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K Mückenhoff

Publications and source records attributed to K Mückenhoff.

At least 19 recordsLinked to original sources
PubMed2006

[Interactions of chemosensitive and vagal afferents in respiratory motor activity and work of breathing].

BACKGROUND: The central interaction of intracranial and arterial chemoreceptors plays an important role for the generation of respiratory motor activity. Whereas the inspiratory inhibitory effect of lung stretch receptors is largely described, relatively little is known about their influence on expiration and on the pathomechanisms of obstructive airway disease. MATERIAL AND METHODS: In anaesthetised cats, electrical activity of inspiratory and expiratory muscles (IM, EM) has been studied along with tidal volume and the position of breathing in eupnoea and increased respiratory drive before and after bilateral vagotomy. In order to mimic slowly adapting lung stretch receptor activity (SAR), the distal ends of vagal nerves (VN) were stimulated electrically using a frequency modulated signal derived from the respiratory alterations in oesophageal pressure. RESULTS: With intact VN, electrical stimulation of carotid sinus nerves or inhalation of 5 % CO (2) in O (2) resulted in an increased activation of IM in inspiration and activation of EM in expiration. Elimination of lung stretch receptor activity during quiet breathing resulted in prolongation and increase in IM activity without much effecting EM, since expiration is passive in eupnoeic breathing. After bilateral vagotomy, the electrical activity of the expiratory muscles was effectively reduced and the activity of the inspiratory muscles was largely enhanced. The effects of vagotomy could be completely reversed by electrical VN stimulation. Simulating an increased tidal volume by increasing the end inspiratory stimulation frequency to more than 100 Hz resulted in a decrease in IM activity and tidal volume. On mimicking an increased end expiratory lung inflation by end expiratory frequencies up to 50 Hz, expiration was prolonged and expiratory muscles activated. In consequence the position of breathing shifted to below the functional residual capacity (FRC). CONCLUSION: The results of these acute investigations show, that besides terminating inspiration, lung stretch receptors cause a facilitation of expiration during increased respiratory drive and expiratory airflow limitation by activation of expiratory muscles.

Afferent Pathways↗
PubMed2004

Rhythms, synchrony and electrical coupling in the Locus coeruleus.

Electrical coupling of neurones is believed to promote synchronized activity. It may, however, also be a requirement for the maintenance of endogenous rhythmic activity in some systems. In en bloc isolated brainstem-spinal cord of the neonatal rat simultaneous whole cell recordings from pairs of LC neurones (n = 47 pairs) disclosed for the most part strongly synchronized activity which could take the form of tonic spiking or phasic bursts. Simultaneous whole cell recording from LC neurones and glia also revealed synchronized waves of depolarization in 7 of 17 pairs. This synchrony was partly due to respiratory-phased synaptic input and partly due to mechanisms, which were not dependent on chemical synapses. The gap junction uncoupler carbenoxolone suppressed non-synaptic rhythmic activity in LC neurones, but did not suppress either respiratory-phased synaptic input to these neurones or their excitatory response to increased CO(2). We give preliminary direct evidence for the existence of a current pathway between LC neurones, which is inhibited by carbenoxolone. Within the LC nucleus carbenoxolone-sensitive electrical coupling, which may involve neurone-glia as well as neurone-neurone interactions, may be required not just for synchronization, but also for the maintenance of rhythm.

Action Potentials↗
PubMed2001

Synchronized rhythms in chemosensitive neurons of the locus coeruleus in the absence of chemical synaptic transmission.

The activity of locus coeruleus (LC) neurons was examined in the en bloc isolated brainstem-spinal cord of the neonatal rat using paired whole cell or whole cell plus extracellular recording. In artificial cerebrospinal fluid (ACSF) LC neurons were synchronized by their respiratory innervation and in some neurons showing tonic or burst patterns of discharge these patterns of discharge could also be synchronized. Replacing ACSF with low Ca(2+)-high Mg(2+) generated synchronized rhythmic bursts which remained synchronized at high CO(2) (up to 20%). This rhythm was suppressed by TTX. Substitution of Ba(2+) for Ca(2+) in ACSF generated a synchronized rhythm which was TTX-insensitive. The frequency of this rhythm increased by 31+/-16% on raising CO(2) concentration from 2 to 10%. We conclude that the capacity of chemosensitive LC neurons to generate a synchronized rhythm depends on their electrical coupling, but not on chemical synaptic transmission.

