Is central chemoreceptor sensitive to intracellular rather than extracellular pH?

Neurodegenerative disease of the brain: a survey of interdisciplinary approaches

Neurodegenerative diseases of the brain pose a major and increasing global health challenge, with only limited progress made in developing effective therapies over the last decade. Interdisciplinary research is improving understanding of these diseases and this article reviews such approaches, with particular emphasis on tools and techniques drawn from physics, chemistry, artificial intelligence and psychology.

Two central pattern generators from the crab, Cancer borealis, respond robustly and differentially to extreme extracellular pH

The activity of neuronal circuits depends on the properties of the constituent neurons and their underlying synaptic and intrinsic currents. We describe the effects of extreme changes in extracellular pH – from pH 5.5 to 10.4 – on two central pattern generating networks, the stomatogastric and cardiac ganglia of the crab, Cancer borealis. Given that the physiological properties of ion channels are known to be sensitive to pH within the range tested, it is surprising that these rhythms generally remained robust from pH 6.1 to pH 8.8. The pH sensitivity of these rhythms was highly variable between animals and, unexpectedly, between ganglia. Animal-to-animal variability was likely a consequence of similar network performance arising from variable sets of underlying conductances. Together, these results illustrate the potential difficulty in generalizing the effects of environmental perturbation across circuits, even within the same animal.

Anuric Acute Kidney Injury Induced by Acute Mountain Sickness Prophylaxis With Acetazolamide

Acetazolamide (ACZ) is a sulfonamide derivative that inhibits carbonic anhydrase and is the mainstay for prevention and treatment of acute mountain sickness (AMS). Acute kidney injury (AKI) is not well recognized as a complication of ACZ ingestion, especially when low doses are used for short periods of time. We report a case of a healthy, middle-aged man who developed severe AKI after the ingestion of ACZ for AMS prophylaxis. The patient presented with bilateral flank pain and anuric AKI without radiographic signs of obstructive uropathy. All blood and urine testing to determine the cause of AKI were negative or normal. The patient required 2 sessions of hemodialysis due to worsening metabolic derangements, which included severe anion gap metabolic acidosis and hyperphosphatemia. Renal function returned to baseline after 96 hours of supportive care. The pathogenesis of AKI in our patient was attributed to ACZ-induced sulfonamide crystalluria causing intratubular obstruction and retrograde urine flow, but not intraureteric precipitation or obstructive uropathy. This classic presentation of anuric AKI and renal colic has been previously described with higher doses of ACZ for prolonged periods of time but never with low doses for AMS prophylaxis such as in our patient (total dose of 1250 mg within 48 hours). Our case highlights the risk of adverse renal outcomes following ACZ ingestion, even in previously healthy individuals, and suggests that increased fluid intake may be advisable for travelers taking ACZ prophylaxis.

Measurement of cerebral tissue oxygenation in young healthy volunteers during acetazolamide provocation: a transcranial Doppler and near-infrared spectroscopy investigation

Recent advances in near-infrared spectroscopy (NIRS) allow measurements of absolute tissue oxygen saturation (TOI) using spatially resolved spectroscopy (SRS), while enabling better depth sensitivity. However concerns remain regarding the relative contribution of the extracranial circulation to the cerebral NIRS TOI signal. In this study we investigated this during a period of selective rise in cerebral blood flow (CBF) produced by the administration of acetazolamide (ACZ) in 10 healthy volunteers. A two channel spectrometer (NIRO 300, Hamamatsu Photonics KK) was used to measure absolute cerebral TOI over the frontal cortex using the SRS technique using an optode spacing of 5 cm and 1.5 cm for channel 1 and 2 respectively. After ACZ administration we were able to observe a significant increase in the velocity of middle cerebral artery (V(mca), measured with the transcranial Doppler (TCD)) which was accompanied by an increase in TOI as monitored by the NIRO 300 with an optode spacing of 5 cm but not with an optode spacing of 1.5 cm. Furthermore a direct relationship was seen between the V(mca) and the TOI measured at 5 cm optode spacing. This work suggests that using this commercial NIRS instrument with an optode spacing of 5 cm one is able to detect the intracranial changes.

