In vivo micropunctureP CO 2 measurements

Summary An electrode is presented which permits in vivoP _{CO 2} measurements using a micropuncture technique. The tip diameter of the electrode is only a few microns, the tip is specially designed for measuringP _{CO 2} in small tissue compartements.This work was supported by grants from the Swedish Medical Research Council, The Bergwall Foundation and the Medical Faculty of Uppsala, Sweden.     

Interstitial PCO2 and pH in rat hippocampal slices measured by means of a novel fast CO2/H+-sensitive microelectrode based on a PVC-gelled membrane

We describe here the construction and properties of a double-barrelled microelectrode (tip diameter 4–10 m) which permits simultaneous measurements of PCO₂ and pH, and which has a 90% response time of only one or a few seconds for a step change in PCO₂. The fast response of the CO₂-sensitive barrel is due to (i) the use of a PVC-gelled (tridodecylamine-containing) membrane solution which enables the construction of extremely short ( 4 m), yet mechanically stable, membrane columns, and (ii) the presence of carbonic anhydrase in the filling solution. Recordings made in the pyramidal layer of area CA1 in rat hippocampal slices showed that the deviation in the acid direction of the basal interstitial pH (pH₀) from that of the perfusion solution was attributable to a higher PCO₂ level within the tissue. Most of the late acid shift evoked by stimulation of the Schaffer collaterals (5- to 20-s trains at 10 Hz) could also be explained on the basis of an accumulation of interstitial CO₂ at a constant HCO ₃ ^– concentration. This conclusion was supported by the finding that inhibition of extracellular carbonic anhydrase activity by 10 M benzolamide completely abolished the activity-induced fall in pH₀, but not the increase in PCO₂. The initial stimulus-induced alkalosis was accompanied by a slight decrease in PCO₂ only, implying a parallel increase in the interstitial HCO ₃ ^– concentration. Benzolamide produced a dramatic enhancement of the early alkaline shift as well as of the simultaneous fall in PCO₂. The latter effect of the drug unmasks a cellular CO₂ sink that is induced by neuronal activity.     

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.     

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).     

Comparison of arterial,end-tidal and transcutaneous PCO2 during moderate exercise and external C02 loading in humans

Summary Static relationships between arterial, transcutaneous[/p] and end-tidal PCO₂ (P _aCO₂, P _tc CO ₂, P _etCO₂) as well as the dynamic relationship between P _etCO₂ and P _tcCO₂ were studied during moderate bicycle ergometer exercise with and without external C0₂ loading. The exercise pattern consisted of 5-min intervals of constant power at 40 W and 100 W and 900 s of randomised changes between these two power levels. The external CO₂ loading was achieved by means of controlled variations of inspiratory gas compositions aimed at a constant P _etCO₂ of 6.5 kPa (49 mm Hg). The P_etO2 was regulated at 17.3 kPa (130 mm Hg). Under steady-state conditions all PCO₂ parameters showed close linear relationships. P _aCO₂/P _tcCO₂ was near to identity while the P _etCO₂ systematically overestimated changes in P _aCO₂. No relationship showed a significant influence of the exercise intensity. Transients of P _tcCO₂ are considerably slower than P _etCO₂ transients. The dynamic relationship between both parameters was found to be independent of whether internal or external C0₂ loadings were applied. It is concluded that the combination of P _etCO₂ and P _tcCO₂ measurements allows an improved non-invasive assessment of P _aCO₂. While P _etC0₂ better reflects the transients, P _tcCO₂ can be employed to determine slow changes of the absolute P _aCO₂.     

CO2 responsivity in the mouse measured by rebreathing

We have modified the rebreathing method to study CO₂ responsivity in very small mammals. Tidal volume (V _T) and frequency (f) of pentobarbital-anesthetized mice were measured during rebreathing from a closed circuit, primed with 95% O₂, 5% CO₂, through which the gas was constantly circulated at 0.5 l·min^–1. The circuit consisted of T-tube from a plethysmograph, Tygon tubing with compliant element, CO₂ analyzer and pump, in series. CircuitPCO₂ (PctCO₂), which was recorded continuously during spontaneous breathing, rapidly equilibrated with end-tidalPCO₂. CO₂ response curves were constructed from extrapolated minute ventilation ( ),V _T,f and parameters of breath-to-breath timing, respectively, onPctCO₂. Analyses of slopes of the response curves, change from onset of rebreathing to peak response, andPctCO₂ at which the response peaked revealed that CO₂ stimulates by increasingf andV _T and that this is effected by facilitation of central inspiratory-expiratory phase switching and inspiratory drive mechanisms. However, the stimulatory effect of CO₂ on phase switching was not sustained, with maximal effect occurring before peak . The advantages and facility of the modified rebreathing method make it suitable for studies of other small mammals, including neonates.     

