Autonomic and ventilatory components of heart rate and blood pressure variability in freely behaving rats

The relative role of parasympathetic, sympathetic, and ventilatory influences in the genesis of blood pressure and R-R interval variability is controversial. In 13 freely behaving WKY rats instrumented with venous and arterial catheters and chest electrodes, mean arterial pressure (MAP, mmHg), R-R interval (ms), and respiratory fluctuations were monitored for 90 min in the control condition and after intravenous atropine (0.75 mg/kg) and/or propranolol (1 mg/kg). Spectral power (pw) in the 0.25- to 0.75-Hz (midfrequency, MF) and the 0.75- to 3.0-Hz (high-frequency, HF, respiratory- synchronous) bands was computed in sequences of 400 heartbeats by use of a combined autoregressive analysis. Atropine reduced but did not abolish HF R- R interval pw (from 1.73 ± 0.50 to 0.39 ± 0.27 ms ², P <0.01) and halved HF MAP pw (from 0.41 ± 0.30 to 0.21 ± 0.12 mmHg ², P <0.05), whereas propranolol did not affect HF pw of the R-R interval or MAP. Propranolol also failed to significantly modify MF R-R interval pw (from 0.48 ± 0.44 to 0.40 ± 0.34 ms ², P = NS) or MF MAP pw (from 0.54 ± 0.39 to 0.42 ± 0.20 mmHg ², P = NS), whereas atropine virtually abolished MF R-R interval pw (from 0.48 ± 0.44 to 0.01 ± 0.01 ms ², P <0.01) and also significantly reduced MF MAP pw (from 0.54 ± 0.39 to 0.33 ± 0.24 mmHg ², P <0.01). The effects of combined blockade were similar to those of atropine alone. It is concluded that, in the unanesthetized rat, efferent vagal influences are responsible for a large fraction of HF R-R interval power, but a sizable amount of such fluctuations persists after atropine and has a ventilatory, rather than an efferent vagal, origin. Vagal influences also contribute to HF MAP power. Vagal (but not sympathetic) influences are important determinants of MF R-R interval fluctuations and also contribute significantly to MF MAP fluctuations.