Mechanisms of action of acetazolamide in the prophylaxis and treatment of acute mountain sickness

Acetazolamide, a potent carbonic anhydrase (CA) inhibitor, is the most commonly used and best-studied agent for the amelioration of acute mountain sickness (AMS). The actual mechanisms by which acetazolamide reduces symptoms of AMS, however, remain unclear. Traditionally, acetazolamide's efficacy has been attributed to inhibition of CA in the kidneys, resulting in bicarbonaturia and metabolic acidosis. The result is offsetting hyperventilation-induced respiratory alkalosis and allowance of chemoreceptors to respond more fully to hypoxic stimuli at altitude. Studies performed on both animals and humans, however, have shown that this explanation is unsatisfactory and that the efficacy of acetazolamide in the context of AMS is likely due to a multitude of effects. This review summarizes the known systemic effects of acetazolamide and incorporates them into a model encompassing several factors that are likely to play a key role in the drug's efficacy. Such factors include not only metabolic acidosis resulting from renal CA inhibition but also improvements in ventilation from tissue respiratory acidosis, improvements in sleep quality from carotid body CA inhibition, and effects of diuresis.

Cellular mechanisms involved in CO(2) and acid signaling in chemosensitive neurons

An increase in CO(2)/H(+) is a major stimulus for increased ventilation and is sensed by specialized brain stem neurons called central chemosensitive neurons. These neurons appear to be spread among numerous brain stem regions, and neurons from different regions have different levels of chemosensitivity. Early studies implicated changes of pH as playing a role in chemosensitive signaling, most likely by inhibiting a K(+) channel, depolarizing chemosensitive neurons, and thereby increasing their firing rate. Considerable progress has been made over the past decade in understanding the cellular mechanisms of chemosensitive signaling using reduced preparations. Recent evidence has pointed to an important role of changes of intracellular pH in the response of central chemosensitive neurons to increased CO(2)/H(+) levels. The signaling mechanisms for chemosensitivity may also involve changes of extracellular pH, intracellular Ca(2+), gap junctions, oxidative stress, glial cells, bicarbonate, CO(2), and neurotransmitters. The normal target for these signals is generally believed to be a K(+) channel, although it is likely that many K(+) channels as well as Ca(2+) channels are involved as targets of chemosensitive signals. The results of studies of cellular signaling in central chemosensitive neurons are compared with results in other CO(2)- and/or H(+)-sensitive cells, including peripheral chemoreceptors (carotid body glomus cells), invertebrate central chemoreceptors, avian intrapulmonary chemoreceptors, acid-sensitive taste receptor cells on the tongue, and pain-sensitive nociceptors. A multiple factors model is proposed for central chemosensitive neurons in which multiple signals that affect multiple ion channel targets result in the final neuronal response to changes in CO(2)/H(+).

Central sympathetic chemosensitivity and Kir1 potassium channels in the cat

The possible involvement of potassium channels in central chemosensitivity, with special reference to the Kir1.1 potassium channel, was investigated by studying the CO(2) response of presympathetic neurons in the rostroventrolateral medulla (RVLM) in the absence or presence of various K(+) channel inhibitors. Synaptic input to RVLM neurons was blocked by local injection of omega-agatoxin and omega-conotoxin. Activity of RVLM neurons was measured by recording the electrical activity in preganglionic (WR-T(3)) or postganglionic (renal) sympathetic nerves after perfusion of the lower brainstem via the left vertebral artery with CO(2)-enriched saline solution. Unspecific K(+) channel blockade by BaCl(2) reduced the excitatory response of sympathetic activity after CO(2)-perfusion to 56% of control. A quantitatively similar inhibition of the central CO(2) response was obtained after administration of 9-fluorenylmethylchloroformate (FMOC-Cl) which eliminates pH sensitivity of Kir1 and Kir4.1. Furthermore, two structurally different Kir1 inhibiting toxins, tertiapin and Lq2, also reduced the central CO(2) response to approximately 50% of control. In contrast, charybdotoxin (CTX) had no effect on the CO(2) response. Using RT-PCR the expression of mRNA homologous to rat Kir1 mRNA was identified in the cat medulla oblongata. These data suggest that a modulation of potassium channel activity possibly via Kir1 may contribute to central chemosensitivity.

Role of intracellular and extracellular pH in the chemosensitive response of rat locus coeruleus neurones