Water and total CO2 reabsorption along the rat proximal convoluted tubule

Micropuncture experiments were performed on rat kidney to evaluate the profile of water and total CO₂ reabsorption along the proximal tubule. Three to eight samples were collected along the same nephron and the puncture-to-glomerulus distances were measured for each site. In Munich rats with accessible glomeruli, the water reabsorption rate was found to be constant all along the first five millimeters of proximal tubule. In Sprague Dawley rats with no accessible glomerulus, the same observation was made for these five millimeters, and the water reabsorption rate per mm along this segment was found to be a function of the glomerular filtration rate. For the two last millimeters of tubule accessible at the kidney surface, the water reabsorption rate was found to decrease in 5 out of the 21 tubules studied and ranged from 0.15–3.5 nl·min^–1·mm^–1.In Sprague Dawley rats the fall in the luminal total CO₂ concentration (CO₂)_t along the tubular length was nearly constant (21 mmole·l^–1) between Bowman's capsule and the end proximal tubule, irrespective of the plasma (CO₂)_t value. The distance needed to reach half-maximum total CO₂ reabsorption varied from 1.1–1.9 mm from one tubule to another, as a function of the total CO₂ filtered load. These data suggest that the tubular length involved in avid bicarbonate reabsorption increases as a function of the filtered load and that in the first millimeters of tubule, bicarbonate reabsorption depends on a rapidly saturable mechanism. However, no close relationship was found between total CO₂ movement or the calculated transepithelial chloride gradient on the one hand and water reabsorption along the convoluted proximal tubule on the other.     

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     

The effects of ventilation pattern on carbon dioxide transfer in three computer models of the airways

We investigate the effects on arterial P(CO(2)) and on arterial-end tidal P(CO(2)) difference of six different ventilation patterns of equal tidal volume, and also of various combinations of tidal volume and respiratory rate that maintain a constant alveolar ventilation. We use predictions from three different mathematical models. Models 1 (distributed) and 2 (compartmental) include combined convection and diffusion effects. Model 3 incorporates a single well-mixed alveolar compartment and an anatomical dead-space in which plug flow occurs. We found that: (i) breathing patterns with longer inspiratory times yield lower arterial P(CO(2)); (ii) varying tidal volume and respiratory rate so that alveolar ventilation is kept constant may change both PA(CO(2)) and the PA(CO(2))-PET(CO(2)) difference; (iii) the distributed model predicts higher end-tidal and arterial P(CO(2)) than the compartmental models under similar conditions; and (iv) P(CO(2)) capnograms predicted by the distributed model exhibit longer phase I and steeper phase II than other models.     

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).     

Ventilatory response to oscillating and non-oscillatingPa CO 2 in the anesthetized cat

Summary In 11 adult cats, lightly anesthetized with chloralose-urethane, blood from both common carotid arteries was led into a plastic chamber of 15–20 ml and returned to the carotids at a point 1.5 cm more cranial. By doing so arterial blood was assumed to pool within the chamber and lose itsP _{CO 2} oscillations which are normally known to exist as a result of the respiratory cycle. In control periods blood bypassed the chamber, thus maintaining respiratoryP _{CO 2} oscillations. Spontaneous ventilation was measured spirometrically. The animals were breathing pure O₂.Results. 1. When the sinus (carotid) nerves were intact or sectioned there was no significant difference in ventilation before or after switching from non-oscillating to oscillatingPa _{CO 2}. 2. When the vertebral arteries were ligated a drop in ventilation occurred after turning to oscillatingPa _{CO 2} which was followed by a slight rise above control values after 30–50 sec. This phenomenon was independent of sinus nerve integrity. Thus in hyperoxie condition the smallPa _{CO 2} oscillations known to occur in phase with respiration do not seem to provide a respiratory stimulus to resting ventilation above that generated by the mean level ofPa _{CO 2}. The ventilatory depression after vertebral artery ligation must at this time remain unexplained.     

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)     

Influence of the bicarbonate pool and on the occurrence of 13CO2 in exhaled air

Summary In ¹³C0₂ breath tests, based on ¹³C:¹²C ratio measurements, the appearance of ¹³C in exhaled C0₂ was monitored after the administration of a ¹³C-la-belled compound. Independently of the substrate used, the existence of a bicarbonate pool into which the C0₂ produced enters before being exhaled, imposes a delay on the appearance of changes in the ¹³C:¹²C ratio. To estimate the nature and magnitude of this delay, we applied a two-compartment model to describe the kinetics of the body bicarbonate pool and we evaluated the ¹³C:¹²C ratio of C0₂ entering that pool from the measured ¹³C:¹²C ratio in the exhaled C0₂ after an oral intake of naturally labelled ¹³C-glucose. Our results demonstrated that discrepancies between total and exogenous glucose oxidation in relation to the peak occurrence time, as well as the absolute quantities, could be adequately explained by the interference of the bicarbonate stores.     

