Absence of long-term modulation of ventilation by dead-space loading during moderate exercise in humans

The stability of arterial PCO₂ (P_aCO₂) during moderate exercise in humans suggests a CO₂-linked control that matches ventilation (_E) to pulmonary CO₂ clearance (CO₂). An alternative view is that _E is subject to long-term modulation (LTM) induced by hyperpnoeic history. LTM has been reported with associative conditioning via dead-space (V_D) loading in exercising goats (Martin and Mitchell 1993). Whether this prevails in humans is less clear, which may reflect differences in study design (e.g. subject familiarisation; V_D load; whether or not _E is expressed relative to CO₂; choice of P_aCO₂ estimator). After familiarisation, nine healthy males performed moderate constant-load cycle-ergometry (20 W-80 W-20 W; <lactate threshold, _L): day 1, pre-conditioning, n=3; day 2, conditioning (V_D=1.59 l, doubling _E at 20 W and 80 W), n=8 with 10 min rest between tests; and, after 1 h rest, post-conditioning, n=3. Gas exchange was determined breath-by-breath. Post-conditioning, neither the transient [phase 1, phase 2 (1, 2)] nor steady-state _E exercise responses, nor their proportionality to CO₂, differed from pre-conditioning. For post-conditioning trial 1, steady-state _E was 28.1 (4.7) l min^–1 versus 29.1 (3.8) l min^–1 pre-conditioning, and mean-alveolar PCO₂ (a validated P_aCO₂ estimator) was 5.53 (0.48) kPa [41.5 (3.6) mmHg] versus 5.59 (0.49) kPa [41.9 (3.7) mmHg]; the 1 _E increment was 4.2 (2.9) l min^–1 versus 5.2 (1.9) l min^–1; the 2 _E time-constant () was 64.4 (24.1) s versus 64.1 (25.3) s; _E/CO₂ was 1.12 (0.04) versus 1.10 (0.04); and the _E-CO₂ slope was 21.7 (3.4) versus 21.2 (3.2). In conclusion, we could find no evidence to support ventilatory control during moderate exercise being influenced by hyperpnoeic history associated with dead-space loading in humans.     

Analysis of alveolar PCO 2 control during the menstrual cycle

We attempted to analyze how is regulated during progesterone-induced hyperventilation in the luteal phase. A model for the CO₂ control loop was constructed, in which the function of the CO₂ exchange system was described as and that of the CO₂ sensing system as . Using this model, we estimated (1) the primary increase in produced by progesterone stimulation and (2) the effectiveness (E) of the loop to regulateP _{A CO 2}, defined as P _{A CO 2} (op)/P _{A CO 2} (cl) in which op signifies open-loop and cl, closed-loop. These respiratory variables were investigated throughout the menstrual cycle in 8 healthy women. During the luteal phase, on average, increased by 9.4% andP _{A CO 2},B andH decreased by 0.33 kPa (2.5 mm Hg), 0.47 kPa (3.5 mm Hg) and 13.6%, respectively, whileS and did not change significantly. (op) increased progressively on successive days of the luteal phase whileE remained unchanged at a value of 7.9, thus there was a progressive decrease inP _{A CO 2}. The decrease inH was considered to lessen P _{A CO 2} (op) and so reduce the final deviation ofP _{A CO 2} (P _{A CO 2} (cl)) during the luteal phase. The decrease inB was found to be dependent on (op).     

Physiological responses to intermittent and continuous exercise at the same relative intensity in older men

We investigated the physiological responses in older men to continuous (CEx) and intermittent (IEx) exercise. Nine men [70.4 (1.2) years, O_2peak: 2.21 (0.20) l min^–1; mean (SE)] completed eight exercise tests (two CEx and six IEx) on an electronically braked cycle ergometer in random order. CEx and IEx were performed at 50% and 70% O_2peak. IEx was performed using 60sE:60sR, 30sE:30sR and 15sE:15sR exercise to rest ratios. The duration of exercise was adjusted so that the total amount of work completed was the same for each exercise test. Oxygen uptake (O₂), minute ventilation (_E) and heart rate (HR) were measured at the mid-point of each exercise test. Arterialised blood samples were obtained at rest and during exercise and analysed for pH and PCO₂. At the same relative intensity (50% or 70% O_2peak), IEx resulted in a significantly lower (P<0.01) O₂, _E and HR than CEx. There were no significant differences (P>0.05) in O₂, _E and HR measured at the mid point of the three exercise to rest ratios at 50% and 70% O_2peak. pH and PCO₂ during CEx and IEx at 50% O_2peak were not significantly different from rest. CEx performed at 70% O_2peak resulted in significant decreases (P<0.05) in pH and PCO₂. There was a significant decrease (P<0.05) in pH only during the 60sE:60sR IEx at 70% O_2peak. Changes in arterialised PCO₂ during the 60sE:60sR, 30sE:30sR and 15sE:15sR at both 50% and 70% O_2peak exercise tests were not significant. When exercising at the same percentage of O_2peak and with the total amount of work fixed, IEx results in significantly lower physiological responses than CEx in older men. All results are given as mean (SE).     