The chemosensitive response of locus coeruleus (LC) neurones to changes in intracellular pH (pH(i)), extracellular pH (pH(o)) and molecular CO(2) were investigated using neonatal rat brainstem slices. A new technique was developed that involves the use of perforated patch recordings in combination with fluorescence imaging microscopy to simultaneously measure pH(i) and membrane potential (V(m)). Hypercapnic acidosis (15 % CO(2), pH(o) 6.8) resulted in a maintained fall in pH(i) of 0.31 pH units and a 93 % increase in the firing rate of LC neurones. On the other hand, isohydric hypercapnia (15 % CO(2), 77 mM HCO(3)(-), pH(o) 7.45) resulted in a smaller and transient fall in pH(i) of about 0.17 pH units and an increase in firing rate of 76 %. Acidified Hepes (N-2-hydroxyethylpiperazine-N'-2- ethanesulfonic acid)-buffered medium (pH(o) 6.8) resulted in a progressive fall in pH(i) of over 0.43 pH units and an increase in firing rate of 126 %. Isosmotic addition of 50 mM propionate to the standard HCO(3)(-)-buffered medium (5 % CO(2), 26 mM HCO(3)(-), pH(o) 7.45) resulted in a transient fall in pH(i) of 0.18 pH units but little increase in firing rate. Isocapnic acidosis (5 % CO(2), 7 mM HCO(3)(-), pH(o) 6.8) resulted in a slow intracellular acidification to a maximum fall of about 0.26 pH units and a 72 % increase in firing rate. For all treatments, the changes in pH(i) preceded or occurred simultaneously with the changes in firing rate and were considerably slower than the changes in pH(o). In conclusion, an increased firing rate of LC neurones in response to acid challenges was best correlated with the magnitude and the rate of fall in pH(i), indicating that a decrease in pH(i) is a major part of the intracellular signalling pathway that transduces an acid challenge into an increased firing rate in LC neurones.

Ventrolateral neurons of medullary organotypic cultures: intracellular pH regulation and bioelectric activity

The hypothesized role of the intracellular pH (pH(i)) as a proximate stimulus for central chemosensitive neurons is reviewed on the basis of data obtained from organotypic cultures of the medulla oblongata (obex level) of new born rats (OMC). Within OMC a subset of neurons responds to hypercapnia as do neurons in the same (or similar) brain areas in vivo. Maneuvers altering intra- and/or extracellular pH (pH(o)) such as hypercapnia, bicarbonate-withdrawal, or ammonium pre-pulses, evoked well defined changes of the neuronal pH(i). During hypercapnia (pH(o) 7.0) or bicarbonate-withdrawal (pH(o) 7.4) most ventrolateral neurons adopted a pH(i) which was < or = 0.2 pH units below the steady state pH(i), while signs of pH(i)-regulation occurred only in a small fraction of neurons. During all treatments leading to intracellular acidosis, bioelectric activity of chemosensitive neurons increased and was often indistinguishable from the response to hypercapnia, regardless of whether pH(o) was unchanged, decreased or increased during the treatment. These data strongly suggest that the pH(i) acts as proximate stimulus. The mode of acid extrusion of chemosensitive neurons is, therefore, of major importance for the control of central chemosensitivity. Immunocytochemical data, pH(i) measurements and neuropharmacological studies with novel drugs pointed to the Na(+)/H(+) exchanger subtype 3 (NHE3) as a main acid extruder in ventrolateral chemosensitive neurons. Possible functions and neuropharmacological strategies arising from this very local NHE3 expression are discussed.

A novel inhibitor of the Na+/H+ exchanger type 3 activates the central respiratory CO2 response and lowers the apneic threshold

Cultured CO2-sensitive neurons from the ventrolateral medulla of newborn rats enhanced their bioelectric activity upon intracellular acidification induced by inhibition of the Na+/H+ exchanger type 3 (NHE3). Now we detected NHE3 also in the medulla oblongata of adult rabbits. Therefore, this animal model was employed to determine whether NHE3 inhibition also affects central respiratory chemosensitivity in vivo. Seven anesthetized (pentobarbital), vagotomized, paralyzed rabbits were artificially ventilated with O2-enriched air. From the phrenic nerve compound discharge, integrated burst amplitude (IPNA), respiratory rate (fR), and phrenic minute activity (IPNA. fR) were taken as measures of central respiratory rhythm and drive. Effects of potent NHE3 inhibition with the novel brain permeant substance S8218 were studied by comparing respiratory characteristics before and after up to 9.2 +/- 1.1 mg/kg cumulative drug application, yielding average plasma concentrations of 0.9 +/- 0.2 microg/ml. In response to S8218, the baseline level of IPNA. fR was significantly enhanced by an average of 51.0 +/- 6.4% (n = 27, p < 0.0001). The influence of NHE3 inhibition on the respiratory CO2 response was studied at plasma concentrations of S8218 maintained in the range of 0.3 microg/ml (10(-6) M). Although the metabolic acid-base status thereby remained widely unchanged, the group mean apneic threshold PaCO2 was significantly lowered by 0.45 +/- 0.11 kPa (n = 7, p < 0.01), whereby in four of seven animals even strong hyperventilation failed to suppress phrenic nerve rhythmicity completely. Likewise, S8218 significantly augmented IPNA. fR, in the range of PaCO2 between 1 and 6 kPa above threshold, by an average of 38.0 +/- 8.5% (n = 35, p < 0.0001). These in vivo results are compatible with the effects of NHE3 inhibition on chemosensitive brainstem neurons in vitro. Moreover, rhythmogenesis is supported through NHE3 inhibition by lowering the threshold PCO2 for central apnea.

