The roles of medullary extracellular and cerebrospinal fluid pH in control of respiration

Cerebrospinal Fluid and Arterial Acid-Base Equilibrium of Spontaneously Breathing Patients with Aneurismal Subarachnoid Hemorrhage

BACKGROUND

Hyperventilation resulting in hypocapnic alkalosis (HA) is frequently encountered in spontaneously breathing patients with acute cerebrovascular conditions. The underlying mechanisms of this respiratory response have not been fully elucidated. The present study describes, applying the physical-chemical approach, the acid-base characteristics of cerebrospinal fluid (CSF) and arterial plasma of spontaneously breathing patients with aneurismal subarachnoid hemorrhage (SAH) and compares these results with those of control patients. Moreover, it investigates the pathophysiologic mechanisms leading to HA in SAH.

METHODS

Patients with SAH admitted to the neurological intensive care unit and patients (American Society of Anesthesiologists physical status of 1 and 2) undergoing elective surgery under spinal anesthesia were enrolled. CSF and arterial samples were collected simultaneously. Electrolytes, strong ion difference (SID), partial pressure of carbon dioxide (PCO₂), weak noncarbonic acids (A_TOT), and pH were measured in CSF and arterial blood samples.

RESULTS

Twenty spontaneously breathing patients with SAH and 25 controls were enrolled. The CSF of patients with SAH, as compared with controls, was characterized by a lower SID (23.1 ± 2.3 vs. 26.5 ± 1.4 mmol/L, p < 0.001) and PCO₂ (40 ± 4 vs. 46 ± 3 mm Hg, p < 0.001), whereas no differences in A_TOT (1.2 ± 0.5 vs. 1.2 ± 0.2 mmol/L, p = 0.95) and pH (7.34 ± 0.06 vs. 7.35 ± 0.02, p = 0.69) were observed. The reduced CSF SID was mainly caused by a higher lactate concentration (3.3 ± 1.3 vs. 1.4 ± 0.2 mmol/L, p < 0.001). A linear association (r = 0.71, p < 0.001) was found between CSF SID and arterial PCO₂. A higher proportion of patients with SAH were characterized by arterial HA, as compared with controls (40 vs. 4%, p = 0.003). A reduced CSF-to-plasma difference in PCO₂ was observed in nonhyperventilating patients with SAH (0.4 ± 3.8 vs. 7.8 ± 3.7 mm Hg, p < 0.001).

CONCLUSIONS

Patients with SAH have a reduction of CSF SID due to an increased lactate concentration. The resulting localized acidifying effect is compensated by CSF hypocapnia, yielding normal CSF pH values and resulting in a higher incidence of arterial HA.

Fluid and ion transfer across the blood-brain and blood-cerebrospinal fluid barriers; a comparative account of mechanisms and roles

The two major interfaces separating brain and blood have different primary roles. The choroid plexuses secrete cerebrospinal fluid into the ventricles, accounting for most net fluid entry to the brain. Aquaporin, AQP1, allows water transfer across the apical surface of the choroid epithelium; another protein, perhaps GLUT1, is important on the basolateral surface. Fluid secretion is driven by apical Na+-pumps. K+ secretion occurs via net paracellular influx through relatively leaky tight junctions partially offset by transcellular efflux. The blood-brain barrier lining brain microvasculature, allows passage of O2, CO2, and glucose as required for brain cell metabolism. Because of high resistance tight junctions between microvascular endothelial cells transport of most polar solutes is greatly restricted. Because solute permeability is low, hydrostatic pressure differences cannot account for net fluid movement; however, water permeability is sufficient for fluid secretion with water following net solute transport. The endothelial cells have ion transporters that, if appropriately arranged, could support fluid secretion. Evidence favours a rate smaller than, but not much smaller than, that of the choroid plexuses. At the blood-brain barrier Na+ tracer influx into the brain substantially exceeds any possible net flux. The tracer flux may occur primarily by a paracellular route. The blood-brain barrier is the most important interface for maintaining interstitial fluid (ISF) K+ concentration within tight limits. This is most likely because Na+-pumps vary the rate at which K+ is transported out of ISF in response to small changes in K+ concentration. There is also evidence for functional regulation of K+ transporters with chronic changes in plasma concentration. The blood-brain barrier is also important in regulating HCO3- and pH in ISF: the principles of this regulation are reviewed. Whether the rate of blood-brain barrier HCO3- transport is slow or fast is discussed critically: a slow transport rate comparable to those of other ions is favoured. In metabolic acidosis and alkalosis variations in HCO3- concentration and pH are much smaller in ISF than in plasma whereas in respiratory acidosis variations in pHISF and pHplasma are similar. The key similarities and differences of the two interfaces are summarized.

