CO2-dependent component of the neurogenic vascular tone in the cat

Integrated cardiorespiratory system model with short timescale control mechanisms

A compartmental model of the cardiorespiratory system featuring pulsatile blood flow and gas transport, as well as closed loop mechanisms of cardiorespiratory regulation is presented. Short timescale regulatory action includes baroreflex, peripheral and central chemoreflex feedback. The cardiorespiratory model is composed by compartments to describe blood flow and gas exchange in the major systemic and pulmonic regions. The control systems include formulations to afferent activity of arterial baroreceptor and peripheral and central chemoreceptors. Simulations described here include situations of hypoxia, hypercapnia, and hemorrhage. The overall responses of our simulations agree with physiological (experimental) and theoretical data. Our results suggest that the present model could be used to further understand the interplay among major regulatory mechanisms in the functioning of the cardiovascular and respiratory systems in cases of normal and abnormal physiological conditions.

A comprehensive cardiopulmonary simulation model for the analysis of hypercapnic respiratory failure

We developed a new comprehensive cardiopulmonary model that takes into account the mutual interactions between the cardiovascular and the respiratory systems along with their short-term regulatory mechanisms. The model includes the heart, systemic and pulmonary circulations, lung mechanics, gas exchange and transport equations, and cardio-ventilatory control. Results show good agreement with published patient data in case of normoxic and hyperoxic hypercapnia simulations. In particular, simulations predict a moderate increase in mean systemic arterial pressure and heart rate, with almost no change in cardiac output, paralleled by a relevant increase in minute ventilation, tidal volume and respiratory rate. The model can represent a valid tool for clinical practice and medical research, providing an alternative way to experience-based clinical decisions.

A mathematical model of CO2 effect on cardiovascular regulation

The effect of changes in arterial CO2 tension on the cardiovascular system is analyzed by means of a mathematical model. The model is an extension of a previous one that already incorporated the main reflex and local mechanisms triggered by O2 changes. The new aspects covered by the model are the O2-CO2 interaction at the peripheral chemoreceptors, the effect of local CO2 changes on peripheral resistances, the direct central neural system (CNS) response to CO2, and the control of central chemoreceptors on ventilation and tidal volume. A statistical comparison between model simulation results and various experimental data has been performed. This comparison suggests that the model is able to simulate the acute cardiovascular response to changes in blood gas content in a variety of conditions (normoxic hypercapnia, hypercapnia during artificial ventilation, hypocapnic hypoxia, and hypercapnic hypoxia). The model ascribes the observed responses to the complex superimposition of many mechanisms simultaneously working (baroreflex, peripheral chemoreflex, CNS response, lung-stretch receptors, local gas tension effect), which may be differently activated depending on the specific stimulus under study. However, although some experiments can be reproduced using a single basal set of parameters, reproduction of other experiments requires a different combination of the mechanism strengths (particularly, a different strength of the local CO2 mechanism on peripheral resistances and of the CNS response to CO2). Starting from these results, some assumptions to explain the striking differences reported in the literature are presented. The model may represent a valid support for the interpretation of physiological data on acute cardiovascular regulation and may favor the synthesis of contradictory results into a single theoretical setting.

Organization of the sympathetic innervation of the forelimb resistance vessels in the cat

Detailed information on the outflow pathway of sympathetic vasoconstrictor fibers to the upper extremity is lacking. We studied the organization of the sympathetic innervation of the forelimb resistance vessels and of the sinoatrial (SA) node in the decerebrated, artificially respirated cat. The distal portion of sectioned individual rami T1-8 and the sympathetic chain immediately caudal to T8 on the right side were electrically stimulated while the right forelimb perfusion pressure (forelimb perfused at constant flow) and heart rate were recorded. Increases in perfusion pressure were evoked by stimulation of T2-8 (maximal response T7: 55 +/- 2.3 mm Hg). Responses were still evoked by stimulation of the sympathetic chain immediately caudal to T8 (44 +/- 15 mm Hg). Increases in heart rate were evoked by the stimulation of more rostral rami (T1-5; maximal response T3: 55.2 +/- 8 bpm). These vasoconstrictor and cardioacceleratory responses were blocked by the cholinergic antagonists hexamethonium and scopolamine. Sectioning of the vertebral nerve and the T1 ramus abolished the vasoconstrictor response. Stimulation of the vertebral nerve and of the proximal portion of the sectioned T1 ramus increased perfusion pressure (69 +/- 9 and 34 +/- 14 mm Hg, respectively), which was unaffected by ganglionic cholinergic block. These data suggest that forelimb resistance vessel control is subserved by sympathetic preganglionic neurons located mainly in the middle to caudal thoracic spinal segments. Some of the postganglionic axons subserving vasomotor function course through the T1 ramus, in addition to the vertebral nerve.

