Catecholamine response to maximal exercise following 16 days of simulated microgravity
Engelke, K.A.; Convertino, V.A.; Convertino, V.A.
Aviation Space and Environmental Medicine 67(3): 243-247
1996
ISSN/ISBN: 0095-6562 PMID: 8775402 Document Number: 467969
Introduction: We tested the hypothesis that the elevated peak heart rate (HR) response to maximal exercise following microgravity exposure is associated with increased plasma levels of catecholamines. Methods: To do this, plasma norepinephrine (NE) and epinephrine (E) were measured in venous blood samples obtained from 7 subjects before and immediately after graded supine cycle exercise to volitional fatigue performed prior to and at the conclusion of 16 d of 6 degree head-down tilt (HDT). Results: Resting HR was increased (p = 0.014) following HDT (56 +- 3 to 63 +- 2 bpm), but pre-exercise NE concentration (217 +- 18 pg cntdot ml-1) was not different (p = 0.224) from that measured before H DT (183 +- 23 pg cntdot ml-1). Peak exercise HR was greater (p = 0.001) after simulated microgravity exposure compared to pre-HDT (181 +- 3 bpm vs. 172 +- 4 bpm). NE measured at volitional fatigue was 64% higher (p = 0.004) after HDT compared to that measured prior to HDT (1337 +- 181 vs. 2191 +- 189 pg cntdot ml-1 for pre- and post-HDT, respectively). Plasma E concentration at exhaustion was unchanged by microgravity exposure. Conclusions: Our observation of greater levels of plasma NE and peak exercise HR suggests that sympathetic influences on cardiac chronotropic control may be altered by exposure to prolonged microgravity.