The effects of hindlimb unweighting on the capacitance sf rat small mesenteric veins

Microgravity is associated with an impaired cardiac output response to orth ostatic stress. Mesenteric veins are critical in modulating cardiac filling through venoconstriction. The purpose of this study was to determine the e ffects of simulated microgravity on the capacitance of rat mesenteric small veins. We constructed pressure-diameter relationships from vessels of 21-d ay hindlimb-unweighted (HLU) rats and control rats by changing the internal pressure and measuring the external diameter. Pressure-diameter relationsh ips were obtained both before and after stimulation with norepinephrine (NE ). The pressure-diameter curves of HLU vessels were shifted to larger diame ters than control vessels. NE (10(-4) M) constricted veins from control ani mals 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, unstre ssed vascular volume may be increased and can no longer decrease in respons e to sympathetic stimulation. This may partially underlie the mechanism lea ding to the exaggerated fall in cardiac output and stroke volume seen in as tronauts during an orthostatic stress after exposure to microgravity.