IMPLICATIONS

Forelimb vasculature is controlled by sympathetic preganglionic neurons located in middle to caudal thoracic spinal segments and by postganglionic axons carried in the T1 ramus and vertebral nerve. This helps to provide the anatomical substrate of interruption of sympathetic outflow to the upper extremity produced by major conduction anesthesia of the stellate ganglion or spinal cord.

Chemoreceptors and cardiovascular control in acute and chronic systemic hypoxia

This review describes the ways in which the primary bradycardia and peripheral vasoconstriction evoked by selective stimulation of peripheral chemoreceptors can be modified by the secondary effects of a chemoreceptor-induced increase in ventilation. The evidence that strong stimulation of peripheral chemoreceptors can evoke the behavioural and cardiovascular components of the alerting or defence response which is characteristically evoked by novel or noxious stimuli is considered. The functional significance of all these influences in systemic hypoxia is then discussed with emphasis on the fact that these reflex changes can be overcome by the local effects of hypoxia: central neural hypoxia depresses ventilation, hypoxia acting on the heart causes bradycardia and local hypoxia of skeletal muscle and brain induces vasodilatation. Further, it is proposed that these local influences can become interdependent, so generating a positive feedback loop that may explain sudden infant death syndrome (SIDS). It is also argued that a major contributor to these local influences is adenosine. The role of adenosine in determining the distribution of O2 in skeletal muscle microcirculation in hypoxia is discussed, together with its possible cellular mechanisms of action. Finally, evidence is presented that in chronic systemic hypoxia, the reflex vasoconstrictor influences of the sympathetic nervous system are reduced and/or the local dilator influences of hypoxia are enhanced. In vitro and in vivo findings suggest this is partly explained by upregulation of nitric oxide (NO) synthesis by the vascular endothelium which facilitates vasodilatation induced by adenosine and other NO-dependent dilators and attenuates noradrenaline-evoked vasoconstriction.

Stimulation of sympathetic activity by carbon dioxide in patients with autonomic failure compared to normal subjects

In vivo studies selectively assessing preganglionic and central autonomic nervous system activity in patients with autonomic failure have so far been limited to testing pituitary function. In animal experiments carbon dioxide (CO2) selectively stimulates central sympathetic nuclei in the ventrolateral medulla and preganglionic sympathetic neurons in the cervical trunk. This central stimulation seems to overrule less pronounced peripheral vasodilatatory effects. This study addressed the question of whether hypercapnea is a suitable challenge procedure to test preganglionic and central autonomic activity in healthy subjects and in patients with autonomic failure of preganglionic and central origin. Seven patients with multiple system atrophy (MSA) and 30 age-matched healthy volunteers underwent a protocol including a Valsalva manoeuvre (VM) under normo- and hypercapnic conditions and exposure to hypercapnea under supine resting conditions. Blood pressure (BP), heart rate (HR) and end-tidal CO2 partial pressure were measured continuously and non-invasively. In normal controls hypercapnea induced significantly higher BP values in phases II, IIe, III and IV of the VM compared to the normocapnic VM and a significant increase in BP during steady-state supine exposure compared to normocapnic baseline. HR increased significantly only after 40 s of steady-state hypercapnea during the latter challenge. In patients with MSA and autonomic failure, in whom a predominantly preganglionic lesion of the autonomic nervous system is established, no significant effects of hypercapnea on the cardiovascular parameters were found. Although this non-invasive challenge procedure cannot differentiate between pre- and postganglionic autonomic failure, exposure to hypercapnea enables the investigation of efferent autonomic activity to vasoconstrictors generated from autonomic centres in the brainstem and cervical trunk.