Animals↗
PubMed1999

Locus coeruleus neurones in vitro: pH-sensitive oscillations of membrane potential in an electrically coupled network.

The response to hypercapnic acidosis (2-8% CO2, bath pH 7.8-7.2) was examined in whole cell recordings from neonatal (P1 to P5) rat Locus coeruleus (LC) neurones in the in vitro brainstem-spinal cord preparation exposed to low Ca2+ (0.2 mM)-high Mg2+ (5 mM). This medium suppressed chemical synaptic transmission and resulted in a pattern of subthreshold oscillations of membrane potential and rhythmic burst discharge which was synchronized throughout the network. The oscillation was suppressed, and the discharge of individual neurones desynchronized, by the gap junction uncoupler, carbenoxolone, indicating that in low Ca2+-high Mg2+ LC neurones form an electrically coupled network. Switching from 2 to 8% CO2 decreased the oscillation amplitude and increased its frequency. The oscillation was suppressed by external Cd2+ and by TTX. but persisted during injection into the cell soma of QX-314. We conclude that in LC neurones acidosis increases the frequency of a Ca2+- and Na+-dependent dendritic oscillator which is synchronized by gap junction coupling throughout the network. This coupling is retained during acidosis.

Acidosis↗
PubMed1998

Respiration-modulated membrane potential and chemosensitivity of locus coeruleus neurones in the in vitro brainstem-spinal cord of the neonatal rat.

1. The activity of locus coeruleus (LC) neurones (n = 126) was examined in whole-cell (conventional and amphotericin B-perforated patch) recordings, and the relationship of this activity to the respiratory discharge recorded on the C4 or C5 phrenic nerve roots was determined at different CO2 concentrations (2 and 8 %; bath pH 7. 8 and 7.2) in the in vitro brainstem-spinal cord preparation of the neonatal rat (1-5 days old). 2. In most neurones (n = 105) ongoing activity was modulated at respiratory frequency. Typically, this consisted of a phase of depolarization and increased discharge frequency synchronous with the phrenic burst, followed by a phase of hyperpolarization and inhibition of discharge (n = 94 of 105). The incidence of respiratory modulation decreased from 91 % on P1 to 57 % on P5. 3. Bath application of the non-NMDA receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX; 5 microM) or the NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV; 100 microM) abolished both phases of respiratory modulation. The hyperpolarizing phase alone was abolished by the adrenoceptor antagonists idazoxan (5 microM) or phentolamine (0.8 microM). These results indicate that excitatory amino acid pathways are involved in the transmission of both the excitatory and inhibitory components and that the latter involves in addition an alpha2-adrenoceptor-mediated pathway. 4. Increasing the CO2 concentration from 2 to 8 % resulted in a shortening of expiratory duration and weakening or loss of respiratory-phased inhibition; this was accompanied by depolarization, increased discharge frequency and, in those neurones where they were initially present (60 %), an increase in the frequency of subthreshold membrane potential oscillations. The depolarizing response was retained in the presence of tetrodotoxin (TTX, 0.2-1.0 microM). 5. These results indicate that in this neonatal preparation LC neurones form part of the synaptically connected brainstem respiratory network, and that the LC constitutes a site of CO2- or pH-dependent chemoreception.

Acidosis↗
PubMed1998

Role of the pons in hypoxic respiratory depression in the neonatal rat.

The main purpose of this study was to evaluate the role of the pons in hypoxic respiratory depression (HRD) of the neonatal rat. Experiments were conducted using the isolated brainstem-spinal cord preparation of the neonatal rat (1-3 days old). The brainstem was transected at various levels. We found that ablation of the diencephalon decreased respiratory frequency (fR), and conversely, that ablation of the midbrain or pons increased fR. In the preparation with the pons intact (without the midbrain), hypoxia (superfusate PO2 = 56 mmHg) caused strong depression of respiratory activity, which was characterized by a steady decrease in fR and in integrated inspiratory burst amplitude (integral of Phr). In the preparation with the intact ventral pons (without midbrain and dorsal pons) we observed similar, though weaker, HRD. When the entire pons was ablated, integral of Phr was little depressed by hypoxia and thus, HRD was further attenuated. We conclude that the pons contributes importantly to the induction of hypoxic respiratory depression in the neonatal rat. Both the ventral and dorsal portions of the pons are involved in the control of hypoxic respiratory depression. In addition, we show that the respiratory modulatory functions of the diencephalon (facilitating) and midbrain (inhibitory) are already expressed at the time of birth.

