Sympathetic, parasympathetic and non-autonomic contributions to cardiovascular spectral powers in unanesthetized spontaneously hypertensive rats

Objective: To determine whether spectral powers of blood pressure and pulse interval can specifically reflect sympathetic and parasympathetic effects in unanesthetized, free-moving spontaneously hypertensive rats (SHR). Design: Spectral powers were observed before and after various autonomic interventions in chronically instrumented rats. Materials and methods: Chemical sympathectomy was produced in 12-week-old SHR by repeated injections of 6-hydroxydopamine, while control rats were given vehicle alone. Chronic arterial and venous catheters were inserted in the femoral artery and vein. Blood pressure was recorded beat-to-beat for 90 min in free-moving rats; further recording sessions were obtained under additional a-receptor blockade with phenoxybenzamine at 1 mg/kg and/or additional cholinergic blockade with atropine at 0.8 mg/kg. Off-line computer analysis (fast Fourier transform) provided estimates of low- (0.025-0.1 Hz), mid- (0.1-0.6 Hz) and high-frequency (0.8-3.0 Hz) powers for blood pressure and pulse interval over consecutive periods of 100 s. Results: The most noticeable findings were that sympathectomy produced a striking increase in the low-frequency power of blood pressure and a tendency (borderline statistical significance) to reduce the mid-frequency power of blood pressure. Additional a-receptor blockade had no effect on any spectral power whereas additional cholinergic blockade caused a further increase in the low-frequency blood pressure power and a drastic reduction in all pulse interval powers. Conclusions: In the unanesthetized SHR, sympathetic activity opposes low-frequency and marginally promotes mid-frequency blood pressure fluctuations; the pulse interval spectral expression of vagal effects is spread throughout the range of frequencies explored and is not confined to the high-frequency band. These data indicate that in SHR no spectral power can specifically reflect the effects of either autonomic limb.