Local cerebral blood flow responses in rats to hypercapnia and hypoxia in the rostral ventrolateral medulla and in the cortex

The effects of hypercapnia and hypoxia on two local cerebral blood flows in the parietal cortex (PC-BF) and rostral ventrolateral medulla (RVLM-BF) were examined using laser Doppler flowmetry in anesthetized rats. Hypercapnia for 45 s duration at the end-tidal CO2 between 5% and 10%, induced by increasing inspiratory CO2, increased both cerebral blood flows and systemic blood pressure in a degree-dependent manner. The response of RVLM-BF was significantly stronger than that of PC-BF. Both cerebral blood flow responses to hypercapnia were not influenced by cutting peripheral chemoreceptor afferent nerves. Hypoxia for 45 s duration at the end-tidal O2 between 12% and 6%, induced by decreasing inspiratory O2, produced an increase of similar magnitude in both RVLM and PC local blood flows in a degree-dependent manner and a decrease in systemic blood pressure. The responses of both PC-BF and RVLM-BF to hypoxia were significantly diminished after cutting peripheral chemoreceptor afferent nerves. It is concluded that: (1) the RVLM-BF is much more sensitive to hypercapnia than the PC-BF; and (2) activation of peripheral arterial chemoreceptors possibly contributes to hypoxia-induced increase in the RVLM-BF and PC-BF.

Chemosensitivity of sympathoexcitatory neurones in the rostroventrolateral medulla of the cat

The hypothesis that sympathoexcitatory neurones within the rostroventrolateral medulla (RVLM) may be chemosensitive was tested in chloralose-anaesthetized cats by artificial perfusion of the RVLM via the left vertebral artery. The baroreceptors and peripheral chemoreceptors were denervated by bilaterally dissecting the carotid sinus and vagus nerves. Either white ramus T3 (WR-T3) or the renal nerve was recorded to monitor sympathetic activity. Perfusion with saline or Ringer solution bubbled with CO2 (10%-100%) produced a rapid and pronounced increase in sympathetic activity and blood pressure. Solutions adjusted to the same pH (pH 5.2 for 100% CO2) with HCl resulted in a much weaker excitation. A linear relationship between PCO2 and sympathetic activity was demonstrated. During prolonged perfusion (90 s) sympathetic activity returned to the control level after initial excitation and fell below control levels when perfusion ceased. The sympathetic activity response to CO2-bubbled solutions was unaffected by blockade of synaptic input by microinjection of CoCl2 into the RVLM, whereas spontaneous sympathetic activity and the supraspinal somato-sympathetic reflex from intercostal nerve T4 to WR-T3 were markedly reduced. It is therefore concluded that sympathoexcitatory bulbospinal neurones in the RVLM are directly chemosensitive to changes in arterial PCO2 and pH.

Depolarization and stimulation of neurons in nucleus tractus solitarii by carbon dioxide does not require chemical synaptic input

The effects of elevated CO2 (i.e. hypercapnia) on neurons in the nucleus tractus solitarii were studied using extracellular (n = 82) and intracellular (n = 33) recording techniques in transverse brain slices prepared from rat. Synaptic connections from putative chemosensitive neurons in the ventrolateral medulla were removed by bisecting each transverse slice and discarding the ventral half. In addition, the response to hypercapnia in 20 neurons was studied during high magnesium-low calcium synaptic blockade. Sixty-five per cent of the neurons (n = 75) tested were either insensitive or inhibited by hypercapnia. However, 35% (n = 40) were depolarized and/or increased their firing rate during hypercapnia. Nine out of 10 CO2-excited neurons retained their chemosensitivity to CO2 in the presence of high magnesium-low calcium synaptic blockade medium. Our findings demonstrate that many neurons in the nucleus tractus solitarii were depolarized and/or increased their firing rate during hypercapnia. These neurons were not driven synaptically by putative chemosensitive neurons of the ventrolateral medulla since this region was removed from the slice. Furthermore, because chemosensitivity persisted in most neurons tested during synaptic blockade, we conclude that some neurons in the nucleus tractus solitarii are inherently CO2-chemosensitive. Although the function of dorsal medullary chemosensitive neurons cannot be determined in vitro, their location and their inherent chemosensitivity suggest a role in cardiorespiratory central chemoreception.

