Effect of simulated microgravity on sympathetic outflow to the mesentery during an orthostatic challenge

Female astronauts suffer from orthostatic hypotesion upon return to a normal gravitational field. Recent work from our laboratory has shown that female rats exposed to 2 weeks of simulated microgravity (hindlimb unloading; HU) also experience orthostatic hypotension during an active orthostatic challenge (head‐up tilt; HUT; i.e., volunatary mounting of a 45° slope). The simulated microgravity abolishes a reflex mesenteric vasoconstriction that occurs in response to HUT, in part due to a decreased sensitivity of mesenteric arteries to adrenergic stimulation. We examined if HU also attenuates sympathetic outflow to the mesentery induced by HUT. Virgin female rats were chronically instrumented with recording electrodes (Telemetry Research) on a nerve fiber branching from the coelic ganglion and innervating the mesentery, as well as a blood pressure transducer (DSI). Nerve activity, mean arterial pressure (MAP) and heart rate ( f H ) responses to HUT were measured prior to, immediately following and 3 h post‐HU. Compared to pre‐HU, HUT immediately following HU induced increased sympathetic outflow to the mesentery in both amplitude and frequency, whereas MAP and f H responses were similar (immediate and sustained increases). Thus, increased nerve activity compensated for deficient mesenteric vascular constriction. However, by 3 h post‐HU, the increased nerve activity was attenuated and the tachycardia less. We propose that an inability to maintain a compensatory increase in sympathetic outflow to the mesentery in face of an impaired end organ response to adrenergic stimulation contributes to orthostatic hypotesion in females following simulated microgravity. AHFMR