CO(2)/H(+) chemoreception in the cat pre-Bötzinger complex in vivo

We examined the effects of focal tissue acidosis in the pre-Bötzinger complex (pre-BötC; the proposed locus of respiratory rhythm generation) on phrenic nerve discharge in chloralose-anesthetized, vagotomized, paralyzed, mechanically ventilated cats. Focal tissue acidosis was produced by unilateral microinjection of 10-20 nl of the carbonic anhydrase inhibitors acetazolamide (AZ; 50 microM) or methazolamide (MZ; 50 microM). Microinjection of AZ and MZ into 14 sites in the pre-BötC reversibly increased the peak amplitude of integrated phrenic nerve discharge and, in some sites, produced augmented bursts (i.e., eupneic breath ending with a high-amplitude, short-duration burst). Microinjection of AZ and MZ into this region also reversibly increased the frequency of eupneic phrenic bursts in seven sites and produced premature bursts (i.e., doublets) in five sites. Phrenic nerve discharge increased within 5-15 min of microinjection of either agent; however, the time to the peak increase and the time to recovery were less with AZ than with MZ, consistent with the different pharmacological properties of AZ and MZ. In contrast to other CO(2)/H(+) brain stem respiratory chemosensitive sites demonstrated in vivo, which have only shown increases in amplitude of integrated phrenic nerve activity, focal tissue acidosis in the pre-BötC increases frequency of phrenic bursts and produces premature (i.e., doublet) bursts. These data indicate that the pre-BötC has the potential to play a role in the modulation of respiratory rhythm and pattern elicited by increased CO(2)/H(+) and lend additional support to the concept that the proposed locus for respiratory rhythm generation has intrinsic chemosensitivity.

Regional differences in the CBF and BOLD responses to hypercapnia: a combined PET and fMRI study

Previous fMRI studies of the cerebrovascular response to hypercapnia have shown signal change in cerebral gray matter, but not in white matter. Therefore, the objective of the present study was to compare (15)O PET and T *(2)-weighted MRI during a hypercapnic challenge. The measurements were performed under similar conditions of hypercapnia, which were induced by inhalation of 5 or 7% CO(2). The baseline rCBF values were 65.1 ml hg(-1) min(-1) for temporal gray matter and 28.7 ml hg(-1) min(-1) for white matter. By linear regression, the increases in rCBF during hypercapnia were 23.0 and 7. 2 ml hg(-1) min(-1) kPa(-1) for gray and white matter. The signal changes were 6.9 and 1.9% for the FLASH sequence and were 3.8 and 1. 7% for the EPI sequence at comparable echo times. The regional differences in percentage signal change were significantly reduced when normalized by regional flow values. A deconvolution analysis is introduced to model the relation between fMRI signal and end-expiratory CO(2) level. Temporal parameters, such as mean transit time, were derived from this analysis and suggested a slower response in white matter than in gray matter regions. It was concluded that the differences in the magnitude of the fMRI response can largely be attributed to differences in flow and that there is a considerable difference in the time course of the response between gray and white matter.

CO2, brainstem chemoreceptors and breathing

The regulation of breathing relies upon chemical feedback concerning the levels of CO2 and O2. The carotid bodies, which detect O2, provide tonic excitation to brainstem respiratory neurons under normal conditions and dramatic excitation if O2 levels fall. Feedback for CO2 involves the carotid body and receptors in the brainstem, central chemoreceptors. Small increases in CO2 produce large increases in breathing. Decreases in CO2 below normal can, in sleep and anesthesia, decrease breathing, even to apnea. Central chemoreceptors, once thought localized to the surface of the ventral medulla, are likely distributed more widely with sites presently identified in the: (1) ventrolateral medulla; (2) nucleus of the solitary tract; (3) ventral respiratory group; (4) locus ceruleus; (5) caudal medullary raphé; and (6) fastigial nucleus of the cerebellum. Why so many chemoreceptor sites? Hypotheses, some with supporting data, include the following. Geographical specificity; all regions of the brainstem with respiratory neurons contain chemoreceptors. Stimulus intensity; some sites operate in the physiological range of CO2 values, others only with more extreme changes. Stimulus specificity; CO2 or pH may be sensed by multiple mechanisms. Temporal specificity; some sites respond more quickly to changes on blood or brain CO2 or pH. Syncytium; chemosensitive neurons may be connected via low resistance, gap junctions. Arousal state: sites may vary in effectiveness and importance dependent on state of arousal. Overall, as judged by experiments of nature, and in the laboratory, central chemoreceptors are critical for adequate breathing in sleep, but other aspects of the control system can maintain breathing in wakefulness.