The effects of CO2 and pH on the spontaneous activity of the taenia coli of guinea-pig

Summary Mechanical and electrical activity of the isolated guinea-pig taenia coli were recorded in bicarbonate-, Tris- or phosphate-buffered test solutions. In normal solution (pCO₂ 5%, pH 7.4), typical minute-rhythmical fluctuations of activity occurred, whereby activity was present for approximately 40% of the total time (active time), the other 60% being activity-free intervals. At constant extracellular pH, low CO₂ content increased active time, high CO₂ content reduced it. Extracellular alkalinization at constantpCO₂ also diminished active time, even in CO₂-free medium, whereas acidification raised it, sometimes causing continuous activity. At constant bicarbonate-buffer, changes of CO₂ content, i.e. accompanied by corresponding changes in pH, affected the active time much less than did CO₂ alterations in an isohydric medium. The test solutions had no major effect on frequency of synchronized spike discharges, in contrast to their actions on the minuterhythmical activity. Since CO₂ can pass through the cell membrane more easily than ions regulating intra- and extracellular pH, the observed effects are best explained by changes in the transmembrane pH-gradient. A drop in active time would be due to a relative intracellular acidification; continuous activity, on the other hand, due to an opposite change in transmembrane pH-gradient, i.e. a relative intracellular alkalinization.This work was supported by the Deutsche Forschungsgemeinschaft     

Cellular mechanism of force development in cat middle cerebral artery by reducedPCO2

These studies were undertaken to determine the effect of reducing aPCO₂ below physiological levels on cat middle cerebral artery. Upon reduction ofPCO₂ from 37 to 14 torr (pH 7.4) we observed membrane depolarization and force development. ReducingPCO₂ decreased the slope of theE _m vs. log [K]_o curve and increased the slope of the steady-state I/V relationship suggesting that the change inE _m was due to reduction of outward K⁺ conductance (g _k). Elevation of pH from 7.37 to 7.6 had a very similar effect on these cerebral arterial muscle cells, depolarizing the muscle membrane (reducing theE _m vs. log [K]_o curve) and increasing the slope of the I/V relationship to statistically equivalent values as reduction ofPCO₂. ReturningPCO₂ from 14 to 37 torr rapidly relaxed these preparations, but only transiently. This relaxation was followed by a rebound contraction within 3 min, demonstrating a transient nature for the action of elevatingPCO₂ in cerebral arteries. The response to changing pH_o followed a slower time course but did not change with time. These studies demonstrate that both elevated pH_o and reducedPCO₂ activate cerebral arterial muscle by a mechanism which includes reduction ing _k. However, it can not be determined if these similar responses and reduction, ofg _k are mediated by changing pH_i or mediated through different mechanisms. It is possible that pH_o andPCO₂ can modify cerebral arterial tone by direct mechanisms and not necesarily by their effect on pH_i. It is clear, however, that reduction ofPCO₂ and elevation of pH_o both activate cerebral arterial muscle by a mechanism which includes reduction ofg _k.This study was supported by NIH grant no. HL-32871. Dr. Harder is an established investigator of the American Heart Association     

The role of fitness on VO2 and VCO2 kinetics in response to proportional step increases in work rate

Summary The purpose of this study was to determine the effect of fitness and work level on the O₂ uptake and CO₂ output kinetics when the increase in work rate step is adjusted to the subject's maximum work capacity. Nine normal male subjects performed progressive incremental cycle ergometer exercise tests in 3-min steps to their maximum tolerance. The work rate step size was selected so that the symptom-limited maximum work rate would be reached in four steps at 12 min in all subjects. Oxygen consumption (VCO₂) and carbon dioxide production VCO₂ were calculated breath by breath. For the group, the time (mean, SEM) to reach 75% of the 3-min response (T _0.75) for VO₂ increased significantly (P<0.01) at progressively higher work rate steps, being 53.3 (5.5) s, 63.5 (4.6) s, 79.5 (5.0) s, and 94.5 (5.8) s, respectively. In contrast, T _0.75 for VCO₂ did not change significantly [74.9 (7.4) s,. 75.6 (5.0) s, 85.1 (5.3) s, and 89.4 (6.3) s, respectively]. VCO₂ kinetics were slower than VO₂ kinetics at the low fractions of the subjects' work capacities but were the same of faster at the high fractions because of the slowing of VO₂ kinetics. The first step showed the fastest rise in VO₂. While VO₂ kinetics slowed at each step, they were faster at each fraction of the work capacity in the fitter subjects. The step pattern in VO₂ disappeared at high work rates for the less fit subjects. The heart rate response paralleled that of VO₂. We conclude that VO₂ and VCO₂ kinetics are slower in the less fit subjects but only VO₂ kinetics are significantly attenuated in response to proportional step increases in work rate.     

