RESUMO
The aim of the present study was to further elucidate the mechanisms of vascular adaptation to microgravity and its gravity-based countermeasure by a biomechanical approach. Active (the dissected vessel segment was superfused with PPS) and passive (while it was superfused with Ca(2+)-free PPS) biomechanical properties of mesenteric third-order small arteries and middle cerebral arteries isolated from 3-day simulated microgravity (SUS), countermeasure (STD, daily 1 h of -G(x) gravitation), and control (CON) groups of rats were studied. The following mechanical parameters were calculated: the overall stiffness parameter of passive vessels (beta), circumferential stress (sigma(theta))-strain (epsilon(theta)) relationship, and pressure-dependent incremental elastic modulus (E(inc,p)) of both active and passive vessels, and vascular smooth muscle (VSM) activity-dependent incremental modulus (E(inc,a)). Results from the analysis of active biomechanical properties revealed the contribution of vascular smooth muscle (VSM) tone during the early adaptation to microgravity: (1) For mesenteric small arteries, active circumferential sigma(theta) -epsilon(theta) curve of SUS group was comparable with that of the passive vessels, indicating that the function of VSM to restore the normal stress distribution is compromised; however, this mal-adaptation was fully prevented by the countermeasure of daily 1 h of -G(x) gravitation; (2) For the middle cerebral arteries, active circumferential sigma(theta) -epsilon(theta) relation of SUS group was shifted to the left side of the passive curve and epsilon(theta) was kept at a nearly constant level with the corresponding sigma(theta) being at its normal range; furthermore, the enhanced myogenic tone responsiveness was not prevented by daily short-duration -G(x). Analysis of the passive biomechanical properties has suggested remodeling changes in matrix components of different types of vessels, which might be significant if the exposure duration was further prolonged. In brief, studies of vascular biomechanics are of particular importance in elucidating the mechanisms underlying vascular adaptation to microgravity and its gravity-based countermeasure.