The caudal solitary complex is a site of central CO(2) chemoreception and integration of multiple systems that regulate expired CO(2)

The solitary complex is comprised of the nucleus tractus solitarius (NTS, sensory) and dorsal motor nucleus of the vagus (DMV, motor), which functions as an integrative center for neural control of multiple systems including the respiratory, cardiovascular and gastroesophageal systems. The caudal NTS-DMV is one of the several sites of central CO(2) chemoreception in the brain stem. CO(2) chemosensitive neurons are fully responsive to CO(2) at birth and their responsiveness seems to depend on pH-sensitive K(+) channels. In addition, chemosensitive neurons are highly sensitive to conditions such as hypoxia (e.g., neural plasticity) and hyperoxia (e.g., stimulation), suggesting they employ redox and nitrosative signaling mechanisms. Here we review the cellular and systems physiological evidence supporting our hypothesis that the caudal NTS-DMV is a site for integration of respiratory, cardiovascular and gastroesophageal systems that work together to eliminate CO(2) during acute and chronic respiratory acidosis to restore pH homeostasis.

Carotid body denervation eliminates apnea in response to transient hypocapnia

We determined the effects on breathing of transient ventilatory overshoots and concomitant hypocapnia, as produced by pressure support mechanical ventilation (PSV), in intact and carotid body chemoreceptor denervated (CBX) sleeping dogs. In the intact dog, PSV-induced transient increases in tidal volume and hypocapnia caused apnea within 10-11 s, followed by repetitive two-breath clusters separated by apneas, i.e., periodic breathing (PB). After CBX, significant expiratory time prolongation did not occur until after 30 s of PSV-induced hypocapnia, and PB never occurred. Average apneas of 8.4 +/- 1-s duration after a ventilatory overshoot required a decrease below eupnea of end-tidal Pco(2) 5.1 +/- 0.4 Torr below eupnea in the intact animal and 10.1 +/- 2 Torr in the CBX dog, where the former reflected peripheral and the latter central dynamic CO(2) chemoresponsiveness, as tested in the absence of peripheral chemoreceptor input. Hyperoxia when the dogs were intact shortened PSV-induced apneas and reduced PB but did not mimic the effects of CBX. We conclude that, during non-rapid eye movement sleep, carotid chemoreceptors are required to produce apneas that normally occur after a transient ventilatory overshoot and for PB.

Effects of extracellular calcium and magnesium on central respiratory control in the brainstem-spinal cord of neonatal rat