Dihydropyridines,phenylalkylamines and benzothiazepines block N-, P/Q- and R-type calcium currents

We compared the effects of representative members of three major classes of cardiac L-type channel antagonists, i.e. dihydropyridines (DHPs), phenylalkylamines (PAAs) and benzothiazepines (BTZs) on high-voltage-activated (HVA) Ca²⁺ channel currents recorded from a holding potential of –100 mV in rat ventricular cells, mouse sensory neurons and rat motoneurons. Nimodipine (DHP), verapamil (PAA) and diltiazem (BTZ) block the cardiac L-type Ca²⁺ channel current (EC₅₀: 1 M, 4 M and 40 M, respectively). At these concentrations, the drugs could also inhibit HVA Ca²⁺ channel currents in both sensory and motor neurons. Large blocking effects (> 50%) could be observed at 2–10 times these concentrations. The -conotoxin-GVIA-sensitive (-CTx-GVIA, N-type), -agatoxin-IVA-sensitive (-Aga-IVA, P- and Q-types) and non-L-type -CTx-GVIA-, -Aga-IVA-insensitive (R-types) currents accounted for more than 90% of the global current. Furthermore, our data showed that CTx-GVIA and -Aga-IVA spare L-type currents and have only additive blocking effects on neuronal HVA currents. We conclude that DHPs, PAAs and BTZs have substantial inhibitory effects on neuronal non-L-type Ca²⁺ channels. Inhibitions occur at concentrations that are not maximally active on cardiac L-type Ca²⁺ channels.     

CO2 laser radiant heat pulses activate C nociceptors in man

Cerebral potentials evoked by noxious CO₂ laser stimuli in man have been referred to nociceptive A units. This paper shows 1) that ultra short (5–50 ms) high power ( 50 W) CO₂ laser skin stimuli are able to activate nociceptive C units in man, and 2) that these C nociceptors have to be assumed to terminate in the very superficial skin layer (300 m depth).     

Enhancement of O2 and CO2 Transfer Through Microporous Hollow Fibers by Pressure Cycling

An intravascular gas exchange device for the treatment of respiratory failure consisted of a multitude of blind-ended hollow fibers glued in a pine-needle arrangement to a central gas supply catheter. It has previously been shown that gas desorption rates can be significantly enhanced by cycling gas pressure between a hypobaric level of 130 and an ambient level of 775 Torr. In this study, influences of the cycling frequency (f) and the cycle fraction during which hypobaric pressure is applied () were investigated. Rates of O₂ desorption from O₂-saturated water and CO₂ desorption from CO₂-saturated water into a manifold containing 198 fibers, 380 m in diameter, were measured over a range of f from 0.2 to 1.0 Hz, from 0.1 to 0.8, and fiber lengths from 4 to 16 cm. Relative to operation at ambient pressure, pressure cycling increased O₂ transfer 3–4 times and CO₂ transfer 4–6 times when the water flowed over the fiber manifold at 2.3 l/min. Transfer rates were relatively insensitive to f and with 80–90% of maximum enhancement obtained when was as low as 0.2. Transfer rates increased continuously with fiber length, implying that pressure cycling reduced the intra-fiber resistance to gas diffusion. A mathematical diffusion model which utilized only two adjustable parameters, a mass transfer coefficient for O₂ and for CO₂, simulated the trends exhibited by desorption data. © 1998 Biomedical Engineering Society. PAC98: 8745Hw, 8790+y     

The effectiveness of the control of pH in the extracellular fluid of the brain by the respiratory control system