Tissue oxygen partial pressure distribution within the human skeletal muscle during hypercapnia

In ten subjects CO2-inhalation elicited a significant increase in mean oxygen partial pressure within biceps muscle by more than 35%. Though mean oxygen partial pressure within biceps muscle increased, the distribution of oxygen partial pressure (pO2-histogram) did not change suggesting a physiological distribution of oxygen delivery within biceps muscle during hypercapnia. Buffering the blood pH did not abolish the effects of the CO2-inhalation. Therefore, a decrease of peripheral blood pH could not account for the hypercapnia induced increase of mean oxygen partial pressure within biceps muscle. Our data suggest that oxygen delivery to skeletal muscle was increased during hypercapnia, most probably due to a hypercapnia induced rise of mean capillary blood flow.

Effects of carbon dioxide on hindlimb vascular resistance in the acute spinal cat

A cervical (C2) spinal section was carried out in anaesthetized, neuromuscularly blocked cats and the vascularly isolated hindlimbs were independently perfused at constant flow with blood taken from the abdominal aorta. One leg was denervated by sectioning the lumbar sympathetic chain. The animals were hyperventilated in 100% O2 and 5% or 10% CO2 in O2 was administered without altering the rate or tidal volume of the respirator. Increasing paCO2 (mm Hg) from 16.36 +/- 0.84 to 37.48 +/- 1.03 and to 62.23 +/- 2.23, induced a significant early vasoconstriction (P1) followed by a later more prolonged vasoconstriction (P2) in the innervated leg, while only a significant P2 response was present in the sympathetically denervated leg. A significant increase in systemic arterial pressure (SAP) was also observed with no change in heart rate (HR). After bilateral adrenalectomy increasing paCO2 from 17.16 +/- 0.66 to 37.96 +/- 1.21 and to 64.20 +/- 1.55, induced smaller but significant P1 and P2 responses in the innervated leg but only a significant P2 response was induced in the denervated leg. These results suggest that the early vasoconstriction was mainly due to activation of lumbar sympathetic neurons, while the late vasoconstriction was caused by the release of adrenal catecholamines and possibly other unidentified vasoconstrictor substances.

Function of the ventrolateral medulla in the control of the circulation

The CNS control of the cardiovascular system involves the coordination of a series of complex neural mechanisms which integrate afferent information from a variety of peripheral receptors and produce control signals to effector organs for appropriate physiological responses. Although it is generally thought that these control signals are generated by a network of neural circuits that are widely distributed in the CNS, over the last two decades a considerable body of experimental evidence has accumulated suggesting that several of these circuits involve neurons found on or near the ventral surface of the medulla oblongata. Neurons in the VLM have been shown to be involved in the maintenance of vasomotor tone, in baroreceptor and chemoreceptor (central and peripheral) reflex mechanisms, in mediating the CIR and somatosympathetic reflexes and in the control of the secretion of vasopressin. These physiological functions of VLM neurons have been supported by neuroanatomical and electrophysiological studies demonstrating direct connections with a number of central structures previously implicated in the control of the circulation, including the IML, the site of origin of sympathetic preganglionic axons, and the SON and PVH, the site of origin of neurohypophyseal projecting axons containing AVP. Considerable suggestive evidence has also been obtained regarding the chemical messengers involved in transmitting information from VLM neurons to other central structures. There have been developments suggesting a role for monoamines and neuropeptides in mediating the neural and humoral control of SAP by neurons in the VLM. This review presents a synthesis of the literature suggesting a main role for VLM neurons in the control of the circulation.