The influence of extracellular Ca2+ and Mg2+ concentrations ([Ca2+]ECF and [Mg2+]ECF, respectively) on central respiratory control was analyzed using the isolated brainstem-spinal cord of the neonatal rat. Central respiratory activity was recorded from the C4 ventral roots. The depth profile of [Ca2+]ECF below the ventral medullary surface was measured with ion-sensitive electrodes. The gradient in [Ca2+]ECF disappeared about 1 h after changing superfusate Ca2+ ([Ca2+]CSF) from 2 to 0.5 mM, but not even in 2 h after switching to Ca2+-free superfusate. High [Ca2+]CSF (4 mM) or high [Mg2+]CSF (4, 8 mM) decreased respiratory frequency (fR), whereas low [Ca2+]CSF (0.5 mM) increased fR and augmented the respiratory CO2 responsiveness. High [Ca2+]CSF as well as low [Mg2+]CSF (0.5 mM) disturbed respiratory rhythm and pattern, which were markedly restored by high CO2. The depressing effect of high [Ca2+]ECF and the stimulating effect of low [Ca2+]ECF on the medullary neuronal activity were confirmed by perforated patch recordings. These results suggest that [Ca2+]ECF and [Mg2+]ECF determine the excitability of the respiratory neuron network by modulating the neuronal surface potential, transmembrane Ca2+ influx, Ca2+-sensitive cation channel gating, and synaptic transmission. Furthermore, some of these actions appear to be antagonized by CO2/H+.

In vitro study of H+-sensitive neurons in the ventral medullary surface of neonate rats

We hypothesized that the direct stimulus of the central chemoreceptor neurons is the CO2/H+-induced change in intracellular pH (pHi). If it is true, pHi responses during hypercapnic stimulation should be exhibited in the central chemoreceptor neurons in the ventral medullary surface (VMS) and some neurons in the CO2/H+ sensitive regions such as the nucleus tractus solitarii of the medial dorsal medulla (MDM). To test this hypothesis, the cultured VMS and MDM neurons (control) derived from one day-old neonate rats were labeled with H+-sensitive fluorescent indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), and were exposed to perfusate of various pHs. The H+-sensitive neurons were determined by a rapid decrease in the intracellular BCECF fluorescence intensity. In almost all the MDM neurons (99.6%) and 94% of the VMS neurons, the intracellular BCECF fluorescence intensity remained unchanged when the extracellular pH (pHo) was decreased. In contrast, in 0.4% of the MDM neurons (8/1800) and in 6% of the VMS neurons (111/1800), the intracellular BCECF fluorescence intensity decreased when the pHo was decreased from 7.4 to 7.2. This subpopulation of MDM and VMS neurons were considered to be H+-sensitive neurons. The H+-sensitive neurons in the VMS showed positive immunoreactivity to glutamate (57%, 17/30) and glutamic acid decarboxylase (23%, 7/30), but no immunoreactivity to choline acetyltransferase, tyrosine hydroxylase, phenylethanolamine N-methyltransferase, somatostatin, serotonin and substance P. These results indicate that the H+-sensitive neurons are present specifically in the VMS, and are mainly glutamatergic and GABAergic.

Changes in medullary extracellular pH, sympathetic and phrenic nerve activity during brainstem perfusion with CO2 enriched solutions

Measurements are presented of sympathetic nerve activity (SNA), phrenic nerve activity (PNA), and local extracellular pH (ECF pH) within the rostral ventrolateral medulla (RVLM) in response to perfusions of the RVLM with CO2-enriched saline. Experiments were performed on cats anaesthetized with chloralose. The ventrolateral medullary surface was exposed, and a catheter was placed in the left vertebral artery from the axilla to allow perfusion of the RVLM. Baroreceptor and peripheral chemoreceptor denervations were performed by cutting the vagal, aortic and carotid sinus nerves. The activities of the renal and the phrenic nerve were recorded, in some experiments in parallel with the cardiac nerve. Recordings of the pH were done with ion-sensitive theta-microelectrodes. A linear relationship between the CO2 concentration of the perfusate and the evoked changes in ECF pH was found. The ECF pH did not change systematically in one or the other direction within depths between 1 and 3 mm below the surface of the medulla. The various patterns of interaction of ECF pH, SNA, and PNA are described in detail. Phrenic nerve response to perfusions was very variable; a more prolonged increase in amplitude of phasic discharges compared to the duration of changes in SNA and ECF pH was the most frequent finding, but non-phasic tonic activation and complete silence were also seen during perfusions. SNA could also deviate from ECF pH both with regard to its latency and to its time course in response to perfusions. Therefore, this study provides further evidence for deviations of cardiorespiratory adaptation from ECF pH, corroborating the notion that this parameter is not the decisive one for central chemoreception.