Summary The effectiveness of the respiratory control system as a regulator of the pH in the extracellular fluid of the brain is defined by pH_{ECF op}/pH_{ECF cl} where pH_{ECF op} means the primary or open loop shift and pH_{ECF cl} the final or closed loop shift of brain extracellular fluid pH. The analysis of a steady state model described in a preceding paper (Loeschcke, 1973) allows, in the limits of the suppositions and simplifications, to calculate the effectiveness of the feedback regulator in the cases of increased metabolism, metabolic acidosis-alkalosis and inhalation of CO₂. The effectiveness is diminished if CO₂ production is increased, it drops in metabolic acidosis and rises in metabolic alkalosis and it drops steeply if CO₂ is inhaled. The effectiveness of this control system depends on the controlling action of the controlling element (the ventilation) rather than on varying sensitivity of the sensing element. The controlling effect is defined asC=–d P _CO2/d V _A orC=d pH_ECF/d V _A.Im Rahmen des Programms des Sonderforschungsbereiches 114 (Bionach) der Deutschen Forschungsgemeinschaft.     

The involvement of expiratory termination in the vagally mediated facilitation of ventilatory CO2 responsiveness during hyperoxia

In anaesthetized rabbits the influence of differential vagal cold blockade on the ventilatory response to inhaled CO₂ during hyperoxia was investigated.Following total inactivation, the relationship between ventilation ( ) and arterialPCO₂ (P _aCO₂) was shifted to the left and steepened slightly over a range of modest hypercapnia, but was progressively flattened as hypercapnia intensified. The latter effect, suggestive of a vagally mediated facilitation of ventilatory CO₂ responsiveness, was studied further.Differential vagal cold blockade to a temperature (5–11°C) which abolished the Breuer-Hering inflation reflex (end-inspiratory tracheal occlusion no longer eliciting a prolongation of expiratory duration,T _E) had no effect on either during normocapnia or at a substantial level of hypercapnia. Only with further vagal cooling to 0°C did the ventilatory depression during hypercapnia emerge, largely becauseT _E failed to shorten in response to the hypercapnic stimulus.It is concluded that the integrity of expiratory-terminating mechanisms is crucial for the manifestation of the vagally mediated facilitation of and its CO₂ responsiveness which is evident during hyperoxic hypercapnia. A possible role is suggested for lung epithelial irritant receptors or for the tonic late-expiratory activity from pulmonary stretch receptors.Supported by the Deutsche Forschungsgemeinschaft, SFB 114Preliminary reports of this work have been presented in Pflügers Arch 355: (Suppl) R47 (1975); 377: (Suppl) R54 (1978) and in Proc. XXVIII. Int. Congr. of Physiol. Sciences, Budapest, Vol VIV, 515 (1980)     

A comparison of the abilities of CO2/HCO 3 − , protonophores and changes in solution pH to release Ca2+ from the SR of barnacle myofibrillar bundles

Increases in solution pH from 6.5 to 7.0 to 7.5 at 0.1 M free Ca²⁺ concentration had no effect on the isometric tension of barnacle myofibrillar bundles in relaxing solutions containing 0.1–0.16 mM BAPTA. Decreases in pH in the same range were also without effect. Under the same conditions CO₂-induced Ca²⁺ release from the SR could be readily obtained by replacing the Cl^–-containing relaxing solution with one containing HCO ₃ ^– and 100% CO₂ at the same pH. At a higher free Ca²⁺ of 2.5 M, there was a contraction on increasing the pH of the Cl^–-containing solution from 7.0 to 7.5. This reponse could be abolished by 1 mM procaine suggesting that it was due to Ca²⁺ release from the SR. The protonophores monensin, gramicidin, CCCP and FCCP at concentrations of 10–100 M had no effect on resting tension at either free Ca²⁺ concentration and did not inhibit the response to 100% CO₂. It is concluded that dissipation of a possible pH gradient across the SR membrane by protonophores does not release Ca²⁺ from the SR of barnacle muscle. Since both CO₂ (by possibly lowering SR pH) and an increase in solution pH can release Ca²⁺ at 2.5 M free Ca²⁺, the existence of a Ca²⁺ release channel which is opened by a change in the trans-SR pH gradient cannot be discounted. In a separate series of experiments, the CO₂-releasable Ca²⁺ store was first depleted by exposure of bundles to 100% CO₂ in the presence of 1 mM BAPTA for 10 min and then partially reloaded by 15 s exposure to a free Ca²⁺ of 0.6 M buffered by 2 mM EGTA (total) in the Cl^– relaxing solution. The same level of reloading was obtained when the loading solution contained HCO ₃ ^–/100% CO₂; this result tends to discount inhibition of the Ca²⁺ uptake pump as a possible mechanism for CO₂-induced Ca²⁺ release. Loading at 2.6 M free Ca²⁺ for 1 min resulted in almost complete recovery of the CO₂ response to its value before depletion of the store.     

