Attenuated baroreflex control of sympathetic nerve activity after cardiovascular deconditioning in rats

J. A. Moffitt, C. M. Foley, J. C. Schadt, M. H. Laughlin, E. M. Hasser
American Journal of Physiology - Regulatory, Integrative and Comparative Physiology Published 1 May 1998 Vol. 274 no. 5, R1397-R1405

  This study used male rats to measure the effect of 14 days of hindlimb unloading (HU) on mean arterial pressure (MAP) vs heart rate(HR) , renal sympathetic nerve activity (RSNA), and lumbar sympathetic nerve activity (LSNA - controls hindlimb skeletal muscle) by intravenously infusing PE and SNP. This gave them the ability to construct baroreflex curves covering roughly 45-170mmHg MAP for each animal. The curve parameters for each animal were then averaged to make group curves and compared between HU and control rats.
  Similar to the other study on HU I blogged (which was done by some of the same people, after this study), HU rats had decreased body weight and soleus muscle mass. They found that HU also caused a resting tachychardia. However,  there were no significant differences in resting MAP, or maximum/minimum HR.
  LSNA was greatly attenuated in the HU group. Where control rats showed a LSNA increase to 350% of baseline at MAP of ~40mmHg, HU rats were only able to increase LSNA to 200%. the midpoint MAP was not different between groups, and the minimum LSNA at high MAP for both groups was at ~0%. This produced a significant reduction in gain as well. When viewed as a percent of maximal nerve activity, HU rats showed a significantly higher resting LSNA than controls, suggesting a reduction in response range rather than a reduction in absolute nerve activity.
  The results of looking at the RSNA were pretty similar to those of the LSNA; reduced maximum as a % of baseline, reduced gain, and a higher resting RSNA (as a percentage of the maximum). However, minimum RSNA was significantly lower in HU rats, where it wasn't in terms of LSNA.
  The take-home message here is that lack of regular exercise, or deconditioning, causes increases in resting SNA at the level of the kidney and skeletal muscle, and that leads to a reduction in the reserve SNA needed to meet the demands of decreases in MAP. -DJH