Sympathetic rhythms during hyperventilation-induced apnea

Impact of the vagal feedback on cardiorespiratory coupling in anesthetized rats

Cardiorespiratory coupling can be significantly influenced by both pontine and vagal modulation of medullary motor and premotor areas. We investigated influences of the pontine intertrigeminal region (ITR) and peripheral vagal pathways on the coupling between systolic blood pressure (SBP) and respiration in 9 anesthetized rats. Glutamate injection into the ITR perturbed both respiration and SBP and decreased SBP-respiratory coherence (0.95±0.01 vs 0.89±0.02; (p=0.01). Intravenous infusion of serotonin (5-HT) produced apnea and hypertension and also decreased SBP-respiratory coherence (0.95±0.01 vs 0.72±0.06; p=0.04). Bilateral vagotomy eliminated the cardiorespiratory coherence perturbations induced by central (glutamate injection into the ITR: 0.89±0.03 vs 0.86±0.03; p=0.63) and peripheral (5-HT infusion: 0.89±0.03 vs 0.88±0.02; p=0.98) pharmacologic manipulations. Glutamate stimulation of the ITR postvagotomy increased the relative spectral power density of SBP in the respiratory frequency range (0.25±0.08 vs 0.55±0.06; p=0.01). The data suggest that SBP-respiratory coupling is largely mediated within the central nervous system, with vagal systems acting in a way that disrupts coherence during transient cardiorespiratory disturbances. Although decreased cardiorespiratory coherence may increase cardiac work during perturbations, this may be physiologically advantageous in restoring homeostatic equilibrium of respiration and blood pressure.

Central respiratory modulation of barosensitive neurones in rat caudal ventrolateral medulla

The sympathetic nerves that maintain blood pressure are modulated by the central respiratory generator. Neurones in the rostral ventrolateral medulla (RVLM) that drive this sympathetic nerve activity (SNA) also display central respiratory drive (CRD)-related activity, suggesting integration of respiratory and cardiovascular regulatory systems within the brainstem. Whether CRD-related activity in the RVLM is due to direct inputs from central respiratory neurones or modulation of cardiovascular-related neurones that influence the RVLM is not known. The caudal ventrolateral medulla (CVLM) contains GABAergic neurones that tonically inhibit presympathetic RVLM neurones and are essential for the production of numerous cardiovascular reflexes. The present study sought to determine whether cardiovascular-related GABAergic neurones in the CVLM display CRD-related activity. The firing patterns of individual barosensitive CVLM neurones were examined in relation to phrenic nerve activity in chloralose-anaesthetized, ventilated, neuromuscularly blocked, vagotomized rats. Histograms of phrenic-triggered CVLM neuronal activity showed that all baro-activated CVLM neurones displayed one of four patterns of CRD-related activity: (i) inspiratory peak (n = 15), (ii) inspiratory depression (n = 15), (iii) inspiratory peak with postinspiratory depression (n = 10), and (iv) postinspiratory peak (n = 9). A subset of each type of CVLM neurone was identified as GABAergic by individually filling the recorded neurone with biotinamide and observing expression of GAD67 mRNA by in situ hybridization (n = 10). These data suggest that the activity of GABAergic neurones in the CVLM is regulated by cardiovascular and respiratory inputs, and baro-activated GABAergic CVLM neurones may contribute to CRD-related modulation of presympathetic RVLM neurones and SNA.

Assessment of cardiac sympathetic regulation by respiratory-related arterial pressure variability in the rat

1. Mechanical ventilation evokes a corresponding arterial pressure variability (APV) which is decreased by beta-adrenoceptor antagonism. Therefore, in this study we set out to determine whether the respiratory-related APV can be used to assess cardiac sympathetic tone. 2. Computer-generated broad-band mechanical ventilation (0-3 Hz) was applied to Sprague-Dawley rats that had been anaesthetized with ketamine and paralysed with pancuronium. APV and its relationship to lung volume variability (LVV-APV) was systematically quantified with auto- or cross-spectral frequency domain analysis. 3. APV and LVV-APV transfer magnitudes between 0.5 and 1.5 Hz showed dose-dependent suppression by propranolol from 0.01 to 1 mg kg-1, while the static value of arterial pressure remained unchanged. Stroke volume variability, assessed by the use of a pulse contour method, exhibited a similar pattern of suppression by propranolol. In contrast, heart rate variability was not lowered with propranolol. 4. The effect of propranolol on respiratory-related APV persisted even in the presence of combined alpha-adrenoceptor and muscarinic receptor blockade by phentolamine and atropine. 5. The frequency range of 0.5-1.0 Hz was optimal for LVV-APV transfer magnitude to correlate with cardiac sympathetic tone. 6. We conclude that respiratory-related APV may provide a valid assessment of cardiac sympathetic regulation which is independent of parasympathetic and vascular sympathetic influences in ketamine-anaesthetized and positive pressure-ventilated rats.

Properties of the inspiration-related activity of sympathetic preganglionic neurones of the cervical trunk in the cat

1. The experiments reported here have examined some temporal characteristics of the inspiration-related sympathetic discharge of the cat in control conditions and during forcing of the respiratory oscillator into marked deviations from its natural frequency. The purpose of these experiments was to establish whether or not the relation of sympathetic to phrenic nerve activity shows properties consistent with the hypothesis that the inspiration-related sympathetic discharge is driven by a neural oscillator, independent of, but coupled and stably entrained to, the brain-stem respiratory oscillator. 2. The electrical activity of the whole cervical sympathetic trunk (n = 26) or of small strands of the cervical trunk containing single units (n = 20) and of the phrenic nerve was recorded in pentobarbitone-anaesthetized, paralysed, artificially ventilated, sino-aortic denervated cats. Most of the cats were bilaterally vagotomized. 3. The onset of the inspiratory burst of the sympathetic preganglionic neurones had a fixed delay from the onset of the phrenic nerve burst. The level of activity within the burst, in whole cervical trunk recording, reached a maximum in early inspiration and then was maintained at approximately this level for the rest of inspiration (twenty-two out of twenty-six cats). In four cats the activity level increased throughout the burst. Individual sympathetic preganglionic neurones displaying inspiration-related burst firing were characteristically recruited in early inspiration and thereafter maintained an approximately constant firing frequency for the rest of inspiration. 4. Electrical stimulation of afferents in the superior laryngeal nerve during various phases of the respiratory cycle caused equivalent, phase-dependent, resetting patterns of both phrenic nerve and inspiration-related sympathetic discharge. 5. In cats with intact vagus nerves, entrainment of the brain-stem respiratory oscillator to the frequency of the respiratory pump was used to change the frequency of the former, within limits, by changing the frequency of the latter. Over the range of frequencies tested, the pump-to-phrenic delay varied as a function of frequency, while the delay between phrenic and sympathetic burst onset was essentially independent of frequency. 6. In hyperthermic, hypocapnic cats phrenic nerve burst frequency increased up to about 300 bursts/min from a value of 15 bursts/min in normothermia-normocapnia. At all frequencies within this range the sympathetic burst maintained a delay, with respect to the phrenic burst, which was essentially independent of frequency.(ABSTRACT TRUNCATED AT 400 WORDS)