CO2 fixation in the nervous tissue IV. CO2 fixation and citrate metabolism in lobster nerve

Summary The specific activity of citric acid of the lobster nerve which was incubated in Ringer-bicarbonate solution containing ¹⁴C-bicarbonate was determined. The specific activity of citric acid was higher than that of malic acid by a factor of 2.5. The citric acid obtained from the lobster nerve was degraded with an improved method which is described in this paper. The ratio of the radio-activity of C-6 to C-1 of citric acid was 11. Aspartic acid obtained from the lobster nerve was also degraded, and the ratio of the radioactivity of C₄ to C₁ was almost 101.From these results, it is assumed that CO₂ fixation in the lobster nerves occurs at the oxalosuccinate level and at the oxaloacetate level and that the rates of these fixations were almost the same. Thus the active backwards reaction from -ketoglutaric acid to citric acid in lobster nerve was confirmed. It was also possible that both the activity of the citrate cleavage enxyme and the mixing of the dicarboxylic acid carboxyl groups were minimal.The concentration of oxaloacetic acid was estimated to be 2 mmole/mg protein, or 4 mmoles for a 50 mg nerve.Fellow of the Rockefeller Foundation     

Fast measurement of the CO2 partial pressure in gases and fluids

A CO₂-electrode system consisting of a membrane covered pH electrode, an electronic antilog modul and a special electronic analog circuit is described. Since the electrode output signal is a logarithmic function of the CO₂ partial pressure the output signal of the antilog module is proportional to the CO₂ partial pressure. The time course of the electrode signal has been analyzed after a step change of . This step response may be approximated by a sum of three exponential functions. Knowing the dynamic behaviour, the transfer function is formulated mathematically and a special analog circuit is constructed with a frequency response inverse to the frequency response of the electrode. Using this device the response time (T ₉₅) of the electrode system is diminished from 11,5 s to 750 ms after a step change of in gas (Luttmann, et al., 1974). If the time for the hydration of CO₂ is decreased by the addition of carbonic anhydrase the response time of the electrode is diminished to 6.5 s. Using the analog circuit yields a response time of 200 ms.Further studies were made to analyze the transient response in fluids at various flow velocities and various mountings. In order to analyze the influence of the fluid boundary layer on the surface of the electrode a photometric method has been developed (Luttmann and Mückenhoff, 1975), which allows to estimate the time course of the CO₂ partial pressure independently of and simultaneously with the electrode measurement.The experimental data are compared with a theory based on theoretical considerations of Schuler and Kreuzer (1967) and Crank (1956).List of Symbols A _i gain factor of thei-th compartment - A _i gain factor of the simulation network - C _i capacity - D diffusion coefficient - d electrode diameter - fraction of CO₂ - F _c (j) frequency response of the linearizing network - F _g (j) frequency response of the boundary layer - F _M (j) frequency response of the measuring system - F _M frequency response of the simulation network - I _i impedance transformer - j Gauss number (j ²=–1) - (j) frequency function of the CO₂ partial pressure - (t) time function of the CO₂ partial pressure - R _i electrical resistance - T _i time constant of the electrode - T _i time constant of the simulation network - T ₉₅ time for reaching 95% of the total difference - V voltage gain factor - v velocity of the streaming fluid - x coordinate - x(t) time function - X(j) corresponding frequency function - dilution factor - inverse time constant - thickness of boundary layer - ^* kinematic viscosity - thickness of diffusion layer - radian frequency Supported by the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 114 (Bionach)     

Roles of inositol trisphosphate and protein kinase C in the spontaneous outward current modulated by calcium release in rabbit portal vein