Neural respiratory and circulatory interaction during chemoreceptor stimulation and cooling of ventral medulla in cats

The effects on respiratory and sympathetic neural activity, measured as integrated phrenic and cervical nerve activities respectively, during changing input from the central chemoreceptors was studied in anaesthetized, paralysed cats whose carotid sinus nerves and vagus nerves had been cut. Central respiratory drive was altered either by graded cold block of the intermediate areas, located bilaterally near the ventral surface of the medulla oblongata, or by step increases in end-tidal PCO2. Cervical nerve activity showed both a tonic (or mean) level of activity and a prominent cyclic discharge that was in phase with phrenic nerve activity. Graded focal cooling of the intermediate areas to 20 degrees C when end-tidal PCO2 was kept constant caused progressive decreases in phrenic activity, the amplitude of the inspiratory related discharge and mean arterial pressure, but only a small decrease in mean cervical nerve activity. Cooling the intermediate areas in the absence of the inspiratory related discharge (i.e. when phrenic activity was below the apnoeic threshold) led to a much smaller decrease in arterial pressure. Step increases of end-tidal PCO2 caused progressive increases of both cervical and phrenic nerve activities. The increase in cervical activity was due primarily, if not wholly, to a progressive increase in the amplitude of the inspiratory related discharge. These findings show that the predominant effect on sympathetic activity during stimulation of the central chemoreceptor and graded cold block of the intermediate areas is a change in the amplitude of the inspiratory related discharge and suggest that the change in arterial pressure that accompanies central chemoreceptor stimulation and graded cold block of the intermediate areas is mediated by the inspiratory related discharge rather than by an increase in the mean level of sympathetic activity. When phrenic activity was lowered to below apnoeic threshold by cooling the intermediate areas, step increases in end-tidal PCO2 caused inhibition rather than stimulation of cervical nerve activity. This finding indicates that sympathetic neurones are not activated by central chemoreceptor input directly, but rather indirectly via intracranial connexions with neuronal networks involved in regulation of respiration.

Regional hemodynamic effects of changes in PaCO2 in the vagotomized, sino-aortic de-afferented rat

Rats were anesthetized with urethane (1.5 g/kg i.p.) paralyzed with gallamine (3 mg/kg, i.v.), artificially ventilated and the vagi, carotid sinus and aortic nerves were cut. PaCO2 levels of 16.4 +/- 0.8, 23.3 +/- 1.6, 32.3 +/- 1.8 and 51.9 +/- 2.2 mm Hg were obtained by hyperventilation in 0%, 3%, 5% and 8% CO2 in O2, respectively. Radioactive microspheres labelled with 57Co or 113Sn were injected into the left ventricle and cardiac output and regional blood flows were determined by the 'arterial reference sample' method. Increasing PaCO2 induced an increase in systemic arterial pressure which was predominantly due to a significant increase in total peripheral resistance, while the increase in cardiac output was much less pronounced and no changes in heart rate were observed. The effect of increasing PaCO2 on regional vascular resistance (VR) was not uniformly distributed. CO2 induced a dilatation in the cerebral, bronchial and hepatic artery vascular beds. Coronary VR was not affected while vasoconstriction was induced by CO2 in the other vascular territories. This vasoconstriction was most significant in skeletal muscle, skin, pancreas, large intestine and kidneys. In most of these territories the vasoconstrictor effect of CO2 was observed at PaCO2 levels above 23.3 mm Hg, while between 16.4 and 23.3 mm Hg there was either no change or a decrease in VR. Propranolol and phentolamine (1 mg/kg and 10 mg/kg, i.v., respectively), which caused a 78% +/- 2% adrenergic blockade, significantly reduced the CO2 pressor and vasoconstrictor effects. Our experiments show that, after peripheral chemoreceptor denervation in the rat: (a) there is a direct relationship between PaCO2 and VR mediated by the sympathetic nervous system over the whole range of PaCO2 values from hypocapnia to hypercapnia, and (b) the various vascular territories contribute to a different extent to the CO2-induced changes in total peripheral resistance.

