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1 Department of Biomedical Engineering and 2 Department of Anesthesiology and Critical Care Medicine, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205
Microgravity is associated
with an impaired cardiac output response to orthostatic stress.
Mesenteric veins are critical in modulating cardiac filling through
venoconstriction. The purpose of this study was to determine the
effects of simulated microgravity on the capacitance of rat mesenteric
small veins. We constructed pressure-diameter relationships from
vessels of 21-day hindlimb-unweighted (HLU) rats and control rats by
changing the internal pressure and measuring the external diameter.
Pressure-diameter relationships were obtained both before and after
stimulation with norepinephrine (NE). The pressure-diameter curves of
HLU vessels were shifted to larger diameters than control vessels. NE
(10
4 M) constricted veins from control animals such that
the pressure-diameter relationship was significantly shifted downward
(i.e., to smaller diameters at equal pressure). NE had no effect on
vessels from HLU animals. These results indicate that, after HLU,
unstressed vascular volume may be increased and can no longer decrease
in response to sympathetic stimulation. This may partially underlie the
mechanism leading to the exaggerated fall in cardiac output and stroke
volume seen in astronauts during an orthostatic stress after exposure
to microgravity.
microgravity; pressure-diameter relationships; sympathetic nervous system
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