We examined the effects of heparin, guanosine nucleotides, protein kinase C (PKC) modulators, such as phorbol 12,13-dibutylate (PDBu) and H-7 on Ca²⁺-dependent K⁺ currents in smooth muscle cells of the rabbit portal vein using the whole-cell patch-clamp technique, to explore the effects of PKC on the oscillatory outward current (I _oo). Neomycin (30 M), an inhibitor of phospholipase C, and intracellular applications of heparin (10 g/ml) and guanosine 5-O-(2-thiodiphosphate) (GDP[S]; 1 mM) partly but consistently inhibited the generation of I _oo, whereas a higher concentration of heparin (100 g/ml) transiently enhanced then suppressed the generation of I _oo. Inhibition of I _oo generation by heparin was more powerful at the holding potential of + 20 mV than at –20 mV. Inositol 1,4,5-trisphosphate (InsP ₃; 30 M) continuously generated I _oo at holding potentials more positive than –60 mV. Noradrenaline (10 M) and caffeine (3–20 mM) transiently augmented, then reduced the generation of I _oo. Heparin (10 g/ml) completely inhibited responses induced by InsP ₃ and noradrenaline, but not those induced by caffeine. Intracellular application of guanosine 5-triphosphate (GTP; 200 M) or low concentrations of guanosine 5-O-(3-thiotriphosphate) (GTP[S]; 3 M) continuously augmented the generation of I _oo. High concentrations of GTP[S] (10 M) transiently augmented, then inhibited I _oo. Neither GTP[S] nor noradrenaline induced the transient augmentation or the subsequent inhibition of I _oo when applied in the presence of GDP[S] (1 mM), neomycin (30 M) or heparin (10 g/ml). PDBu (0.1 M) reduced the generation of I _oo but failed to produce an outward current following application of caffeine (3–5 mM). This action of PDBu was inhibited by pretreatment with H-7 (20 M). In the presence of H-7, GTP[S] continuously enhanced the generation of I _oo. The suppression of the generation of I _oo during application of noradrenaline (10 M) was reduced by pretreatment with H-7. Thus both InsP₃ and protein kinase C contribute to the generation of I _oo in smooth muscle cells of the rabbit portal vein and heparin is not a specific InsP ₃ antagonist on the InsP ₃-induced Ca²⁺-release channel (PIRC). InsP ₃ opens PIRC and protein kinase C may deplete the stored Ca²⁺ by either inhibiting the reuptake of Ca²⁺ or by enhancement of the releasing actions of InsP ₃.     

Bell-shaped activation of inositol-1,4,5-trisphosphate-induced Ca2+ release by thimerosal in permeabilized A7r5 smooth-muscle cells

There is no consensus about the different types of Ca²⁺ transport processes in the endoplasmic reticulum that are targeted by the sulphydryl reagent thimerosal. We have therefore investigated how thimerosal affects the various Ca²⁺ transport processes in permeabilized A7r5 smooth-muscle cells, using an unidirectional ⁴⁵Ca²⁺ flux technique. Thimerosal up to a concentration of 32 M did not have an effect on the passive ⁴⁵Ca²⁺ leak from the stores, while higher concentrations increased this aspecific leak. Thimerosal inhibited the endoplasmic reticulum Ca²⁺ pump with an EC⁵⁰ of 9 M. Thimerosal exerted a biphasic effect on the Ca²⁺ release induced by inositol 1,4,5-trisphosphate [Ins(1,4,5)P ₃] with a stimulation of the release at thimerosal concentrations below 10 M, and an inhibitory effect at higher concentrations. Thimerosal (2.5–250 M) did not exert an effect on the specific binding of [³H]Ins(1,4,5)P ₃ to its receptor, indicating that it probably did not act at the level of the binding site. This finding contrasts with the effect of the closely related sulphydryl reagent parachloromercuriphenylsulphonate, which, at high concentrations, inhibited [³H]Ins(1,4,5)P ₃ binding. The effects of thimerosal were largely prevented by the sulphydryl reducing agent dithiothreitol (3 mM). We conclude that thimerosal concentrations ranging from 0.32 to 1 M can stimulate the Ins(1,4,5)P ₃-induced Ca²⁺ release without inhibiting the Ca²⁺ pumps or without increasing the passive Ca²⁺ permeability of the endoplasmic reticulum.     

Diffusionsfehler und Eigenverbrauch der Pt-elektrode beipO2-Messungen im steady state

Zusammenfassung Im steady state beeinflussen Diffusionsfehler und Eigenverbrauch der Pt-Elektrode als systematische Fehler O₂-Partialdruckmessungen. Sie sind abhängig von den geometrischen Eigenschaften der Elektrode, den Diffusionseigenschaften der Membran sowie den Diffusions- und Konvektionseigenschaften des Meßmediums. Das Diffusionsfeld vor der Pt-Oberfläche und das dadurch bestimmte stationäre Meßsignal werden für gasförmige und nicht gasförmige Medien mit und ohne Konvektion berechnet. Daraus resultieren quantitative Aussagen über die systematischen Fehler. Speziell für Messungen in durchbluteten Geweben (z. B. Hirnrinde und Myokard) wird der Einfluß des Eigenverbrauchs von Pt-Elektroden auf den intracapillärenpO₂-Abfall in Durchblutungsrichtung und das intercapillärepO₂-Feld am Meßort der Elektrode ermittelt. Diese Berechnungen erfolgten mit Hilfe eines Digitalmodells.

