until an angle of 80° from the vertical was reached. The mechanisms underlying the release of these hormones are discussed in terms of the circulatory disturbances produced by acceleration. Finally, the endocrine effects of positive pressure breathing (PPB), with and without counterpressure (CP) have been compared with the use of an anti-G suit alone in subjects exposed to +6Gz for one minute. Arterial oxygen saturation (SaO[2]) was monitored continuously by ear oximetry. SAO[2] fell during +Gz acceleration with only anti-G suit protection but this was ameliorated significantly by PPB. Further benefit was obtained with CP. No differences in endocrine responses were observed between these three conditions which suggested that hypoxia was not the stimulus for hormonal responses to high G. This assertion was confirmed by the failure of hypoxia, induced by breathing a gas mixture of low oxygen content, to induce hormonal changes in the subjects.