Acid-Base curve nomogram for dog blood

Summary Pooled canine blood of different hemoglobin concentration was equilibrated with two carbon dioxide tensions and the resulting pH's of the samples were measured at 38°C. Readings obtained provided data for the construction of a cartesian nomogram for the dog, based on the pH/logpCO₂ coordinate system. The nomogram can be used to evaluate both respiratory and non-respiratory acid-base parameters. For contrast, a control nomogram on human blood was also constructed. Both nomograms show broad similarity. This is to be expected as the normal criteria used to define base excess relating to dog blood are the same as those used for human blood. Nonetheless, base excess and buffer base calculations using the nomogram for the dog and that for man show a deviation of 2.5–10%, and we consider this deviation to be more due to a species difference than to any summated experimental error.     

A Miniaturized Carbon Dioxide Gas Sensor Based on Sensing of pH-Sensitive Hydrogel Swelling with a Pressure Sensor

A measurement concept has been realized for the detection of carbon dioxide, where the CO₂ induced pressure generation by an enclosed pH-sensitive hydrogel is measured with a micro pressure sensor. The application of the sensor is the quantification of the partial pressure of CO₂ (Pco₂) in the stomach as diagnosis for gastrointestinal ischemia. The principle is put to the proof by examining the sensor response to changes in Pco₂. Furthermore, the response time, temperature-sensitivity and resolution are determined. The sensor responds well to changes in Pco₂ with a maximum pressure generation of 0.29× 10⁵ Pa at 20 kPa CO₂. The 90% response time varies between 1.5 and 4.5 minutes at 37^∘C. The sensor shows a linear temperature-sensitivity which can easily be compensated for, and enables detection of Pco₂ changes as small as 0.5 kPa CO₂.     

Sex steroids and their receptors in patients with tumors of the pancreas

A tendency toward an increase in serum content of sex steroid-binding globulin and a decrease in the total concentration of testosterone and its free and globulin-bound fractions and in the index of free androgens was observed in patients with pancreatic cancer. There were no statistically significant differences in baseline serum levels of hypophyseal hormones and prolactin in male patients and in the control. One or both types of steroid hormone receptors, the content of androgen receptors being higher than that of estrogen receptors, varying in a wide range were detected in 71% of the patients with malignant neoplasms. The occurrence of steroid hormone receptors was higher in adenocarcinomas, while their content was significantly higher in normal pancreatic tissues. The occurrence of androgen receptors was higher in male patients at the early stages of the disease, predominantly in highly differentiated adenocarcinomas located in the body of the pancreas. The content of androgen receptors was higher in male patients, in metastasizing tumors, in adenocarcinomas located in the body of the pancreas, and in patients without clinical and laboratory evidence of the mechanical jaundice syndrome. The content of androgen receptors positively correlated with the maximum size of the tumor. Translated fromByulleten' Eksperimental'noi Biologii, I Meditsiny, Vol. 126, No. 8, pp. 197–200, August, 1998     

Effect of H+ and elevated PCO 2on membrane electrical properties of rat cerebral arteries

Some membrane electrical properties of muscle cells from the middle cerebral artery of the rat were recorded with intracellular microelectrodes. The resting membrane potential (E _m) of this preparation was –63 mV. Reduction of extracellular pH to 7.0 in the face of a constantP _{CO 2}of 40 mm Hg had no significant effect onE _m. Similarly the slope of the steady-state voltage/current curves was not different at pH 7.0 compared to control at pH 7.4. In marked contrast, whenP _{CO 2}was elevated to around 60 to 70 mm Hg there was a rapid hyperpolarization and reduction in the slope of the voltage current curve suggesting an increased conductance for one or more ionic species. In addition elevation ofP _{CO 2}increased the slope of theE _m vs. log[K]₀ curve from 46 mV/decade to 59 m V/decade which is in good agreement with a Nernstian potential for a K⁺ selective membrane. These data suggest that while the smooth muscle cells of rat cerebral arteries are relatively insensitive to a small reduction in extracellular pH; reduction of intracellular pH by elevatingP _{CO 2}induces hyperpolarization by increasing K⁺ conductance (g _k). However, it is not clear from these experiments if theP _{CO 2}effects are mediated entirely by changes in pH or if there is a direct membrane action of CO₂.This work is supported by Grant no. HL27862