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Erklärung der Symbole A O₂-Verbrauch des Gewebes - Bunsenscher Löslichkeitskoeffizient des Mediums - _m Bunsenscher Löslichkeitskoeffizient der Membran - C ₁,C ₁,C ₂,C ₂,C ₃ Konstanten - D Diffusionskoeffizient des Mediums - DF, DF Diffusionsfehler bei einfacher und doppelter Membran - DGl Differentialgleichung - d Capillarabstand - d _h Dicke der hydrodynamischen Grenzschicht - d _m ,d _m Dicke der Membranen - , , , , , dimensionslose Parameter - exp Exponentialfunktion - F Faradaykonstante - grad Gradient - I _o stationäres Meßsignal in Medien ohne Konvektion - I stationäres Meßsignal in Gasen - I _o stationäres Meßsignal in Flüssigkeiten mit Konvektion - J _o nullte Bessel-Funktion - K Diffusionsleitfähigkeit des Mediums - KE, KE Konvektionseffekt bei einfacher und doppelter Membran - K _m ,K _m Diffusionsleitfähigkeit der Membranen - l Capillarlänge - l Capillarabschnitt - Viscosität des Mediums - p, pO₂,p(r), p(r,z) O₂-Partialdruck - p mittlerer Partialdruck - P _a O₂-Partialdruck am arteriellen Capillarende - p _c konstanter Partialdruck - P/r _o +d _m O₂-Partialdruck an der Grenze Membran/Medium - P _v O₂-Partialdruck am venösen Capillarende - P _g relativer O₂-Partialdruckabfall im Gewebe - P _v relativer O₂-Partialdruckabfall am venösen Capillarende - R Radius der ebenen kreisförmigen Elektrode - RB Randbedingung - RDF, RDF restlicher Diffusionsfehler einfacher und doppelter Membranen - r _o Radius der Elektrode mit halbkugelförmiger Pt-Oberfläche - r, z Zylinderkoordinaten - r _K Capillarradius - S Sättigungsabfall im Capillarblut ohne Elektrode - S Sättigungsabfall im Capillarblut mit Elektrode - u O₂-Konzentration - V Diffusionsgesamtfluß - V _K Diffusionsfluß aus einem Capillarabschnitt - v _r ,v _z Komponenten des Stromdichtevektors inr- bzw.z-Richtung (Zylinderkoordinaten) - mittlere Stromdichte - Stromdichtevektor des Flusses der O₂-Moleküle - v _c konstante Geschwindigkeit des bewegten Mediums - x, y, z Kartesische Koordinaten - Integrationsvariable - ² Laplace-Operator - partielle Ableitung nach der Zeit     

Einstellzeit der Pt-Elektrode bei Messungen nicht stationärer O2-Partialdrucke

Zusammenfassung Das Meßsignal bei sprunghaftenpO₂-Änderungen wird anhand des Diffusionsfeldes der Elektrode beschrieben. Es wird das zeitliche Verhalten des Meßsignals von blanken und membranbespannten Elektroden in gasförmigen und nicht gasförmigen Meßmedien betrachtet. Aus dem Verhalten des Meßsignals kann jeweils die Einstellzeit alssystematischer Meßfehler abgeleitet werden. In nicht gasförmigen Medien (z. B. biologisches Gewebe) übersteigt das Meßsignal nach einempO₂-Sprung zu höheren Werten das stationäre Endsignal. Daraus ergibt sich eine besondere Betrachtung der Einstellzeit in solchen Medien.Die Einstellzeit für Pt-Elektroden mit einfacher und doppelter Membran wird explizit angegeben. Schließlich wird für biologische Medien die Einstellzeit mit dem Diffusionsfehler [8] verglichen. Die Forderungen an eine Membran der Pt-Elektrode mit kleiner Einstellzeit und gleichzeitig kleinem Diffusionsfehler sind zusammengestellt.