Central sleep apnea after interrupting longterm acetazolamide therapy

One month administration of acetazolamide (ACET) (at sea level) improves periodic breathing and decreases the number of central apneas (CA) (De Backer et al., 1995 Am. J. Respir. Crit. Care Med. 151, 87-91) in nonhypercapnic central apnea syndrome. It remains unclear whether cessation of therapy would provoke recurrence of symptoms. In the present study we evaluated the number of CA after 1 and 6 months interruption of ACET therapy. Eight patients with central sleep apnea were included [central apnea index (CAI) > 5 or apnea and hypopnea index (AHI) > 10 and obstructive apnea index (OAI) < 5]. Polysomnography was repeated once after 1 month treatment (N2), after 1 month off treatment (N3) and after 6 months off treatment. CAI (25 +/- 10 at N1) decreased during N2 (4 +/- 2) and N3 (5 +/- 3) and remained low after N4 (3 +/- 1). However an increase in the number of obstructive apneas and central hypopneas could be observed together with a shift from central apnea to hypopnea after N4. Maybe ACET induces a long lasting resetting of the CO2 threshold which is still present after interruption of the therapy.

Assessment of cerebrovascular reactivity by dynamic susceptibility contrast-enhanced MR imaging

In patients with cerebrovascular disease the acetazolamide (ACZ) test is performed to evaluate the decrease in cerebral perfusion pressure (CPP) through the investigation of the vasomotor reactivity (VMR). This latter is currently assessed with ACZ with several methods. Recently, magnetic resonance imaging (MRI) techniques have been developed that are sensitive to stimulus-induced changes in blood flow. Dynamic susceptibility contrast material-enhanced gradient-echo MRI techniques (DSC-MRI) might be an attractive tool to assess VMR. We aimed to test the ability of DSC-MRI in the assessment of VMR. Relative hemodynamic parameters rCBV, MTT, and rCBF were evaluated at baseline after the first injection of gadopentetate dimeglumine and 10 min after the intravenous administration of ACZ (1 g) with a second bolus of contrast agent. Assessment of hemodynamic parameters was performed over the whole hemisphere and also within regions of interest. The significances of the mean differences, before and after ACZ, were assessed with repeated-measures ANOVA with two within factors: laterality (right-left) and ACZ. DSC-MRI with ACZ test was performed in ten healthy controls (aged 51.4+/-16.2 years). The cerebral hemispheric ratio for the three parameters (cerebral blood volume (CBV), mean transit time (MTT), and cerebral blood flow (CBF)) ranged between 1.01 and 1.03. The mean gray matter-to-white matter ratio for CBV, CBF and MTT were 2.44, 2.41 and 1.05, respectively. As the laterality effect was not significant, left and right hemispheric values were averaged. A significant increase of all hemodynamic parameters was observed after ACZ (P<0.01-0.001). The same changes for CBV, CBF and MTT were observed after ACZ according to the regions of interest (P<0.006-0.015). DSC-MRI is a non-invasive method which enables the assessment of VMR. This technique may be added to any conventional MRI in order to detect a hemodynamic impact of an ICA stenosis. Therefore, it might be useful in determining the appropriate management when the indication for surgical versus medical therapy is in question.

Focal central chemoreceptor sensitivity in the RTN studied with a CO2 diffusion pipette in vivo

We describe and use a CO2 diffusion pipette to produce a quickly reversible focal acidosis in the retrotrapezoid nucleus region of the rat brain stem. No tissue injection is made. Instead, artificial cerebrospinal fluid (aCSF) equilibrated with CO2 circulates within the micropipette, providing a source for continued CO2 diffusion into the tissue from the pipette tip. Tissue pH electrodes show the acidosis is limited to 500 micron from the tip. In controls (aCSF equilibrated with air), 1-min pipette perfusions increased tissue pH slightly and decreased phrenic nerve amplitude. In moderate- and high-CO2 groups (aCSF equilibrated with 50 or 100% CO2), 1-min perfusions significantly decreased tissue pH and increased phrenic nerve amplitude in a dose-dependent manner. The responses developed and reversed within minutes. Compared with our prior use of medullary acetazolamide injections to produce a focal acidosis, in this approach the acidosis 1) arises and reverses quickly and 2) its intensity can be varied. This allows study of sensitivity and mechanism. We conclude from this initial experiment that retrotrapezoid nucleus region chemoreceptors operate within the normal physiological range of CO2-induced tissue pH changes.