Abdominal vascular responses to changes in carbon dioxide tension in the cephalic circulation of anaesthetized dogs

Dogs were anaesthetized with chloralose, the regions of both carotid sinuses were vascularly isolated and perfused with arterial blood and both cervical vagosympathetic trunks were cut above the nodose ganglia. The cephalic circulation was perfused through the brachiocephalic and left subclavian arteries with blood which was equilibrated with various levels of CO2. The abdomen was vascularly isolated, perfused through the aorta at constant flow and drained through the inferior vena cava at constant pressure. Changes in vascular resistance were determined from changes in abdominal aortic perfusion pressure and changes in capacitance from the integral of the changes in venous outflow. An increase in PCO2 in the cephalic perfusate resulted in an increase in abdominal vascular resistance and a decrease in capacitance. However, when carotid sinus pressure was high, the response of resistance to an increase in cephalic PCO2 was abolished and that of capacitance was significantly reduced. The reflex responses of both vascular resistance and capacitance to a change in carotid sinus pressure were enhanced when the cephalic PCO2 was raised. However, the effect on the reflex capacitance response from stimulation of baroreceptors was obtained only when PCO2 was changed below 5 kPa whereas the effect on resistance occurred at higher values of PCO2. The interaction between the effects of changes in cephalic PCO2 and the carotid sinus reflex and the differential effect on resistance and capacitance vessels have been explained in terms of the known difference in the sensitivities of these vessels to sympathetic nerve activity.

Cardiac inotropic responses from changes in carbon dioxide tension in the cephalic circulation of anaesthetized dogs

Experiments were performed on anaesthetized dogs to determine the effects of moderate changes in PCO2 in the cephalic circulation on the inotropic state of the heart and on the reflex inotropic responses from changes in carotid sinus pressure. The cephalic circulation was perfused, through the brachiocephalic and left subclavian arteries, with blood taken from the superior vena cava and equilibrated with various gas mixtures in a gas exchange unit. The carotid sinus regions were vascularly isolated and perfused with arterial blood at controlled pressures. Cardiac inotropic responses were assessed from the maximum rate of change of left ventricular pressure (dP/dtmax) with heart rate and mean aortic pressure held constant. An increase in cephalic blood PCO2 resulted in an increase in dP/dtmax and an increase in the unpaced heart rate. Small, graded changes in cephalic PCO2 resulted in graded responses of dP/dtmax. A change in carotid sinus pressure resulted in a significantly greater response of dP/dtmax when cephalic PCO2 was high. After interruption of the left cardiac sympathetic nerves, the responses of dP/dtmax to changes in cephalic PCO2 and carotid sinus pressure were nearly abolished. These results indicate that the tension of carbon dioxide in the cephalic circulation is likely to be of importance in the control of the inotropic state of the heart. They also imply that, in studies of cardiovascular reflex responses, it is important to control the carbon dioxide tension in the arterial blood.

Pressor effect of CO2 in the rat: different thresholds of the central cardiovascular and respiratory responses to CO2

