APPLICATION OF ACUTE MAXIMAL EXERCISE TO PROTECT ORTHOSTATIC TOLERANCE AFTER SIMULATED MICROGRAVITY

We tested the hypothesis that one bout of maximal exercise performed a t the conclusion of prolonged simulated microgravity would improve blo od pressure stability during an orthostatic challenge. Heart rate (HR) , mean arterial blood pressure (MAP), norepinephrine (NE), epinephrine (E), arginine vasopressin (AVP), plasma renin activity (PRA), atrial natriuretic peptide (ANP), cardiac output (Q over dot), forearm vascul ar resistance (FVR), and changes in leg volume were measured during lo wer body negative pressure (LBNP) to presyncope in seven subjects imme diately prior to reambulation from 16 days of 6 degrees head-down tilt (HDT) under two experimental conditions: 1) after maximal supine cycl e ergometry performed 24 h before returning to the upright posture (ex ercise) and 2) without exercise (control). After HDT, the reduction of LBNP tolerance time from pre-HDT levels was greater (P = 0.041) in th e control condition (-2.0 +/- 0.2 min) compared with the exercise cond ition (-0.4 +/- 0.2 min). At presyncope after HDT, FVR and NE were hig her (P < 0.05) after exercise compared with control, whereas MAP, HR, E, AVP, PRA, ANP, and leg volume were similar in both conditions. Plas ma volume (PV) and carotid-cardiac baroreflex sensitivity were reduced after control HDT, but were restored by the exercise treatment. Maint enance of orthostatic tolerance by application of acute intense exerci se after 16 days of simulated microgravity was associated with greater circulating levels of NE, vasoconstriction, Q over dot, baroreflex se nsitivity, and PV.