Intrapulmonary receptors in the Tegu lizard: II. Functional characteristics and localization;

Intrapulmonary receptors identified in the Tegu lizard by single-unit vagal recording (Fedde et al., 1977) were subjected to a number of stimuli and localized within the lung. Some carbon dioxide receptors could follow periodic changes in intrapulmonary CO2 concentrations as rapidly as 1.3 Hz; No oxygen sensitivity was observed with this receptor type, and halothane markedly depressed the discharge frequency. In response to intravenously injected acetazolamide they increased their discharge frequency and became almost totally insensitive to CO2, suggesting molecular per se is not the direct controller of receptor discharge; These receptors show many of the functional characteristics described for those in the avian lung. Afferent activity from both CO2 and mechanoreceptors could be elicited by electrically stimulating the lung surface. The CO2 receptors appeared to be organized in a receptive field covering more than 1 cm2 of lung surface, multiple receptors being innervated by a single afferent fiber. Activity in afferent fibers from mechanoreceptors could be evoked from only one distinct spot on the lung surface. Conduction velocities of afferent fibers from CO2 receptors ranged from 1 to 3 m-sec-1; from mechanoreceptors, from 1.9 to 5.2 m-sec-1.

Intrapulmonary receptors in the Tegu lizard: I. Sensitivity to CO2

Single unit vagal recordings from intrapulmonary receptors were obtained in decerebrate, paralyzed lizards both during pump ventilation and during unidirectional ventilation on the cannulated, sack-shaped lung. Two types of receptors were identified: (1) CO2-receptors, which increased their discharge frequency as intrapulmonary CO2 concentration decreased but were not sensitive to stretch of the lung. (2) Mechanoreceptors, which rapidly increased discharge frequency when the lung was stretched. These receptors' CO2 sensitivity varied. Lungs of lizards thus appeared to possess both CO2 receptors, which have functional characteristics similar to those in birds, and mechanoreceptors with properties similar to stretch receptors in mammals.

Gas-blood CO2 equilibration in parabronchial lungs of birds

We have conducted two experimental series in the chicken in order to study CO2 exchange in the parabronchial lungs of birds. In the first series, the animals were artifically ventilated and end-expired PCO2, PE'CO2, was measured and compared with mixed venous PCO2, PVCO2. On the average, PECO2 exceeded PVCO2 by 2.8 Torr. In the second series, rebreathing was used to investigate the mechanism of this positive (PE'-PV)CO2 difference. Lung gas PCO2 was found to equilibrate with PVCO2 if both CO2 and O2 exchange in the lung was abolished during rebreathing. Only if O2 uptake continued, we observed a positive gas-to-mixed venous blood PCO2 difference. The results suggest that positive gas-blood PCO2 differences both during rebreathing and steady-state ventilation are brought about by the Haldane effect. Model calculations show that in the homogeneous avian lung, unlike in the alveolar lung, the Haldane effect can produce positive (PE'-PV)CO2 differences during steady-state breathing due to the peculiarities of the crosscurrent arrangement and parabronchial ventilation and blood perfusion.

Study of CO2 sensitive vagal afferents in the cat lung

Single unit activity was recorded in the cat vagus in order to detect possible receptors firing in response to changing lung CO2 concentration. The cats were ventilated at a high rate (60-120 breaths per min) and inspired CO2 concentration was altered between 0 and 8% in a step-like fashion, each phase consisting of about 10 breaths. Thus the effects of changing intrapulmonary CO2 concentration could be differentiated from the effects of stretch of lung tissue. Activity was recorded in 7 cats from 120 units firing in phase with ventilation. Many receptors showed some CO2 sensitivity, but no fiber was found discharging in response to CO2 exclusively. The results provide no evidence for the occurence of specific CO2 receptors in the feline lung with vagal afferents functionally similar to those reported for the avian lung.

A technique for study of lung function in birds by blocking the primary bronchus

The technique described allows in vivo placement of a blocking balloon in the avian primary bronchus between the origins of the two sets of secondary bronchi, the medioventrals and mediodorsals. With caudal air sacs cannulated the animal may then be ventilated with a constant flow of gas passing entirely through the gas-exchanging parabronchi. This technique has been found useful particularly in studies of pulmonary gas exchange in birds.

