Influences of chemical sympathectomy, demedullation, and hindlimb suspension on the V̇O₂max of rats

Results from previous studies have shown that the reduction in maximal oxygen consumption (V̇O₂max) with simulated microgravity is attenuated in chemically sympathectomized rats. To determine the contributions of the catecholamines from the adrenal medulla in this process, investigations were conducted with 65 saline injected (SAL) and chemically sympathectomized (SX) female rats that were either surgically demedullated (DM), or intact (IN). Microgravity conditions were simulated by head-down suspension (HDS) while controls were assigned to individual cages (CC). The experimental period was 14 d. The rats were tested for V̇O₂max, treadmill run time (RT), and submaximal mechanical efficiency (ME) prior to suspension and on days 7 and 14. Saline injected rats that had intact adrenal medullas (SAL-IN) exhibited significantly reduced measures of V̇O₂max after 7 and 14 d by 15% and 21%, respectively. No significant reduction in V̇O₂max was observed with HDS in the SX-IN animals. Sympathectomized rats that were demedullated (SX-DM) also exhibited a significant reduction in V̇O₂max (12%). In addition, HDS was associated with a marked and significant reduction in RT in all groups. ME for submaximal exercise was significantly reduced after HDS in SAL-IN rats but not in the SX-IN rats. SX-DM rats experienced significant reductions in ME similar in magnitude to the SAL-IN rats. These results confirm that chemical sympathectomy attenuates the expected decrease in V̇O₂max with HDS and suggests that circulating epinephrine contributes to this response.