Carbon dioxide regulates the tonic activity of locus coeruleus neurons by modulating a proton- and polyamine-sensitive inward rectifier potassium current

The electrophysiological effects of CO2 on locus coeruleus noradrenergic neurons were investigated in rat brain slices. Under control conditions, when slices were perfused with artificial cerebrospinal fluid containing 24 mM NaHCO3/5% CO2 (pH approximately 7.34, 33 degrees C) and exposed to 5% CO2/95% O2 arriving through an interface chamber, locus coeruleus neurons discharged spontaneously at approximately 1 Hz. Extracellular recordings showed that lowering CO2 that arrived through the chamber below 5% resulted in reductions in firing rate, often with a complete cessation of activity when exogenous CO2 was removed completely. Intracellular recordings revealed that lowering CO2 produced an outward current with an increase in slope conductance and a reversal potential near the potassium equilibrium potential; doubling the concentration of external potassium shifted the reversal potential of the current activated by CO2 removal by approximately +20 mV. Raising CO2 above 5% induced an increase in firing rate, an inward current, a decreased slope conductance at potentials near resting membrane voltage, and an increased slope conductance at more negative potentials. These effects of CO2 were mimicked by other manipulations that are known to affect intracellular pH. For example, NH4Cl, which acutely induces intracellular alkalinization, caused a marked reduction in firing rate, an outward current and an increased slope conductance that reversed near the potassium equilibrium potential. Bath-applied barium blocked the effects induced by removal of CO2 or addition of NH4Cl. The polyamine spermine (tetrahydrochloride) applied via intracellular micropipettes blocked the outward current induced by removal of CO2 or addition of NH4Cl. Spermine (free base) or an equivalent concentration of putrescine failed to alter the CO2 (0%)- or NH4Cl-induced effects. We conclude that CO2 maintains the tonic activity of locus coeruleus neurons by decreasing intracellular pH which, in turn, closes inward rectifier potassium channels, an effect that may be mediated by a protonated polyamine. According to this model, when there is alkalinization of locus coeruleus cells through removal of CO2 or addition of NH4Cl, endogenous spermine or a similar polyamine becomes partially deprotonated, releasing the channel block and allowing the cell to hyperpolarize. The possible implications of these results for the physiological effects of CO2 in the locus coeruleus are discussed.

Dissociation of vasoreactivity to acetazolamide and hypercapnia. Comparative study in patients with chronic occlusive major cerebral artery disease

BACKGROUND AND PURPOSE

The aim of this study was to compare the effect of vasodilative stimuli for the measurement of cerebrovascular reactivity obtained by acetazolamide and hypercapnia in patients with chronic occlusive major cerebral artery disease.

METHODS

We examined 24 patients with unilateral occlusive lesions of a major cerebral artery using the 133Xe inhalation technique and single-photon emission CT. Regional cerebral blood flow (CBF) was measured during a resting state, during inhalation of 5% CO2, and 15 minutes after the administration of acetazolamide consecutively in the same patients. Normative values of resting CBF and acetazolamide reactivity were obtained in 21 normal subjects.

RESULTS

All patients with the exception of 1 showed an increase in CBF during hypercapnia ipsilateral to the occlusive lesion. Ipsilateral acetazolamide reactivity was preserved in 13 patients. Conversely, 11 patients showed an absent response or paradoxical CBF reduction. Ipsilateral CO2 reactivity did not correlate with acetazolamide reactivity when all 24 patients were considered. However, there was a significant correlation between acetazolamide and CO2 in the 13 patients who showed preserved acetazolamide reactivity (r = .60, P < .05). No significant correlation was present in the remaining 11 patients with reduced acetazolamide reactivity. Although significant blood pressure augmentation was observed in hypercapnia, we could not find a correlation between change of blood pressure and CO2 reactivity.

CONCLUSIONS

Acetazolamide identified patients with reduced vasomotor reactivity who appeared to have preserved CO2 reactivity. Acetazolamide testing may be useful in the assessment of cerebral hemodynamics. However, further investigations are necessary to assess the clinical utility of these tests.