Rats were anesthetized with urethane and the vagi, aortic and carotid sinus nerves were sectioned bilaterally. Hypocapnia was induced by artificial hyperventilation with 100% O2. Administration of 5% CO2 in O2, without alteration of the respiratory rate or tidal volume, induced significant increases in mean systemic arterial pressure ( mSAP ) in rats with intact central nervous system (CNS) and after midcollicular section (36 +/- 4 and 34 +/- 2 mm Hg, respectively; mean +/- S.E.). Smaller but significant increases in mSAP (17 +/- 3 mm Hg) were induced by inhalation of 5% CO2 after section of the spinal cord at the C4 level. Ganglionic blockade with hexamethonium completely abolished the pressor response to CO2. In hypocapnic (paCO2 15.5 +/- 0.7 mm Hg) apneic rats with intact CNS, after denervation of the peripheral chemoreceptors, inhalation of 1.5% CO2 in O2 increased paCO2 to 22.3 +/- 1.2 mm Hg and mSAP by 16 +/- 1 mm Hg, but the animals remained apneic for up to 45 min of continuous CO2 administration. Higher FICO2s induced further immediate increases in mSAP and, after delays of up to 6-7 min, also a resumption of central rhythmic respiratory activity (monitored by the intercostal muscles or phrenic nerve electrogram). The paCO2 threshold for this respiratory response was 25 +/- 1 mm Hg. When rhythmic respiratory activity resumed, a further slight increase in mSAP and the appearance of respiratory modulated oscillations of the SAP were observed in most animals. When, after a period of CO2 inhalation, 100% O2 was again administered to the animals mSAP fell immediately, reaching the control level within 20-30 s, while the respiratory activity, if present, disappeared only after 1.5-2 min. From these experiments we conclude that in the hypocapnic rat, after denervation of the peripheral chemoreceptors: (1) CO2 induces a neurogenic hypertensive response even in the absence of rhythmic respiratory activity; (2) the central chemosensitive sites appear to be located in the ponto-medullary region and in the spinal cord; and (3) the central mechanisms responsible for the pressor response have a lower CO2 threshold and a much shorter latency than those responsible for the initiation of the rhythmic respiratory activity.

Responses of sympathetic preganglionic neurons to systemic hypercapnia in the acute spinal cat

The relation between end-tidal (ET) pCO2 and firing rate of sympathetic preganglionic neurons (SPN) of the cervical sympathetic trunk was studied during hyperoxic hypercapnia in acute C1 or C4 spinal cats. The cats were under barbiturate anesthesia or anemically decerebrate. The firing rate of the majority of the tonically active strands (18/22) increased in hypercapnia and showed a continuous relation to ET pCO2 within the range studied. The firing rate of the remaining 4 strands was unaffected. The maximum increase in firing rate of the responsive strands was 3.7 times the control value on average (range 2.5-14.0). Recruitment of units which were silent in control conditions also occurred. These data demonstrate the existence of a spinal mechanism responsible for excitation of SPN during systemic hypercapnia.

Is the central inspiratory activity responsible for pCO2-dependent drive of the sympathetic discharge?

Out of 27 cats anesthetized with chloralose-urethane mixture, paralyzed, vagotomized and artificially ventilated, phrenic nerve response to systemic hypercapnia (7-8 vol.% CO2/O2 mixture) was accompanied by an increase in blood pressure and sympathetic discharge in 19 cats. Out of these 19 cats, 12 were totally debuffered and in the remaining 7 cats one carotid sinus nerve was left intact. Single unit activity in the sympathetic cervical nerve and spontaneous mass activity in the cervical, splanchnic, renal sympathetic and phrenic nerves were recorded. Evoked response in the phrenic nerve was produced by electrical stimulation of the descending bulbospinal inspiratory pathways in the midplane area of the medulla or in the ventrolateral cervical spinal cord. Starting from the control mean end-tidal CO2 concentration of 4.7 vol.% (+/- 1.0 S.D.) a progressing hypocapnia was induced by hyperventilation up to the end-tidal CO2 concentration of 1.3-3.2 vol.% (mean 2.4 vol.% +/- 0.5 S.D.) significantly below paCO2 apneic threshold. In chemo- and baroreceptor denervated cats with a pressor and excitatory sympathetic response to hypercapnia, a hypocapnia resulted in a fall of the arterial blood pressure (mean 16.9 mm Hg +/- 7.5 S.D., 2.2 kpa +/- S.D.). With the increasing paCO2 over the period of hypocapnic apnea a pressor and excitatory sympathetic response preceded, in all experiments, the onset of the phrenic nerve rhythmic activity. The difference between paCO2 threshold for the pressor and sympathetic response (35.7 mm Hg +/- 3.6 S.D., 4.7 kpa +/- 0.5 S.D.) and paCO2 threshold for the reappearance of the phrenic nerve rhythmic activity (43.6 mm Hg +/- 2.6 S.D., 5.8 kpa +/- 0.3 S.D.) was highly significant. If apneic hypocapnia was combined with the continuous stimulation of the afferent fibers of the superior laryngeal nerve the CO2 threshold for phrenic rhythmic activity was significantly increased whereas CO2 threshold for the pressor and sympathetic excitatory response remained unchanged. CO2 administration during hypocapnia apnea caused a progressing reduction of the magnitude of the evoked phrenic nerve response. From these findings it is concluded that the central excitatory effect of CO2 on the sympathetic activity may be accomplished in the absence of the rhythmic respiratory activity and independently of the subthreshold tonic inspiratory activity. Pressor and sympathetic excitatory response to CO2 observed during hypocapnic apnea is presumably caused by a neuronal pool different from that responsible for the central inspiratory activity. It is suggested that this CO2 sensitive neuronal mechanism might be involved in the central generation of sympathetic tone.

