Spectral analysis was recently chosen to characterize the fast oscillations depending on the activity of the sympathetic and parasympathetic nervous systems. Renin stimuli could impinge on different, i.e. low frequency domain since the time lag to renin-angiotensin system activation is significantly larger. This experimental study was designed to analyze low frequency components of short-term variability of blood pressure (BP) of conscious rats in two conditions where renin was activated. First, a combined blockade of autonomic nervous system and circulating vasopressin was achieved with i.v. chlorisondamine plus a selective vascular V1 receptor antagonist. This treatment determined a 4.2-fold increase in plasma renin activity. Spectral powers of the systolic and diastolic BP and heart rate (HR) were computed in the high (respiratory), mid (0.2-0.6 Hz, MF, Mayer waves mainly of sympathetic origin) and low (0.02-0.2 Hz, LF) frequency bands, as detected by the Fast Fourier Transform technique on consecutive 102 s stationary periods. The blockade resulted in a significant decrease in the MF (sympathetic) power of BP (−53%, P < 0.001) and HR (−69%, P < 0.001). A significant increase in the LF component of BP variability was observed (+43%, P < 0.001). ACE inhibition (enalaprilate i.v.) abolished this component. Secondly, we studied a model of renovascular hypertension (two-kidney Goldblatt hypertension) after 3 weeks and we also observed a marked LF component of BP variability when hypertensive rats were compared to sham operated animals (+73%). Interestingly, losartan i.v. markedly reduced this LF component. Losartan also induced an increase in the MF oscillations of BP and HR in the two groups. A reflex sympathetic activation secondary to All antagonism could contribute to the observed effects on MF oscillations.
Thus, an increase in the LF component of BP variability was observed when sympathetic activity was reduced with autonomic blockade and in a model of renin-dependent hypertension. The dependence of these 20 sec period oscillations upon the renin-angiotensin system activity was demonstrated with pharmacological blockade with enalapril or losartan.