In vivo and in vitro responses of neurons in the ventrolateral medulla to hypoxia

Neurons in the ventrolateral medulla (VLM) are known to be involved in several cardiorespiratory reflexes and to provide tonic drive to sympathetic preganglionic neurons. Recent studies have suggested that VLM neurons modulate the respiratory responses to hypoxia and to hypercapnia. The purpose of the present study was to determine with electrophysiological techniques if the discharge of these neurons is altered by hypoxia and/or by hypercapnia both in vivo and in vitro. Extracellular single-unit activity of VLM neurons (n = 39) was recorded during inhalation of a hypoxic gas (10% O2) and during inhalation of a hypercapnic gas (5% CO2) in anesthetized, spontaneously breathing rats (n = 16). Hypoxia elicited an increase in the discharge frequency in 64% of the VLM neurons studied; hypercapnia stimulated 42% of the neurons. Fifty-two percent of the neurons were stimulated by both hypoxia and hypercapnia. Signal averaging revealed that 76% of the hypoxia-stimulated neurons had a resting discharge related to the cardiac and/or respiratory cycle. Similar percentages of VLM neurons (35/54) were stimulated by hypoxia in a second group of animals (n = 14) that were studied after sinoaortic denervation. A rat brain slice preparation was then used to determine if hypoxia exerts a direct effect upon neurons in the VLM. Perfusing a hypoxic gas over the surface of medullary slices evoked an increase in the discharge frequency in the majority (39/49) of VLM neurons studied; responses were graded in relation to the magnitude of the hypoxic stimulus. Similar responses to hypoxia were observed in VLM neurons studied during perfusion with a synaptic blockade medium. Retrograde labeling of VLM neurons with rhodamine tagged microspheres injected into the thoracic intermediolateral cell column demonstrated that the hypoxia sensitive neurons were located in a region of the VLM that projects to the thoracic spinal cord. These results demonstrate that neurons in the ventrolateral medulla are excited by a direct effect of hypoxia; these neurons may play a critical role in the cardiorespiratory responses to hypoxia.

Presence of neuronal cell bodies in the sympathetic pressor areas of dorsal and ventrolateral medulla inhibiting phrenic nerve discharge in cats

To examine whether neuronal cell bodies (perikarya) in the pressor areas of dorsal medulla or ventrolateral medulla also modulate respiratory function, phrenic nerve activity was monitored and correlated with the pressor response in chloralose-urethane anaesthetized cats. The animals were paralyzed and artificially ventilated maintaining the end-tidal fractional concentration of CO2 at 0.04-0.05. The same pressor point in the dorsal or ventrolateral medulla was stimulated electrically (rectangular pulse of 20-200 microA, 80 Hz and 0.5 ms) and then chemically (0.25-0.5 M sodium glutamate, 80-200 nl). Areas producing pressor effects in either the dorsal or ventrolateral medulla produced a current-dependent decrease of phrenic discharge. The decrease in Pna was significant when the electrical current reached 50 microA or above. It occurred immediately before the onset of increase in blood pressure. Injection of glutamate to the same pressor point in either the dorsal or ventrolateral medulla produced a similar decrease in phrenic nerve activity. The results suggest that in the pressor areas of dorsal and ventrolateral medulla there are neuronal perikarya that can increase systemic arterial pressure and inhibit phrenic nerve activity. Whether the same or separate neurones are responsible for these responses is to be determined.

