CATECHOLAMINERGIC EFFECTS OF PROLONGED HEAD-DOWN BED REST

Prolonged head-down bed rest (HDBR) provides a model for examining res ponses to chronic weightlessness in humans. Eight healthy volunteers u nderwent HDBR for 2 wk. Antecubital venous blood was sampled for plasm a levels of catechols [norepinephrine (NE), epinephrine, dopamine, dih ydroxyphenylalanine, dihydroxyphenylglycol, and dihydroxyphenylacetic acid] after supine rest on a control (C) day and after 4 h and 7 and 1 4 days of HDBR. Urine was collected after 2 h of supine rest during da y C, 2 h before HDBR, and during the intervals 1-4, 4-24, 144-168 (day 7), and 312-336 h (day 14) of HDBR. All subjects had decreased plasma and blood volumes (mean 16%), atriopeptin levels (31%), and periphera l venous pressure (26%) after HDBR. NE excretion on day 14 of HDBR was decreased by 35% from that on day C, without further trends as HDBR c ontinued, whereas plasma levels were only variably and nonsignificantl y decreased. Excretion rates of dihydroxyphenylglycol and dihydroxyphe nylalanine decreased slightly during HDBR; excretion rates of epinephr ine, dopamine, and dihydroxyphenylacetic acid and plasma levels of cat echols were unchanged. The results suggest that HDBR produces sustaine d inhibition of sympathoneural release, turnover, and synthesis of NE without affecting adrenomedullary secretion or renal dopamine producti on. Concurrent hypovolemia probably interferes with detection of sympa thoinhibition by plasma levels of NE and other catechols in this setti ng. Sympathoinhibition, despite decreased blood volume, may help to ex plain orthostatic intolerance in astronauts returning from spaceflight s.