Effects of CO2 on pulmonary air flow resistance in the duck

Effects of CO2 on pulmonary smooth muscle were assessed by measuring the air flow resistance of secondary bronchi and parabronchi in ducks unidirectionally ventilated with a constant gas flow through the parabronchial lung, the bypass of the primary bronchus being occluded by a blocking catheter. Pressure differences across the blocking balloon (deltaP), corresponding to the pressure drop in the gas flowing through the mediodorsal and medioventral secondary bronchi (MD and MV) and parabronchi, were measured at flow rates (V) varied from 0.5 to 3 L-min-1 and at CO2 concentrations of ventilating gas (FICO2) varied from 0 to 10%. 1) deltaP increased more than linearly with V. The resulting flow resistance R(= deltaP/V) averaged 43 and 95 cm H2O-L-1-sec at V = 0.5 and 3 L-min-1, respectively. 2) Step changes in FICO2 at constant V were followed within 0.5 to 5 sec by changes in R. 3) Lowering FICO2 from 5% resulted in marked increases in R, the value at FICO2 = 0% being more than twice the average value at 5%. Raising FICO2 from 5% up to 10% was followed by only slight changes in R. 4) Vagotomy did not consistently change R at any level of CO2; it did, however, slightly increase the delay time for changes in R on step changes of FICO2. 5)The medioventral secondary bronchi and their orifices into the primary bronchus appeared to be mainly responsible for the resistance measured and its changes with CO2. The resistance offered by the parabronchi appeared to be much smaller and much less dependent on CO2. The results suggest importance of lung gas CO2 in aerodynamic valving of respiratory flow in avian lungs during normal breathing and particularly during thermal panting to prevent alkalosis.

Measurement of Krogh's diffusion constant of CO2 in respiring muscle at various CO2 levels: evidence for facilitated diffusion

Krogh's diffusion constant for CO2, KCO2, was determined in respiring muscle tissue at various levels of tissue PCO2, between 10 and 160 torr, using a technique described previously (Kawashiro et atl, 1975). With increasing mean tissue PCO2, KCO2 declined towards an apparently asymptotic value. The relationship between KCO2 (10(-9) mmol-cm(-1)-min(-1)-tor(-1)) and PCO2 (torr) at 37 degrees C could be approximated by the equation KCO2 = 17.3 [1 + 1.72 - exp(--0.027 - PCO2)]-At PCO2 = 0 torr KCO2 exceeded the asymptotic value, which was virtually attained at PCO2 = 100 torr, by more than a factor of two. Thus CO2 diffusion in muscle appears to be facilitated in the low PCO2 range. Specific CO2 production rate of tissue, which was determined simultaneously, did not vary with CO2 in the PCO2 range studied. Effects of facilitated CO2 transport on CO2 exchange in muscle are assessd using simple models. In the presence of CO2 facilitation muscle PCO2 is reduced, particularly during exercise.

Convective and diffusive gas mixing in human lungs: experiments and model analysis

Equilibration of inspired with lung residual gas was studied by a single-breath technique for varying breath-holding time with He, Ar, and SF6 as test gases. The ratio of end-expired (FE') to mean lung concentration after expiration (FL) was always below unity, indicating imperfect mixing of gas in the lung. The ratio of FL/FE' for all gases increased with tB, for any tB the ratio was smallest for SF6 and greatest for He. Similarly, Bohr dead space (VD) at any given tB was greatest for SF6 and smallest for He, with VD decreasing toward an asymptotic value common for all gases as tB increased. The results were analyzed quantitatively on a serial three-compartment model of the lung. Model analysis suggests that both diffusion and convection are effective in equilibrating test gases in the lung during breath holding. Further, stratified inhomogeneities in the absence of convective gas mixing in the alveolar space would seriously limit alveolar respiratory gas exchange; with convection, however, stratification is likely to impose only moderate constraints on resting gas exchange.

Gas exchange in air sacs: contribution to respiratory gas exchange in ducks

Air sac gas exchange was studied in ducks by measuring the rates of inert gas uptake and of O2 and CO2 equilibration in caudal thoracic air sac whose ventilation was prevented by surgival sealing of the ostia. The data were analyzed on a model incorporating three possible routes by which air sac gas could be exchanged with the surrounding tissue: (1) into the blood perfusing the air sac walls; (2) into the adjoining air sac via tissue membranes; (3) into the bronchial system of the lung via diffusion through lung tissue bordering upon the caudal thoracic air sac. Exchange rates of gases via the two latter paths were found to be small as compared with the first route. From application of model parameters to O2 and CO2 exchange in air sacs under physiological conditions the following conclusions were drawn: (1) the caudal thoracic air sac makes the major contribution to total gas exchange between air sacs and blood; (2) this exchange can account for less than 5% of total respiratory gas exchange; (3) the exchange is too small to account for the O2 and CO2 partial pressures in caudal thoracic air sacs of ducks. Other mechanisms like gas exchange in neopulmonic parabronchi, which conduct air to the caudal air sacs during inspiration or re-inspiration of dead space appear to play a more significant role in the deviation of O2 and CO2 partial pressures in the caudal air sacs from those in inspired air.