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Erklärung der Symbole a Verhältnis der Diffusionskoeffizienten zweier Membranen - Bunsenscher Löslichkeitskoeffizient des Mediums - _m Bunsenscher Löslichkeitskoeffizient der Membran - b Verhältnis der Diffusionsleitfähigkeiten von Membran und Medium - C ₁,C ₂ Proportionalitätskonstanten zwischen Meßsignal und O₂-Partialdruck - D Diffusionskoeffizient des Mediums - D _m,D _m Diffusionskoeffizienten der Membranen - Diffusionskoeffizient der effektiven Membran - DF Diffusionsfehler - DGl Differentialgleichung - d _m,d _m Dicke der Membranen - Dicke der effektiven Membran - dimensionsloser Parameter des Diffusionsfehlers - erf Fehlerfunktion - exp Exponentialfunktion - F Faradaykonstante - grad Gradient - I stationäres Meßsignal vor dempO₂-Sprung - I stationäres Meßsignal nach dempO₂-Sprung - I(t), I(),I() instationäres Meßsignal als Funktion der Zeit bzw. zeitabhängiger dimensionsloser Parameter - K Diffusionsleitfähigkeit des Mediums - K _m Diffusionsleitfähigkeit der Membran - Diffusionsleitfähigkeit der effektiven Membran - dimensionsloser Parameter - n Summationsindex - pO₂ O₂-Partialdruck - pO₂ als Funktion von Ort und Zeit bzw. zeitabhängiger dimensionsloser Parameter; Diffusionsfeld der Elektrode - p _c konstanterpO₂ vor dempO₂-Sprung - p _c konstanterpO₂ nach dempO₂-Sprung - p(r ₀+d _m , ) pO₂ an der Grenze Membran/Medium in Abhängigkeit des Zeitparameters - p(r,o) Diffusionsfeld zum Zeitpunkt (t=0) despO₂-Sprunges - p(r₀+d_m, o) pO₂ an der Grenze Membran/Medium zum Zeitpunkt despO₂-Sprunges - R Radius der ebenen, kreisförmigen Elektrode - r ₀ Radius der Elektrode mit halbkugelförmiger Pt-Oberfläche - r Kugelkoordinate - ₁,₂ dimensionslose ortsabhängige Parameter - T ₉₀,T ₉₅ Zeit, bis 90% bzw. 95% des Signalunterschiedes nach dempO₂-Sprung ausgeglichen sind (Einstellzeit) - T ₉₀,T ₉₅ Einstellzeit der Elektrode mit Doppelmembran - T _90*,T _95* Zeit, bis sich das Signal nach Übersteigen des stationären Endwertes diesem auf 10% bzw. 5% angenähert hat - dimensionslose Parameter zu den vorangegangenen Einstellzeiten - t Zeitkoordinate - , dimensionslose zeitabhängige Parameter - t _max, _max Zeit maximaler Signalhöhe nachpO₂-Sprung und zugehöriger dimensionsloser Parameter - V(t) Diffusionsgesamtfluß zur Pt-Oberfläche - Stromdichtevektor der diffundierenden O₂-Moleküle - x, y, z Kartesische Koordinaten - Integrationsvariable - ² Laplace-Operator - partielle Ableitung nach der Zeit - Integral über eine Fläche - gerichtetes Flächenelement     

CO2 fixation in the nervous system V. CO2 fixation and citrate metabolism in rabbit nerve

Summary The extent of CO₂ fixation in the sciatic nerve of the rabbit was determined. The specific activitiy of citric acid was higher than that of glutamic, aspartic, and malic acids, and the specific activity of citric acid obtained from the 2 hour incubation nerve was close to 1/3 of that of the CO₂ in the medium. The ratio of the radioactivity of the C-6 to C-1 of citrate was about 21 in intact nerves and about 11 in damaged nerves, and the ratio of the radioactivitiy of C-4 to C-1 of aspartate was approximately 11 in both cases. These results suggest that in the sciatic nerve of the rabbit: 1) the dicarboxylic acid shuttle was active, 2) the extent of the carboxylation at the oxalosuccinic acid level was 1/2 or more of that at the oxaloacetic acid level, and 3) the CO₂ fixation by the carboxylation of a-ketoglutaric acid might have some relationship to nerve function. The significance of CO₂ fixation, and the possible relationship between the carboxylation of -ketoglutaric acid and the concentrations of citric acid, acetyl-CoA and acetylcholine, and the control of the rate of tricarboxylic acid cycle were discussed.Fellow of the Rockefeller Foundation     

Mechanisms of antagonistic action of internal Ca2+ on serotonin-induced potentiation of Ca2+ currents in Helix neurones

