Mechanisms of vascular smooth muscle hyporesponsiveness after simulated microgravity

Exposure to microgravity causes cardiovascular deconditioning in spaceflown astronauts, and upon return these astronauts develop orthostatic intolerance. One mechanism thought to underlie orthostatic intolerance is vascular hyporesponsiveness, a consequence of both microgravity and simulated microgravity. The goal of the present study was to explore the alterations to vascular smooth muscle that may contribute to vascular hyporesponsiveness. Microgravity was simulated by hindlimb unweighting (HU) male Wistar rats for 20 days. Following the 20 days, abdominal aorta, carotid artery, and femoral arteries were collected and subjected to several analyses including: western blot, quantitative real time RT-PCR (Q-RTPCR), DNA quantification, contractile studies, length-force relationships, and vessel morphometry. In the abdominal aorta, HU decreased the levels of the contractile proteins alpha actin, myosin light chain 20 (MLC-20), and myosin heavy chain (MHC) while increasing levels of the actin-binding protein, heavy caldesmon and also voltage-operated calcium channels (VOCC). HU decreased mRNA expression for MLC-20, but not for MHC or alpha actin. Contractile responses to K+ and calcium were found to be reduced by HU in the abdominal aorta. In the carotid artery, HU decreased levels of MLC-20 and p38 MAP kinase (p38MAPK) but increased levels of c-src and ERK112. HU also decreased compliance of the carotid artery, and also contractile responses to norepinephrine (NE) in endothelium-intact carotid artery rings. In the femoral artery, HU decreased levels of MLC-20, MHC, and p38MAPK and also decreased contractile responses to K+ and NE. In the abdominal aorta the HU-induced decrease in alpha actin, MLC-20, and MHC along with the increase in the actin-binding protein, heavy caldesmon, could contribute to the vascular hyporesponsiveness of that artery. In the femoral artery however, it appears that the decrease in myosin coupled with the decrease in p38MAPK may contribute to the vascular hyporesponsiveness. In the carotid artery, it appears that there are alterations that have opposing results. HU-induced decreases in MLC-20 and p38MAPK may decrease contraction, but the HU-induced increases in c-src and ERK112 coupled with the decreased compliance in that vessel, may explain why there is no observed hyporesponsiveness in the endothelium-denuded carotid artery.