Measurement of diffusivity and metabolic rate of O2 and CO2 in respiring tissue

The method described is apt to measure, at the same time, Krogh's diffusion constant and specific metabolic rates for O2 and CO2 in intact, respiring tissues. Due to metabolism tissue thickness for this method is limited to about 500 mum unless hyperbaric conditions are used. The results suggest that both KO2 and KCO2 are similar in alive and in dead tissue. Due to tissue inhomogeneity and to possible facilitation of O2 or CO2 transport our values of K have to be considered as effective mean values for the physiological range of PO2 and PCO2 in muscle at rest and at exercise.

Problems in determination of oxygen dissociation of avian blood

No appreciable errors are expected in determination of blood gas values and pH using classical techniques provided time of anaerobic storage is kept small and is, if unavoidable performed on ice. In particular, dissociation curves may safely be analyzed with the Van Slyke technique which is in disagreement with the conclusions of Lutz et al. (1973). For measurement of PO2 and PCO2, delay time is mainly dictated by response time of the electrodes; measurements may have to be corrected for metabolism, particularly in high PO2 range.

Determination of diffusivity of oxygen and carbon dioxide in respiring tissue: results in rat skeletal muscle

Gas transfer rates for O2 and CO2 through freshly excised respiring rat abdominal muscle were measured. The tissue separated as a thin membrane two chambers, one of which was ventilated with a constant gas mixture. The other chamber was closed and the time course of changes of PO2 and PCO2, initially set at varied levels, was followed by electrodes. A plot of rate of change of PO2 and PCO2 in the closed chamber against the partial pressure difference across the tissue yielded both Krogh's diffusion constant, KO2 and KCO2, and metabolic rate of tissue, i.e. specific O2 consumption and CO2 production, mo2 and mco2. The mean values at 37 degrees C, KO2 = 1.31 x 10(-9) mMol-cm-1-min-1-torr-1 and KCO2 = 28.0 x 10(-9) mMol-cm-1-min-1-torr-1, did not differ significantly from values determined by other authors in various tissue preparations in which metabolism had been suppressed. Average O2 consumption, mo2 = 0.87 mMol-min-1-L-1, was not different from the values obtained in the same tissue by the Warbung manometric method, 0.74 mMol-min-1-L-1. The mean respiratory quotient, calculated as the ratio of mean CO2 production and mean O2 consumption, was 0.85.

The differential effect of cooling on responses of cerebellar cortex

1. Responses of the cerebellar cortex in anaesthetized cats were evoked by mossy fibre and/or climbing fibre inputs, and the effects of graded cooling of the cerebellar cortex were investigated. Cooling was applied either globally by flooding the exposed cortex with cooled Ringer Locke, or in later experiments locally be passing cooled fluid through a silver tube in contact with the cerebellar cortex. The cortical temperature was continuously monitored by a thermistor inserted to a depth of 0.5 mm close to the recording site. 2. In the granular layer the cooling caused a large increase in the diphasic P1N1 wave generated by the afferent mossy fibre volley. The waves generated by synaptic excitation and discharge of granule cells, N2P2, were not diminished until the temperature fell towards 20 degrees C. In contrast the N3 wave of the molecular layer was largest with cooling in the range of 35 to 25 degrees C, often several times larger than at 38 to 40 degrees C. Associated with the enhanced N3 wave there was an enhanced N4 wave, which indicates an increased discharge by Purkynĕ cells. 3. Climbing fibre inputs generate a negative field potential in the molecular layer due to the powerful excitation of Purkynĕ cells. In contrast to the N3 potential this climbing fibre wave was largest at the higher temperatures 35-40 degrees C and declined progressively with cooling, being usually suppressed at moderate coolings of 31-27 degrees C. Intracellular recording revealed that the diminution was due both to the elimination of all but the first impulses of the normal burst discharge of the climbing fibre impulses and to the diminution of the synaptic excitation of a single climbing fibre impulse. 4. It is shown that the negative potentials produced in the molecular layer by combinations of mossy fibre and climbing fibre inputs can be very effectively distinguished by this differential effect of cooling. 5. The effects of cooling even to a severe level are immediately recoverable on warming. Repeated cooling has no untoward effects and there is no sign of the hysteresis reported for the cuneate nucleus. 6. There is a discussion of the factors that could cause cooling to differentiate between the actions of the mossy fibre and climbing fibre impulses on Purkynĕ cells.

