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1 Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas, TX, USA
2 Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas, Dallas, TX, USA; Department of Internal Medicine, University of Texas Southwestern Medical Center, Dallas, TX, USA
* To whom correspondence should be addressed. E-mail: craigcrandall{at}texashealth.org.
This study tested the hypothesis that head-down tilt bed rest (HDBR) reduces adrenergic and non-adrenergic cutaneous vasoconstrictor responsiveness. Additionally, an exercise countermeasure group was included to identify if exercise during bed rest might counteract any vasoconstrictor deficits that arose during HDBR. Twenty-two subjects underwent 14 days of strict 6° HDBR. Eight of these 22 subjects did not exercise during HDBR, while 14 of these subjects exercised on a supine cycle ergometer for 90 minutes a day at 75% of pre-bed rest heart rate maximum. To assess 
-adrenergic vasoconstrictor responsiveness, intradermal microdialysis was used to locally administer norepinephrine (NE), while forearm skin blood flow (SkBF; laser-Doppler flowmetry) was monitored over the microdialysis membranes. Nonlinear regression modeling was used to identify EC50 and minimum values from the SkBF/NE dose-response curves. In addition, the effects of HDBR on non-adrenergic cutaneous vasoconstriction were assessed via the venoarteriolar response of the forearm and leg. HDBR did not alter the EC50 or the magnitude of cutaneous vasoconstriction to exogenous NE administration regardless of whether the subjects exercised during HDBR. Moreover, HDBR did not alter the forearm venoarteriolar response in either the control or exercise groups during HDBR. However, HDBR significantly reduced the magnitude of cutaneous vasoconstriction due to the venoarteriolar response in the leg, and this response was similarly reduced in the exercise group. These data suggest that HDBR does not alter cutaneous vasoconstrictor responses to exogenous NE administration, whereas cutaneous vasoconstriction of the leg due to the venoarteriolar response is reduced following HDBR. It remains unclear whether attenuated venoarteriolar responses in the lower limbs contribute to reduced orthostatic tolerance following bed rest and space flight.
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