Daily exercise and gender influence arterial baroreflex regulation of heart rate and nerve activity

C. Y. Chen, S. E. DiCarlo
American Journal of Physiology - Heart and Circulatory Physiology Published 1 November 1996 Vol. 271 no. 5, H1840-H1848

  Before this study, groups had found that exercise training lead to differences in renal sympathetic nerve activity and heart rate, but information was lacking in other nerves. The previous studies had also used either all male or mixed males and female rats, but they hadn't looked specifically at males vs females despite the fact that there were some known differences between the sexes in terms of heart rate during changes in posture.
   Rats were allowed 8-9 weeks of sedentary conditions vs daily spontaneous running (DSR, just like our wheel runners) before being fitted with chronic catheters to allow drug infusion and monitor blood pressure and a chronic electrode to record lumbar sympathetic nerve activity (LSNA). Two days after surgery, the venous catheters were used to infuse PE or nitroglycerin to raise or lower blood pressure, respectively, by 25mmHg. Curves were constructed by comparing the drug-induced changes in blood pressure vs heart rate (HR) and LSNA, compared to resting values.
  DSR increased heart to body weight in both males and females, and caused a resting bradycardia in males but not females. Exercise reduced the maximum HR and its range (maximum minus minimum HR) in females, and the maximum and minimum HR, but not the range.  This corresponded with a DSR-induced reduction in maximum HR gain in females but not males. For LSNA, both males and female had an attenuation in maximum nerve activity and its range, and a corresponding decrease in maximum gain.
  I guess this all means that exercise training lets your body meet physiological demand without doing as much "work" in terms of HR or LSNA, which makes sense. There seems to be a disconnect between nerve activity and heart rate between the sexes though, because there were differences between male and female rats in HR but not LSNA. But trying to get at what those differences might be is an entirely different field with entirely different models. - DJH