Animals↗
PubMed1996

Chemosensitive medullary neurones in the brainstem--spinal cord preparation of the neonatal rat.

1. Using the isolated medulla and spinal cord of the neonatal rat, the response to CO2-induced changes in superfusate pH was examined in whole cell and perforated patch recordings from ventral medullary neurones which were identified by injection of Lucifer Yellow. The respiratory response to changing the CO2 concentration (from 2 to 8%) consisted of an increase in phrenic burst frequency, which could be accompanied by an increase, decrease or no change in burst amplitude. 2. Five classes of neurone - inspiratory, post-inspiratory, expiratory, respiration-modulated and ionic - were distinguished on the basis of their membrane potential and discharge patterns. Almost all (112 of 123) responded rapidly to 8% CO2 with a sustained change in membrane potential. Depolarizing responses (3-18 mV) occurred in inspiratory, respiration-modulated and 45% of tonic neurones. Hyperpolarizing responses (2-19 mV) occurred in expiratory and post-inspiratory neurones. The remaining tonic neurones were inhibited or showed no response. 3. In representatives of each class of neurone, membrane potential responses to 8% CO2 were retained when tested in the presence of tetrodotoxin (n = 7), low (0.2 mM) Ca(2+)-high (5 mM) Mg2+ (n = 23) or Cd2+ (0.2 mM) (n = 3)-containing superfusate, implying that they are mediated by intrinsic membrane or cellular mechanisms. 4. Neurones were distributed between 1200 microns rostral and 400 microns caudal to obex, and their cell bodies were located between 50 and 700 microns below the ventral surface (n = 104). Almost all responsive neurones (n = 78) showed dendritic projections to within 50 microns of the surface. 6. These experiments indicate that significant numbers of ventral medullary neurones, including respiratory neurones, are intrinsically chemosensitive. The consistency with which these neurones show surface dendritic projections suggests that this sensitivity may arise in part at this level.

Animals↗
PubMed1995

Theophylline and hypoxic ventilatory response in the rat isolated brainstem-spinal cord.

We have used the isolated brainstem-spinal cord preparation of the neonatal rat to study the effects of theophylline on the ventilatory response to hypoxia. The brainstem-spinal cord was isolated from neonatal rats (0-4 days) and superfused with mock cerebrospinal fluid (CSF), equilibrated with a gas mixture (FO2, 0.90; FCO2, 0.02; FN2, 0.08; control CSF) at 27 degrees C. We recorded phrenic nerve discharge from C4 roots, using suction electrodes, and measured respiratory frequency (fR) and the amplitude of the integrated phrenic neurogram (integral of phr). We examined how theophylline and the specific adenosine antagonist, 8-p-sulfophenyltheophylline (SPT), modify the ventilatory response to hypoxia. The response during superfusion with hypoxic CSF (FO2, 0.06) consisted of a marked decrease in fR (to 60% of control) and a slight decline in integral of phr (to 85% of control). By contrast, in the presence of theophylline (30 mg/L = 165 microM) and SPT (5 mg/L = 15 microM) in the superfusate hypoxia reduced fR only moderately (to 87% of control) and exerted virtually no effect on integral of phr (105% of control). Theophylline and SPT attenuated the rate of decrease in fR and completely blocked the decrease in integral of phr. There was no difference between the effects of theophylline and those of SPT. The results suggest that theophylline attenuates hypoxic respiratory depression, and that this effect is mediated by the blockade of adenosine.

Animals↗
PubMed1994

Activity-related pH changes in respiratory neurones and glial cells of cats.

Intracellular pH (pHi) and membrane potential (Em) were measured in vivo in expiratory neurones and glial cells in the medulla of anaesthetized cats using double-barrelled H(+)-sensitive microelectrodes. In glial cells, stimulation of spinal pathways evoked a depolarization of up to 12 mV amplitude and an increase of pHi (7.25 +/- 0.15) by maximally 0.1 pH unit. IN expiratory neurones, pHi (7.15 +/- 0.18) fell by up to 0.2 pH unit during inspiratory inhibition. In axons of expiratory neurones, pHi remained unaffected during rhythmic action potential discharges. We suggest that the glial alkalinization is due to activation of Na+/HCO3- cotransport, whereas the neuronal acidification is caused by efflux of HCO3- via receptor-coupled anion channels.

Animals↗
PubMed1994

Pump for accurate mixing of three gas components with predetermined fractions.

