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[Differential effect of simulated microgravity on myogenic tone of middle cerebral and mesenteric small arteries in rats].

The aim of the present study was to investigate the effect of a short-term (3-day) simulated microgravity with and without daily dorsoventral gravitation (-G(x)) for 1 h on myogenic tone and vasoconstrictor responsiveness of the middle cerebral artery and mesenteric third-order small artery in rats. The tail-suspension (SUS) model was used to simulate cardiovascular deconditioning due to microgravity. Daily restoring to normal standing (STD) posture for 1 h was adopted to provide -G(x) as the countermeasure. Segments of middle cerebral artery and mesenteric third-order small artery were isolated and cannulated. Vascular diameters in response to increased intraluminal pressure (from 20 mmHg to 120 mmHg, by 20 mmHg steps) of isolated arteries under no-flow conditions were recorded by a Pressure Myograph System in both physiologic salt solution (PSS) (active diameter, Da) and calcium-free PSS (passive diameter, Dp). The myogenic tone was calculated by (Dp-Da)/Dpx100%. Vasoconstrictor responsiveness of the isolated middle cerebral artery to serotonin and that of small mesenteric artery to phenylephrine were assessed in the PSS under an intraluminal pressure of 40 mmHg. The results showed that SUS induced an enhancement of the myogenic tone and vasoconstrictor responsiveness in the isolated middle cerebral artery but a depression of those in the small mesenteric artery. Daily STD for 1 h prevented the depression of myogenic tone and vasoconstrictor responsiveness in the small mesenteric artery, but did not prevent the functional enhancement in the middle cerebral artery. These data suggest that a short-term simulated microgravity may result in different alterations in the function of the cerebral artery and the resistance vessel in the hind-body. Moreover, only the decrease of function in these resistance vessels, not in the cerebral arteries, can be prevented by such a countermeasure of daily STD for 1 h.