The influence of internal Ca²⁺ ions has been investigated during intracellular perfusion of isolated neurones from pedal ganglia of Helix pomatia in which serotonin (5-HT) induces a cyclic-adenosine-monophosphate-(cAMP)-dependent enhancement of high-threshold Ca²⁺ current (I _Ca). Internal free Ca²⁺ ([Ca²⁺]_i) was varied between 0.01 and 10 M by addition of Ca²⁺-EGTA [ethylenebis(oxonitrilo)tetraacetate] buffer. Elevation of [Ca²⁺]_i depressed the 5-HT effect. The dose/ effect curve for the Ca²⁺ blockade had a biphasic character and could be described by the sum of two Langmuir's isotherms for tetramolecular binding with dissociation constants K _d1=0.063 M and K _d2=1 M. Addition of calmodulin (CM) antagonists (50 M trifluoperazine or 50 M chlorpromazine), phosphodiesterase (PDE) antagonists [100 M isobutylmethylxanthine (IBMX) or 5 mM theophylline] and protein phosphatase antagonists [2 M okadaic acid (OA)] in the perfusion solution caused anticalcium action and modified the Ca²⁺ binding isotherm. Using the effect of OA and IBMX, two components of the total Ca²⁺ inhibition were separated and evaluated. In the presence of one of these blockers tetramolecular curves with K _d1=0.04 M and K _d2=0.69 M were obtained describing the activation of the retained unblocked enzyme — PDE or calcineurin (CN) correspondingly. The sum of these isotherms gave a biphasic curve similar to that in control. Leupeptin (100 M), a blocker of Ca²⁺-dependent proteases did not influence the amplitude of 5-HT effect, indicating that channel proteolysis is not involved in the depression. Our findings show that the molecular mechanism of Ca²⁺-induced suppression of the cAMP-dependent upregulation of Ca²⁺ channels is due to involvement of two Ca²⁺-CM-dependent enzymes: PDE reducing the cAMP level, and CN causing channel dephosphorylation. No other processes are involved in the investigated phenomenon at a Ca²⁺ concentration of less than or equal to 10 M.     

Effects of purinergic stimulation on the Ca current in single frog cardiac cells

Ca current (I _Ca) was measured by whole-cell voltage clamp in single cells isolated from frog ventricle, in which the Na current was inhibited by tetrodotoxin (0.3 M) and K currents were blocked by substituting K with 120 mM intracellular and 20 mM extracellular Cs. The influence of stimulation by ATP (0.1–100 M) was assessed in the presence of propranolol (1 M) or pindolol (0.1 M), prazozin (0.1 M) and atropine (10 M). ATP, in the micromolar range, had two types of effect. Like other P₁-purinoagonists, it antagonized the increase in I _Ca elicited by -adrenostimulation. When added alone, 1 M ATP could increase I _Ca up to twofold. An increase in I _Ca was also observed even after it had been maximally enhanced by intracellularly applied cAMP (50 M). Voltage dependence and kinetics of I _Ca were not affected. These effects were considered to be related to P₂-purinoceptor activation. At higher ATP concentrations the increase in I _Ca was less; at 100 M, ATP reduced I _Ca. The ATP-induced increase in I _Ca was prevented by internal perfusion of the cells with GDP [-S] or neomycin, respectively, to block signal transduction to phospholipase C or its phosphodiesterase activity on the polyphosphoinositides. We conclude that P₂purinoceptor stimulation increases the Ca current in frog ventricular cells by a pathway that might involve phosphoinositide turnover.     

APCO2 surface electrode working on the principle of electrical conductivity

APCO₂ electrode working on the principle of electrical conductivity is described. The calibration curve can be linearized according to the formula . This linearity has been tested in thePCO₂ range of 0.93–9.33 kPa (7–70 Torr). For the experiments electrodes are used which have conductivity values of about 50 nS and drifts of maximally 5%/h at aPCO₂ of 5.33 kPa (40 Torr). The response time (T ₉₀) is about 20 s. The temperature sensitivity is 2.4 nS/1 K between 298K–310K. The standard error of the measurements is =0.33 nS. With these electrodes tissuePCO₂ can be measured on the surface of various organs.     

Carbon dioxide,extracellular pH and fibre water in frog skeletal muscle

The fibre water of frog skeletal muscle was increased by exposure of the muscle to CO₂. Exposure to acid at constant PCO₂ caused no change in fibre water although muscle weight decreased, whereas exposure to alkali increased fibre water.Supported by the Schweizerische Stiftung für Midizinisch-Biologische Stipendien