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 NIF R-R interval fluctuations and also contribute significantly to MF MAP fluctuations.