{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,8,27]],"date-time":"2026-08-27T13:11:24Z","timestamp":1787836284756,"version":"build-2784847793"},"reference-count":35,"publisher":"American Physiological Society","issue":"3","content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["American Journal of Physiology-Heart and Circulatory Physiology"],"published-print":{"date-parts":[[2003,9]]},"abstract":"<jats:p>Although orthostatic hypotension is a common clinical syndrome after spaceflight and its ground-based simulation model, 6\u00b0 head-down bed rest (HDBR), the pathophysiology remains unclear. The authors' hypothesis that a decrease in sympathetic nerve activity is the major pathophysiology underlying orthostatic hypotension after HDBR was tested in a study involving 14-day HDBR in 22 healthy subjects who showed no orthostatic hypotension during 15-min 60\u00b0 head-up tilt test (HUT) at baseline. After HDBR, 10 of 22 subjects demonstrated orthostatic hypotension during 60\u00b0 HUT. In subjects with orthostatic hypotension, total activity of muscle sympathetic nerve activity (MSNA) increased less during the first minute of 60\u00b0 HUT after HDBR (314% of resting supine activity) than before HDBR (523% of resting supine activity, P &lt; 0.05) despite HDBR-induced reduction in plasma volume (13% of plasma volume before HDBR). The postural increase in total MSNA continued during several more minutes of 60\u00b0 HUT while arterial pressure was maintained. Thereafter, however, total MSNA was paradoxically suppressed by 104% of the resting supine level at the last minute of HUT ( P &lt; 0.05 vs. earlier 60\u00b0 HUT periods). The suppression of total MSNA was accompanied by a 22 \u00b1 4-mmHg decrease in mean blood pressure (systolic blood pressure &lt;80 mmHg). In contrast, orthostatic activation of total MSNA was preserved throughout 60\u00b0 HUT in subjects who did not develop orthostatic hypotension. These data support the hypothesis that a decrease in sympathetic nerve activity is the major pathophysiological factor underlying orthostatic hypotension after HDBR. It appears that the diminished sympathetic activity, in combination with other factors associated with HDBR (e.g., hypovolemia), may predispose some individuals to postural hypotension.<\/jats:p>","DOI":"10.1152\/ajpheart.00965.2002","type":"journal-article","created":{"date-parts":[[2015,3,3]],"date-time":"2015-03-03T20:11:53Z","timestamp":1425413513000},"page":"H1158-H1167","source":"Crossref","is-referenced-by-count":41,"title":["Pathophysiology of orthostatic hypotension after bed rest: paradoxical sympathetic withdrawal"],"prefix":"10.1152","volume":"285","author":[{"given":"Atsunori","family":"Kamiya","sequence":"first","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Daisaku","family":"Michikami","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Qi","family":"Fu","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Satoshi","family":"Iwase","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Junichiro","family":"Hayano","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Toru","family":"Kawada","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Tadaaki","family":"Mano","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Kenji","family":"Sunagawa","sequence":"additional","affiliation":[],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"24","reference":[{"key":"REF1","doi-asserted-by":"publisher","DOI":"10.1016\/0094-5765(80)90043-0"},{"key":"REF2","doi-asserted-by":"publisher","DOI":"10.1152\/jappl.1996.81.1.7"},{"key":"REF3","doi-asserted-by":"publisher","DOI":"10.1152\/ajpregu.1994.266.6.R1962"},{"key":"REF4","doi-asserted-by":"publisher","DOI":"10.1113\/jphysiol.2001.012574"},{"key":"REF5","doi-asserted-by":"crossref","unstructured":"Eckberg DLand Sleight P.Human Baroreflexes in Health and Disease. 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