Running Title: Simulated Microgravity Alters Arterial Vasoconstriction Correspondence to:

Previous studies have shown that hindlimb unweighting (HU) of rats, a model of microgravity, reduces evoked contractile tension of peripheral conduit arteries. It has been hypothesized that this diminished contractile tension is the result of alterations in the mechanical properties of these arteries (e.g., active and passive mechanics). Therefore, the purpose of this study was to determine whether the reduced contractile force of the abdominal aorta from 2-wk HU rats results from a mechanical function deficit resulting from structural vascular alterations or material property changes. Aortas were isolated from control (C) and HU rats and vasoconstrictor responses to norepinephrine (NE, 10-9 10-4 M) and arginine vasopressin (AVP, 10-9 10-5 M) were tested in vitro. In a second series of tests, the active and passive Cauchy stress-stretch relations were determined by incrementally increasing the uniaxial displacement of the aortic rings. Maximal Cauchy stress in response to NE and AVP were less in aortic rings from HU rats. The active Cauchy stress-stretch response indicated that although maximum stress was lower in aortas from HU rats (C, 8.1 ± 0.2 kPa; HU, 7.0 ± 0.4 kPa), it was achieved at a similar hoop stretch. There were also no differences in the passive Cauchy stress