Central chemoreception in the region of the ventral respiratory group in the rat

We injected acetazolamide (AZ; 5 x 10(-6) M, 1 nl) into the region of the ventral respiratory group (VRG) of anesthetized paralyzed ventilated rats. Control injections (mock cerebrospinal fluid, n = 6, or the inactive AZ analogue 2-acetylamino-1,3, 4-thiadiazole-5-sulfon-t-butylamide, n = 6) did not increase the integrated phrenic neurogram [phrenic nerve amplitude (PNA)]. The AZ injections produced a focal region of tissue acidosis with a radius < 300-400 microns and are used as a probe for sites of central chemosensitivity. Injection location is determined by anatomic analysis. Of 22 VRG injections of AZ, 14 increased the amplitude of the PNA over 15-90 min; 8 had no effect. In 17 cases, we measured medullary tissue pH at the injection center and/or at a distant site and reaffirmed the size of the acidotic region produced by such small AZ injections. Of injections with pH electrodes within 300-400 microns of the injection center, all responders showed an acid pH; three nonresponders showed an acid pH, and one an alkaline pH. In a subgroup of five rats, at VRG sites with known respiratory effects identified by prior glutamate injection (10 nl, 100 mM), all subsequent AZ injections produced a PNA response. Simultaneous measurement of PNA and tissue pH responses at the injection center of eight rats did not show a uniform correlation in time; initially, both changed with a similar time course, but PNA recovered more quickly. We conclude that 1) the region of the VRG contains sites of ventilatory chemoreception, 2) ineffective AZ injections do produce a tissue acidosis but at sites with minimal impact on breathing, and 3) tissue pH does not uniquely represent the chemoreceptor stimulus.

Cerebral vasoreactivity assessed with transcranial Doppler and regional cerebral blood flow measurements. Dose, serum concentration, and time course of the response to acetazolamide

BACKGROUND AND PURPOSE

To improve the assessment of cerebral vasoreactivity using acetazolamide (ACZ), we studied the time course of the response and the relationship between dose, response, and serum concentration.

METHODS

Blood flow velocities were measured with the use of transcranial Doppler ultrasonography in one of the middle cerebral arteries of 48 healthy subjects after the intravenous administration of 1 to 1.6 g ACZ. In 34 subjects (group 1), velocities were measured every second minute to detect the maximum middle cerebral artery velocity increase. We also measured regional cerebral blood flow using single-photon emission computed tomography in 27 of the subjects in group 1 before and approximately 15 to 20 minutes after the ACZ injection. The serum concentration of ACZ was measured in 15 subjects. In the remaining 14 subjects (group 2), middle cerebral artery velocity measurements were made 10, 25, 30, and 45 minutes after ACZ administration to obtain information regarding the late time course of the response.

RESULTS

In group 1 the plateau phase of the velocity response was reached 8 to 15 minutes after ACZ administration. A large range of velocity increase was observed, and a significant correlation was found between the maximum velocity increase and the dose and serum concentration of ACZ. In group 2 subjects, maximum velocities were maintained 30 minutes after the injection, but after 45 minutes velocities had decreased to 68% of their highest level. No significant relationship was found between dose or serum concentration of ACZ and the regional cerebral blood flow increase. The velocity increase after ACZ was similar in both older and younger subjects.

CONCLUSIONS

This study shows that cerebral vasoreactivity is best assessed 10 to 30 minutes after ACZ administration and that the dose should probably exceed 15 mg/kg if a maximum vasodilatory response in the cerebral circulation is to be obtained.

Carbonic anhydrase and CO2 chemoreception in the pulmonate snail Helix aspersa

We have studied the effects of carbonic anhydrase inhibition on the hypercapnic ventilatory response of the pulmonate snail, Helix aspersa, in an isolated brain-pneumostome preparation. We found that the cell permeant carbonic anhydrase inhibitor, acetazolamide (ACTZ), increased pneumostomal opening and ventilation during normocapnia (2-3% CO2) and decreased the rate of pneumostomal response to step changes in CO2 (4.5%), but did not change the steady-state ventilatory response to elevated CO2 (4.5%) compared to the inactive ACTZ analogue, N2-substituted 2-acetylamino-1,3,4-thiadiazole (Cl 13850). In contrast, the cell impermeant carbonic anhydrase inhibitor, quartenary ammonium sulfonilamide (QAS), had no effect on the pneumostomal response to CO2 compared to Cl 13850. Using Hansson's histochemical technique to stain for carbonic anhydrase activity, we identified a small number of neurons in the subesophageal ganglia that exhibited carbonic anhydrase activity. Some of these cells were in the region of CO2-sensitivity. In conclusion, carbonic anhydrase inhibition slows the ventilatory response to rapid changes in CO2, but does not affect the intrinsic ability of H. aspersa to respond to CO2. The ventilatory effects of carbonic anhydrase inhibition may be attributed to the intracellular actions of the carbonic anhydrase enzyme.