A mixing pump that creates an accurate mixture of three gases at predetermined fractional ratios that can be set in steps of 10 ppm is described. A nearly continuous flow of each of the three component gases is produced by pistons driven by stepping motors; the gas mixture is forwarded by a fourth piston. The flow of each component gas is adjusted by the stepping frequency of the motor and a microcomputer system is used to adjust the three frequencies according to the desired fractional concentrations. The total flow of the gas mixture is adjustable between 0.1-500 ml/min and is nearly independent of the after-load. The accuracy of the pump was tested by mixing the respiratory gases, O2 and CO2, with various carrier gases (N, He or Ar) at various fractional ratios and total flow rates. The fractions of O2 and CO2 in the mixture were analysed with the Scholander technique. In the physiological range, the mixing error in the gas fractions was less than 4%. The pump is, thus, suited for producing calibration mixtures.

Carbon Dioxide↗
PubMed1994

Ion-sensitive microelectrode system with short response time.

The measurement of changes in ion activity (e.g., pH) in neurons requires fine tip-sized double-barreled microelectrodes: one channel being equipped with an ion-selective liquid membrane, the other used for measurement of the membrane potential. The limited bandwidth and the differing transfer functions for electrical and ionic signals necessitate frequency response linearization networks to ensure that the output signal of the electrode is a faithful image of the input signal. We have developed a linearization network to ensure a rapid response time for ion-sensitive microelectrodes. To test the response characteristic we have developed a test system that allows the pH at the electrode tip to be changed within 1 ms. Application of these techniques to electrodes of 1 micron tip diameter results in a 90% response time to a pH step of approximately 60 ms and of approximately 2 ms with electrodes with 20 micron tip.

Electrophysiology↗
PubMed1993

Depth profiles of pH and PO2 in the isolated brain stem-spinal cord of the neonatal rat.

We have measured depth profiles of extracellular pH (pHECR) and PO2 (PtO2) as well as the kinetics of changes of pHECR in the isolated brain stem-spinal cord preparation of the neonatal rat using pH and PO2 microelectrodes that entered from the ventral surface. When the preparation was superfused with control mock cerebrospinal fluid (Control mock CSF; pH = 7.5, PO2 = 630 Torr, PCO2 = 28 Torr, at 27 degrees C), the pH in the medulla diminished with a nearly constant gradient from the surface to a depth of about 1000 microns, the slope being about 0.1 pH unit per 100 microns. A similar gradient in the 200 to 300 microns of the CSF above the surface suggested existence of unstirred layers despite continuously flowing superfusate. The pH gradient in the spinal cord was somewhat smaller than that in the medulla. The PO2 gradients in both medulla and spinal cord were about 100 Torr per 100 microns from 200 microns above to 100 to 200 microns below the surface; PO2 reached zero at about 450 (medulla) to 600 microns (spinal cord). Although the preparation was anoxic and acidic except for a small layer below the surface, respiratory activity was recorded for several hours in C4 phrenic roots. The kinetics of changes in pHECF were recorded at 100 and 200 microns depth while rapidly replacing the control mock CSF by more acidic CSF, either with increased PCO2 ("Respiratory acidosis") or by adding fixed acid ("Metabolic acidosis"). The changes in pHECF were smaller than those in pHCSF, particularly during respiratory acidosis, as a result of the buffering of the brain tissue. Our results show the importance of superficial layers of the ventral medulla in producing respiratory rhythmicity; they further suggest that somewhat alkaline CSF (pH about 7.8) should be used in this preparation to ensure physiologic surface pH values despite unstirred surface layers.

Animals↗
PubMed1993

Hypercapnia and medullary neurons in the isolated brain stem-spinal cord of the rat.

We have extracellularly recorded single neuron activity in the ventral medulla of the isolated brain stem-spinal cord preparation of the neonatal rat (37 preparations) in order to test their sensitivity to changes in CO2/H+. Search for neuronal activity was performed when the preparation was superfused with control mock CSF (equilibrated with 2% CO2, 90% O2 in N2; pH = 7.8 at 27 degrees C). Neurons, found down to about 500 microns from the surface, could be classified as R neurons when they showed rhythmic discharge in phase with phrenic activity, recorded from C4 ventral roots; or as Non-R neurons when they did not exhibit such phasic discharge. Among the 89 Non-R neurons, 20 responded to rapidly replacing the control CSF by hypercapnic CSF (8% CO2, 90% O2 in N2; pH = 7.2) with increased, 44 with reduced activity, while 25 did not respond to hypercapnia. Five Non-R neurons became phasic with respiration during hypercapnia. Of the 14 R neurons, 10 fired predominantly in expiration (R-E), 4 in inspiration (R-I). Only one R-E and two R-I neurons were excited by hypercapnia, the remaining were either inhibited or did not respond. Excited Non-R and R neurons were mainly encountered in rostral parts of those areas in the ventral medulla that have been reported as chemosensitive.

