Strain Differences in Baroreflex Inhibition by Centrally Infused Enalapril in Old Rats

To determine whether inhibition of the brain renin-angiotensin system would affect baroreflexes similarly in old rats of different strains, we compared 24-month-old male Fischer 344 and Sprague-Dawley rats. Baroreflex sensitivity was tested while the rats were awake by recording reflex heart rate responses elicited as blood pressure was elevated with phenylephrine or lowered with sodium nitroprusside. Sprague-Dawley rats had higher blood pressures and lower heart rates initially. Chronic infusion of enalapril, a converting enzyme inhibitor, into a lateral cerebral ventricle (ICV) for two weeks lowered blood pressure in Sprague-Dawley but not in Fischer 344 rats. Furthermore, reflex bradycardia was unaffected in either rat strain, but reflex tachycardia was selectively suppressed in Fischer 344 rats. Thus, although time controls were not done to rule out spontaneous changes during the 14-day infusion period, these results suggest that central cardiovascular regulation does not change similarly with age in these two rat strains. As removal of the brain renin-angiotensin system lowered blood pressure in one strain and inhibited reflex tachycardia in the other, the divergence could mean that the brain renin-angiotensin system acts differently to keep blood pressure elevated in Sprague-Dawley rats and modulate reflex tachycardia in Fischer 344 rats.