Central chemoreceptor stimulus in the terrestrial, pulmonate snail, Helix aspersa

We studied the effect of hypercapnic and fixed acid central chemoreceptor stimulation on the pneumostome in the pulmonate snail, Helix aspersa. We found that focal stimulation of the central chemoreceptor area of the pulmonate snail brain with hypercapnic solutions more effectively increased the pneumostomal area than did fixed acid stimulation at the same extracellular pH. Disrupting intracellular pH regulation by inhibiting Cl- transport, either pharmacologically (DIDS) or by ion substitution (Cl(-)-free perfusate), enhanced pneumostomal responses to CO2. While maintaining a constant perfusate pH, addition of NH4Cl to the perfusate resulted in pneumostomal closure; whereas removal of NH4Cl from the bath resulted in pneumostomal opening. In conclusion, the ventilatory response to CO2 in H. aspersa does not require Cl- transport or conductance. Furthermore, changing pHi alone is an adequate stimulus for the central chemoreceptors in the snail.

Hyperosmolality alters the ventilatory response to acute hypercapnia and hypoxia

Acute hyperosmolality in the Pekin duck results in an extracellular acidosis and hypercarbia without any stimulation of ventilation. The development of the extracellular acidosis is accompanied by the concurrent development of an intracellular alkalosis systemically which has been hypothesized to depress ventilation (Kasserra et al., J. Appl. Physiol., 1993). In order to investigate this apparent suppression of ventilation, the ventilatory response to various respiratory challenges (CO2, O2, K+) was studied both before (normosmotic) and after (hyperosmotic) hypertonic sucrose infusion. Increased plasma osmolality caused a significant drop in arterial pH of 0.06 +/- 0.01 units and a 4 Torr increase in PaCO2, yet did not stimulate any significant increase in ventilation despite a significant increase in oxygen consumption. Acute hyperosmolality increased the PaCO2 associated with resting ventilation, and decreased the magnitude of the ventilatory response to a given increase in PaCO2, compared with the response to the same ventilatory challenge in normosmotic animals. Acute hyperosmolality increased the ventilatory response to hypoxia and K+ compared with normosmotic animals. The opposite effect of hyperosmolality on the ventilatory responses to hypercapnia compared with hypoxia suggests that the mechanisms of chemoreception for hypercapnia and hypoxia are different. The depressed ventilatory response curve to increased PaCO2 and decreased arterial pH during hyperosmolality, both alone and during the hypercapnic challenge, suggests that the peripheral chemoreceptor response to pH and CO2 is suppressed. It is hypothesized that the suppression results from the intracellular alkalosis occurring during acute hyperosmolality.

Indomethacin abolishes cerebral blood flow increase in response to acetazolamide-induced extracellular acidosis: a mechanism for its effect on hypercapnia?

Indomethacin is known to attenuate quite markedly the increase in CBF during hypercapnia. Hypercapnia is, in all likelihood, mediated by the acid shift at the level of the smooth muscle cells of the cerebral arterioles. We therefore investigated the effect of indomethacin on the CBF increase caused by acetazolamide (Az), a drug that induces brain extracellular acidosis, which triggers its effect on CBF. We compared the results to the inhibitory effect of indomethacin on the CBF increase during hypercapnia. Indomethacin but not diclofenac, another potent cyclooxygenase inhibitor, was found to block almost completely the CBF increase caused by Az-induced extracellular acidosis or by CO2, but it did not influence the CBF increase produced by sodium nitroprusside or papaverine. The results suggest that indomethacin exerts its action on CO2 reactivity by a nonprostaglandin-mediated mechanism that directly interferes with the regulation of cerebrovascular tone mediated by extracellular pH.

Acid-base disturbance and ventilatory response to changes in plasma osmolality in Pekin ducks

The effects of acute changes in plasma osmolality on blood acid-base status and ventilation were investigated in the Pekin duck, Anas platyrhynchos. Hyperosmolality due to intravenous infusion of hypertonic NaCl or sucrose caused a prolonged acidosis (so-called dilution acidosis), which was attributable to a decrease in estimated strong ion difference due to a fall in the plasma [Na+]:[Cl-] ratio. Ventilation did not increase in response to the acidosis, and was actually depressed in some birds. PaCO2 increased by 3.5 +/- 1.5 Torr and PaO2 decreased by 4 +/- 2 Torr over the 2 h experimental period in all animals. It is suggested that the extracellular acidosis due to hyperosmolality is accompanied by an intracellular alkalosis which may suppress chemoreceptor stimulation, resulting in no ventilatory increase. Hyposmolality had no effect on acid-base status or respiration.