Somatotopic studies on red nucleus: spinal projection level and respective receptive fields

The somatotopic inputs into red nucleus (RN) neurons have been studied with special reference to their level of projection in the spinal cord. As inputs we employed either volleys in predominantly cutaneous nerves of forelimb and hindlimb or cutaneous mechanoreceptor discharges evoked by taps to footpads of forelimb and hindlimb. There has been physiological confirmation of the anatomical findings that RD neurons projecting to the lumbar cord are located in the ventrolateral zone of the pars magnocellularis, whereas in the dorsomedial zone are RN neurons with cervical but not lumbar projection. Somatotopically there was found to be a differentiation of input to RN neurons according as they projected to the lumbar or only to the cervical cord. This finding was presented in the form both of tables and of somatotopic maps. As expected, this discrimination was more restrictive for the more selective inputs from pad taps than for nerve inputs. Nevertheless, forelimb inputs often had a considerable excitatory and inhibitory action on lumbar-projecting RN neurons, and vice versa for cervical-projecting neurons. There were two notable somatotopic findings that suggest specificities of connectivities. First, despite the large convergence of IP neurons onto RN neurons (about 50-fold), the degree of somatotopic discrimination was about the same for interpositus and RN neurons with two testing procedures: between inputs from forelimb and hindlimb; and between inputs from pads on one foot. Second, although there was in the interpositus nucleus a considerable topographical admixture of neurons with dominant forelimb or hindlimb inputs, the axonal projections of these neurons were apparently unscrambled on the way to the target RN neurons, so as to deliver the somatotopic specificities observed for two classes of RN neurons; those projecting down the spinal cord beyond L2 level, and those projecting to C2 but not L2. Finally, there is a general discussion of motor control with reference to the pathway; pars intermedia of anterior lobe of cerebellum leads to interpositus nucleus leads to red nucleus leads to rubrospinal tract leads to spinal motoneurons.

Responses of red nucleus neurons to antidromic and synaptic activation

An account is given of the responses of 432 red nucleus (RN) neurons with axons projecting down the spinal cord. Almost half were in an initial series of 18 experiments on anesthetized cats, and the remainder were in a second series of 12 experiments on decerebrate unanesthetized cats. The differences between the two series were of little significance. All recording was from single neurons using extracellular glass microelectrodes that were inserted throught the right superior colliculus and directed to the right red nucleus at a standard orientation. Identification of RN neurons was both by location, checked by subsequent histology, and by antidromic invasion from the spinal cord. The spinal stimulating electrodes were placed in proximity to the left rubrospinal tract at C2 and L2 segmental levels. Axonal conduction velocities were calculated from the latency differential between the L2 and C2 antidromic responses and were usually in the range 60-130 m/s, 97% of all neurons located in the red nucleus had axons projecting to the C2 level, and 37% projected to the L2 level. The responses of 229 RN neurons were observed with stimulation applied to the contralateral (left) interpositus nucleus. In 10 (5%) there were antidromic responses to both interpositus and C2 stimulation, a finding in good agreement with the anatomical description of rare axon collaterals from rubrospinal fibers to the interpositus nucleus. In 209 (91%), there was a clear monosynaptic excitation. The impulse generation was at a latency usually of 1.0-1.8 ms, which a modal value of 1.4 ms. The afferent inputs to RN neurons were provided by stimulation either of predominantly cutaneous nerves in all four limbs or of cutaneous mechanoreceptors of the contralateral forelimb and hindlimb...

