GENDER DIFFERENCES IN ENDOCRINE RESPONSES TO POSTURE AND 7 DAYS OF -6-DEGREES HEAD-DOWN BED REST

Endocrine regulation of fluids and electrolytes during 7 days of -6-de grees head-down bed rest (HDBR) was compared in male (n = 8) and, for the first time, female (n = 8) volunteers. The subjects' responses to quiet standing for 2 h before and after HDBR were also tested. In both sexes, diuresis and natriuresis were evident during the first 2-3 day s of HDBR, resulting in a marked increase in the urinary Na+-to-K+ rat io and significant Na+ retention on re-ambulation. After the lst day o f HDBR, plasma renin activity (PRA) was increased relative to aldoster one (Aldo), plasma volume was decreased, and the renal response to Ald o appeared to be appropriate. Circulating levels of arginine vasopress in, cortisol, and ACTH were unchanged during HDBR. Plasma testosterone decreased slightly on day 2 of HDBR in males. The ratio of early morn ing ACTH to cortisol was lower in females than in males because ACTH w as lower in females. Urinary cortisol increased and remained elevated throughout the HDBR in males only. There were no gender differences in the responses to 7 days of HDBR, except those in the pituitary-adrena l system; those differences appeared unrelated to the postural change. The provocative cardiovascular test of quiet standing before and afte r HDBR revealed both sex differences and effects of HDBR. There were s ignificant sex differences in cardiovascular responses to standing bef ore and after HDBR. Females had greater PRA and Aldo responses to stan ding before HDBR and larger Aldo responses to standing after HDBR than males. Cardiovascular responses to standing before and after HDBR dif fered markedly; arterial pressure and heart rates increased with stand ing before HDBR; by contrast, after HDBR, arterial pressure decreased, with greater increases in heart rates. In both sexes, all hormonal re sponses to standing were greater after HDBR. The results show clearly that similar responses to standing and HDBR occur in both sexes but th at females exhibit greater PRA and Aldo responses than males.