[16]

EFFECTS OF HINDLIMB UNWEIGHTING ON THE MECHANICAL AND STRUCTURE PROPERTIES OF THE RAT ABDOMINAL AORTA.  A. Papadopoulos and M.D. Delp.  Departments of Health and Kinesiology and Medical Physiology, Texas A&M University, College Station, TX.

   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. Both active and passive Cauchy stress-stretch relations were determined by incrementally increasing the uniaxial displacement of the aortic rings. There were no differences in the passive Cauchy stress-stretch response or gross vascular morphology (e.g., medial cross-sectional area: CON, 0.30 ± 0.02 mm²; HU, 0.32 ± 0.01 mm²) between groups.  In addition, there were no differences in resting or basal vascular tone at the displacement which elicits peak developed tension (Lmax) between groups (resting tension: CON, 1.71 ± 0.06 g; HU, 1.78 ± 0.14 g). The active Cauchy stress-stretch response indicated that although maximum stress was lower in aortas from HU rats (CON, 8.13 ± 0.24 kPa; HU, 7.02 ± 0.44 kPa), it was achieved at a similar uniaxial displacement and stretch. These results indicate that HU does not alter the functional mechanical or gross structural properties of conduit arteries. However, the significantly lower active Cauchy stress by HU aortas confirms a contractile deficit in these arteries. (Supported by NASA grant NAG2-1340 and NSBRI grant NCC9-58-42)