Oxygen and carbon dioxide dissociation of duck blood

Oxygen and CO2 dissociation of duck blood was studied in blood samples equilibrated with known gas mixtures at the bird's body temperature (41 degrees C) and analyzed in the Van Slyke manometric apparatus and in pH electrodes. At various pH values between 7.38 and 7.55 the Hill plots yielded straight and parallel lines over a wide range of O2 saturation, the Hill coefficient being 2.9. Half saturation pressure P50 at pH = 7.50 was 36 torr. The Bohr effect factor was -0.53. Buffering properties were analyzed by equilibrating blood samples with gas mixtures of different PCO2 at 41 degrees C. The buffer value for whole blood in the range of 3-7% CO2 was 19.3 mMol-L-1-pH-1, the buffer value for true plasma 22.9 mMol-L-1-pH-1. The CO2 dissociation curve constructed using the buffer values had a slope of 0.17 mMol-L-1-torr-1 in the PCO2 range from 40 to 50 torr. The CO2 content of oxygenated blood at PCO2 = 40 torr was 21.7 mMol-L-1. The Haldane effect factor at PCO2 = 35 torr equalled 0.30 mMol of combined CO2 per mMol HbO2. With the values of PO2, PCO2 and pH measured in arterial blood of undisturbed and unrestrained, resting ducks effective dissociation curves for both O2 and CO2 were constructed assuming a metabolic R.Q. of 0.8. These curves are expected to resemble closely the actual in vitro dissociation curves of resting ducks.

Metabolic changes in avian blood and their effects on determination of blood gases and pH

Oxygen consumption. MO2 and CO2 production rates, Cco2, of duck blood samples anaerobically stored at 41 degrees C were 0.041 and 0.036 mMol (L blood)minus 1 min minus 1, respectively, and were independent of O2 saturation in the range of 100 to 10% saturation; the resulting metabolic respiratory quotient of blood was 0.88. The pH decreased linearly with time at a rate of 0.0014 unit min minus 1. The lactic acid production rate was independent of Po2, and was about 0.033 MMol L minus 1 min minus 1. Errors were assessed that may be introduced in blood gas analysis when the high metabolic activity of avian blood is not accounted for. Thus the O2 dissociation curve established using the Van Slyke analysis will be shifted to the right; however, the displacement around P50 is only about 1 torr for a time lag of 5 min between sampling and analysis and is even less at higher Po2 values. When using electrodes, P02 will be underestimated and PCO2 overestimated. The magnitude of these errors depends on both delay time and slope of the dissociation curves. It is concluded that the standard blood gas analytical methods are applicable to avian blood, but that in some cases corrections for metabolic effects are neccessary. Any delay between blood sampling and analysis should be kept as short as possible; storage, if neccessary, should be on ice.

Arterial blood gases in undisturbed resting birds: measurements in chicken and duck

Arterial blood was sampled in resting, unrestrained and undisturbed birds using a remote-controlled sampling device. With this technique a blood sample of 0.45 ml was obtained for analysis of PO2, PCO2 and pH at the body temperature of the animal. In 5 domestic hens the following mean values were obtained: PA02-82 torr; PACO2=33 torr; pHA=7.52. The average body temperature was TB=41.0 degrees C. In 6 domestic ducks the analysis yielded values of PAO2=82 torr; PACO2=38 torr; pH2=7.49; Tb=41.0 degrees C. Although these values are close to those reported in the literature some discrepancies may be recognized; they can be attributed to respiratory and metabolic changes of blood composition possibly induced by classical sampling procedures.

Gas transport efficacy of gills, lungs and skin: theory and experimental data

The general functional principles encountered in respiratory organs of vertebrates are investigated. Generally three steps are involved in external gas exchange in vertebrates: (1) convective transport by flow of external respiratory medium, air or water (=ventilation); (2) transfer of gas between external respiratory medium and blood by diffusion (=medium/blood transfer); (3) convective transport by blood flow (=perfusion). According to the arrangement of external medium flow relative to capillary blood flow four construction principles may be distinguished: (a) counter-current system (fish gills), (b) cross-current system (avian lungs), (c) ventilated pool system (mammalian lungs), and (d) infinite pool system (amphibian skin). The gas transfer performance of these systems is analyzed in terms of conductances, relative partial pressure differences and limitations attributable to ventilation. to medium/blood transfer and to perfusion. The theory is applied to analysis of gas exchange data obtained in an elasmobranch fish, domestic fowl, dog and a lungless salamander. The analysis shows that, despite distinct differences in maximum efficiencies of these systems, the differences in efficiency values actually attained are much less pronounced, and may be even less marked when taking functional inhomogeneities into account which are neglected in this study.

Remote-controlled device for sampling arterial blood in unrestrained animals

A device is described that allows remote-controlled sampling of arterial blood in unrestrained animals. An artery and a vein are dissected in local anesthesia and connected by a plastic catheter, the sampling catheter. The flow of arterial blood in this artificial shunt can be blocked by kinking the sampling catheter by a remote-controlled device. The blood thus trapped in the sampling catheter, of 0.45 ml volume is analyzed for PO2, PCO2, and pH using electrodes. The technique has been used in ducks and hens but can be applied to other vertebrate classes and to species of smaller body size.