Chemosensitivity of medullary neurons in explant tissue cultures

To determine whether cultured medulla contains chemosensitive neurons which are excited by CO2 and fixed acid and whether this function is specific to the ventral medulla, tissue explants of ventral and dorsal medulla were prepared from neonatal rats and incubated for two to three weeks. Cultures were superfused with artificial cerebrospinal fluid, maintained at 37 degrees C, and pH of the superfusate was varied either with PCO2 (14-71 Torr) at constant HCO3- (22 mM) or HCO3- (10-30 mM) at constant PCO2 (35 Torr). Spontaneous action potentials were recorded extracellularly in 51 ventral and 23 dorsal medullary neurons. Ventral medullary neurons exhibited a steady baseline firing frequency of 4 +/- 0.8 Hz. In contrast, dorsal medullary neurons exhibited two different patterns of spontaneous activity: 11 fired continuously (7.2 +/- 1.4 Hz) while 12 fired with a bursting pattern. Burst duration was 0.80 +/- 0.14 min and cycle time was 1.74 +/- 0.43 min. Decreasing pH with CO2 caused an increase in the activity of 10 of 27 ventral medullary neurons and two of six dorsal medullary neurons with a mean response of 7.5 Hz/pH unit. Varying pH by changing HCO3- had no effect on firing frequency. These results demonstrate that: (i) chemosensitive neurons are present in both ventral and dorsal medullary explant cultures; (ii) these cells only respond to changes in pH induced with CO2; and (iii) about half of the dorsal medullary neurons fire spontaneously with a regular bursting pattern of activity.

Kainic acid on the rat ventral medullary surface depresses hypoxic and hypercapnic ventilatory responses

Kainic acid, topically applied to the ventral surface of the medulla immediately caudal to the trapezoid body in the urethane/chloralose anaesthetised rat, led to a depression of ventilation and a sustained rise in blood pressure; ventilatory responses to hypercapnia (10% carbon dioxide) and hypoxia (11% oxygen) were slightly depressed. Widespread application of kainic acid to an area at and slightly rostral to the rootlets of the hypoglossal nerve produced a stimulation of ventilation and an unsustained rise in blood pressure. Apnea ensued 12-28 min after application. Ventilatory responses to hypercapnia and hypoxia were markedly attenuated; more discrete bilateral application revealed two regions, one immediately rostral and lateral to the hypoglossal rootlets and the other over the point of exit of the hypoglossal nerve rootlets, which specifically contributed to the diminution of the chemosensory responses. These results raise questions about the medullary circuitry which mediates the chemoreflex regulation of breathing.

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.

CO2 decreases membrane conductance and depolarizes neurons in the nucleus tractus solitarii

To identify central sites of potential CO2/H+-chemoreceptive neurons, and the mechanism responsible for neuronal chemosensitivity, intracellular recordings were made in rat tissue slices in two cardiopulmonary-related regions (i.e., nucleus tractus solitarii, NTS; nucleus ambiguus, AMBc) during exposure to high CO2. When the NTS was explored slices were bisected and the ventral half discarded. Utilizing such "dorsal" medullary slices removed any impinging synaptic input from putative chemoreceptors in the ventrolateral medulla. In the NTS, CO2-induced changes in firing rate were associated with membrane depolarizations ranging from 2-25 mV (n = 15). In some cases increased e.p.s.p. activity was observed during CO2 exposure. The CO2-induced depolarization occurred concomitantly with an increased input resistance ranging from 19-23 M omega (n = 5). The lower membrane conductance during hypercapnia suggests that CO2-induced depolarization is due to a decreased outward potassium conductance. Unlike neurons in the NTS, AMBc neurons were not spontaneously active and were rarely depolarized by hypercapnia. Eleven of 12 cells tested were either hyperpolarized by or insensitive to CO2. Only 1 neuron in the AMBc was depolarized and it also showed an increased input resistance during CO2 exposure. Our findings suggest that CO2/H+-related stimuli decrease potassium conductance which depolarizes the cell and increases firing rate. Although our in vitro studies cannot guarantee the specific function of these cells, we believe they may be involved with brain pH homeostasis and cardiopulmonary regulation.

