Foley CM, Mueller PJ, Hasser EM, Heesch CM.
Am J Physiol Regul Integr Comp Physiol. 2005 Nov;289(5):R1440-7.
In this paper, they looked at the effect of simulated microgravity on renal sympathetic nerve activity (RSNA) and heart rate (HR) and the differences between male and female rats. It was already known that after either circumstance, people were generally less physiologically able to respond to the demands of even simple tasks, like standing up (i.e. they demonstrated orthostatic intolerance) and that women were more likely to have problems than men. This study tried to examine some of the causes of that phenomenon since the effects had been seen, but the causes weren't well demonstrated. They did this by partially suspending rats so that their back legs couldn't touch the ground - in essence, putting the rats through this hindlimb unloading (HU) made them "weightless" for 2 weeks.
Aside from significant reductions in the mass of muscles associated with running, the HU rats had higher resting mean arterial pressure (MAP) and HR compared to controls after they were returned to the ground. They also had a greater maximum HR on the MAP vs HR curve (tested via SNP and PE infusion). When they looked at the baroreflex curves of RSNA, they found that there were reductions in the maximum RSNA during decreases in MAP as a result of both being female and HU, with HU females having the least responsiveness to demand for RSNA. The midpoint of the curves also slightly shifted right (p=0.06), a result of increased in function when there may be no real need for it at higher MAPs.
The takehome message of this paper is a very translational approach to the kind of studies we do - life under different conditions can radically alter the function of the sympathetic nervous system and your ability to respond appropriately to physiological demand for more or less SNA, and these changes may be affect one sex more than the other. -DJH
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