Differential effects of enalapril and hydralazine on short-term variability of blood pressure and heart rate in rats.

Using a spectral procedure, we studied the acute and chronic effects of enalapril and hydralazine on the variability of blood pressure (BP) and heart rate (HR) in conscious Wistar rats. In the acute protocol, rats received two injections 25 min apart (saline followed by enalaprilic acid or hydralazine hydrochloride). In the chronic protocol, animals received oral enalapril maleate, hydralazine hydrochloride, or distilled water. A 5-min recording session was initiated on day 12. Acute enalapril and hydralazine amplified the low-frequency (LF) component of the systolic BP (SBP) spectrum. Chronic enalapril reduced the variability of BP, as indicated by the lower variance in SBP distribution. Chronic enalapril preferentially reduced the amplitude of the 400-mHz oscillations of SBP. Acute administration of enalapril or hydralazine resulted in BP variability profiles, suggesting a reflexly mediated vascular sympathetic activation. In contrast, chronic angiotensin-converting enzyme (ACE) blockade with enalapril caused a significant decrease in the LF oscillations of BP. This could reflect a reduced sympathetic outflow to vascular smooth muscles.