Animals↗
PubMed1989

Afferent vagal activity during hyperthermic polypnea in the pigeon.

Respiration-modulated activity in afferent vagal fibers was recorded in 10 pigeons during euthermic breathing and thermal panting. Of these fibers, 13 were identified as intra-pulmonary chemoreceptors (IPCs), that increased discharge with diminishing lung gas PCO2, and 13 as mechanoreceptors, that increased firing with lung inflation. Two types of IPC were observed that were distinct by their firing pattern during panting. Phasic IPCs displayed phasic discharge within the respiratory cycle, even at respiratory frequencies (fresp) as high as 400 min-1. Tonic IPC fired tonically and increased their discharge as fresp increased. Several IPCs were silent during euthermic breathing, but discharged tonically as fresp increased with thermal polypnea. Discharge of neither type of IPC was consistently related to PaCO2. Discharge from mechanoreceptors was phasic with respiration, up to values of fresp as high as 350 min-1. However, the average number of impulses per breath decreased as fresp increased. We conclude that discharge from phasic intrapulmonary chemoreceptors and mechanoreceptors may contribute to setting the respiratory pattern during hyperthermic polypnea.

Animals↗
PubMed1984

Evidence for a role of NA+/H+ exchange in platelets activated with calcium-ionophore A 23187.

We have investigated the release of protons from human platelets and platelet aggregation induced by the calcium ionophore, A 23187. Addition of the ionophore to suspensions of washed platelets resulted in fast liberation of H+. In the presence of 0.2 mM amiloride, a potent inhibitor of Na+/H+ countertransport, the amount of protons liberated was decreased by 50% and was further reduced to about 10% by 1 mM amiloride. Similar inhibition of H+-release was observed after decreasing Na+ in the incubation medium. Both results suggest that increasing internal Ca2+ by the ionophore induces Na+/H+ exchange in human platelets. Platelet aggregation could be induced by adding the ionophore to the platelet suspension. This aggregation was inhibited by amiloride, at least when induced by low ionophore concentrations. The results suggest that stimulation of Na+/H+ exchange, and the concomitant increase in intraplatelet pH, are important mechanisms in platelet activation.

Amiloride↗
PubMed1984

Thrombin stimulates Na+-H+ exchange across the human platelet plasma membrane.

We have investigated the release of protons from thrombin-stimulated platelets. Addition of thrombin to suspensions of washed platelets resulted in fast liberation of H+. In the presence of 0.1 mM amiloride, a potent inhibitor of the Na+/H+ transport system, the amount of protons liberated was decreased by about 50%, and was further reduced to about 15% by 1 mM amiloride. Similar inhibition of H+ release was observed after Na+ in the incubating medium had been replaced by choline. We conclude that one of the earliest events in thrombin-stimulated platelets consists of the activation of an Na+/H+ countertransport, which leads to an increase in intracellular pH.

Amiloride↗
PubMed1981

The influence of changes in pCO2 on the fractional packed cell volume of whole blood.

In order to investigate the influence of changes in pCO2 on the fractional packed cell volume (FPCV, hematocrit) of whole blood, a device for measuring the conductivity was developed. This method allows an instantaneous and continuous determination of the FPCV, because the erythrocyte membrane has insulating properties, and, consequently, the resistance of blood depends on the relative cell volume. The steady state and transient relationships between FPCV and acid-base levels were investigated by combining this method with simultaneous recordings of pCO2. The experiments showed that addition of CO2 caused an increase in the resistance of whole blood, whereas the resistance of separated plasma decreased slightly and the resistance of true plasma remained almost constant. The change in the FPCV (delta H) can be described by a linear function of pH or log pCO2 (formula: see text). The transient response of the resistance, after a stepwise increase in the CO2 content, was found to be the slowest process in attaining an acid-base equilibrium. In blood with acetazolamide, the time courses of changes in pH and pCO2 were retarded, whereas the time course of the resistance change reflecting the swelling of the erythrocytes was nearly the same (T 50 approximately equal to 4 s). This may indicate a rate-limited water shift due to a slight water permeability of the erythrocyte membrane.

Acetazolamide↗