Two long-lasting central respiratory responses following acute hypoxia in glomectomized cats

1. Central respiratory response to acute (10 min) hypoxia, as measured by phrenic nerve activity, was determined in peripheral chemo-denervated cats. 2. Hypoxia was induced by ventilating cats for 10 min at reduced inspired oxygen levels (inspired O2 fraction, FI,O2 = 0.06-0.15). The degree of hypoxaemia was determined from an arterial blood sample and ranged from 'severe' (arterial O2 pressure, Pa,O2 less than 26 Torr) to 'mild' (Pa,O2 greater than 35 Torr). The respiratory response was monitored for 1 h following return to ventilation with 100% oxygen. 3. The results confirmed the finding of prolonged (greater than 60 min) inhibition of respiration upon return to hyperoxic conditions following severe hypoxia, as reported previously (Millhorn, Eldridge, Kiley & Waldrop, 1984). A new finding was a long-lasting (greater than 60 min) facilitation of respiration following exposure to less severe (Pa,O2 greater than 35 Torr) hypoxia. 4. Medullary extracellular fluid pH was measured in six cats. Changes in pH could not explain either the prolonged inhibition following severe hypoxia or the long-lasting facilitation observed following mild hypoxia. 5. Ablation studies were performed in order to determine the locations of the neuronal substrates for the inhibitory and facilitatory mechanisms. The results of this series of experiments indicate that the mesencephalon is necessary for activation of the inhibitory mechanism, while the facilitatory mechanism requires the presence of higher brain structures, notably the diencephalon. 6. Following removal of the diencephalon, the inhibitory response was seen following even mild hypoxic insults, i.e. those shown to produce facilitation in animals with intact brains. In the absence of the mesencephalon, neither prolonged inhibition nor prolonged facilitation could be produced following hypoxia. 7. It is proposed that there are two centrally mediated long-lasting responses to acute hypoxia. Facilitation is seen following mild hypoxia. Inhibition is more likely following severe hypoxia. However, both mechanisms appear to be triggered simultaneously and the output of the central respiratory controller reflects the influence of each.

Dynamics of medullary hydrogen ion and respiratory responses to square-wave change of arterial carbon dioxide in cats

1. The dynamics of changes of medullary extracellular fluid (ECF) hydrogen ion concentration ([H+]) and respiration, measured as integrated phrenic nerve activity, were determined in anaesthetized, paralysed, vagotomized and glomectomized cats. ECF [H+] was measured directly by means of a small (2 mm diameter) glass pH electrode placed on the ventral surface of the medulla. The variables were measured continuously after a step change of arterial PCO2 produced by abruptly starting or stopping an infusion of hypercapnic fluid into the aortic arch. 2. Alteration of pH in the descending thoracic aorta at the onset or offset of infusion was complete within 1.5 s after the change began, indicating that it was nearly square wave in form. 3. In sixteen experiments, ECF [H+] began to fall within 2 s of offset of infusion, reflecting aortic-medullary circulation time. Thereafter, ECF [H+] decreased to a stable level over the next 5 min; the curve describing the decrease consisted of two exponential functions, one with a time constant (tau) of 9.5 +/- 0.6 s and a second with a tau of 53 +/- 3 s. 4. We interpret the findings at the offset of CO2 infusion in terms of CO2 wash-out from the medullary ECF. The slow function is associated with wash-out during stable medullary blood flow that develops after 1 min. The early fast function is associated with the decreasing medullary blood flow that occurs during the first minute after change from arterial hypercapnia to normocapnia. 5. We have estimated medullary blood flow using a mathematical model incorporating the two functions. The values obtained are consistent with those in the literature where other methods have been used. Changes of blood flow following the step change of CO2 are fairly rapid, half of the response occurring in 13 s. 6. The change of respiratory activity lags the change of stimulus expressed by [H+], throughout the recovery period and respiration requires up to 8 min to reach a stable level. We attribute this slow response to slow central neural respiratory dynamics, the respiratory after-discharge.