Role of carotid and central chemoreceptors in the CO2 response of sympathetic preganglionic neurons

In anesthetized, vagotomized, paralyzed, artificially ventilated cats with aortic nerves cut, we recorded the response of 28 sympathetic preganglionic neurons (SPNs) of the cervical sympathetic trunk of changes in arterial pCO2. We observed the effects on these responses of: (i) surgical denervation of carotid sinus chemoreceptors in normoxia (paO2 110 mm Hg); and (ii) hyperoxia (paO2 greater than 350 mm Hg) which is known to depress peripheral chemoreceptor sensitivity to CO2. Stimulus-response curves, obtained by rebreathing at constant paO2, were used to detect the effects of these manoeuvres. The present experiments have confirmed previous observations demonstrating the CO2-sensitivity of this neuron population. The population average firing rate, as a function of paCO2, describes a sigmoid curve, increasing continuously between 20 and 90 mm Hg and asymptotically approaching plateaus at the highest and lowest paCO2 values. Carotid sinus nerve section caused a decrease of the average response of the population at all paCO2 values, resulting in a displacement to the right of the response curve, in a decrease in slope and maximum values. On the assumption that the CO2 response curve after carotid sinus nerve section is due to central chemoreceptor input, and that there is a simple addition between the effects of central and carotid chemoreceptors, the difference between CO2 response curves ("difference curves") before and after denervation represents the contribution of the carotid chemoreceptors. A comparison of this "difference curve" with the curve obtained after denervation reveals that the contribution of the carotid chemoreceptors is of the same magnitude as that of the central chemoreceptors up to a paCO2 value of 60-70 mm Hg. Beyond this value, the carotid contribution declines and becomes a smaller component of the total response, whereas the contribution of the central chemoreceptors continues to increase. Similar results were obtained with rebreathing in hyperoxia, after correction for the central excitatory effect of hyperoxia. Hyperoxia never caused a depression of the CO2 response of units after section of the carotid sinus nerve. Observation of the effects of the two manoeuvres on individual SPNs leads to the conclusion that in approximately half of the CO2-sensitive units there is an overlap of central and peripheral chemoreceptor input. The remainder of the CO2-sensitive units receive input only from the central chemoreceptors.

Cardiovascular control by medullary surface chemoreceptors

The cardiovascular and respiratory effects of superfusion of the ventral surface of the medulla with acid hypercapnic or alkaline hypocapnic solutions have been studied in anaesthetized, paralyzed, artificially ventilated cats. Peripheral chemoreceptor and baroreceptor denervation was achieved by section of carotid sinus, aortic and cervical vagus nerves. Systemic arterial and central venous pressure, hindquarters blood flow, heart rate and phrenic nerve activity were recorded. Acid hypercapnic (pH 6.8, pCO2 85 mm Hg) superfusion caused increases in systemic arterial pressure, phrenic nerve activity and heart rate, and a decrease in hindquarters blood flow. Alkaline hypocapnic (pH u.i, pCO2 less than 10 mmHg) superfusion caused opposite effects. These experiments indicate a significant role of the chemoreceptors of the ventral surface of the medulla in cardiovascular control.