Temperature effects on CO2-sensitive intrapulmonary chemoreceptors in the lizard, Tupinambis nigropunctatus

Body temperature (Tb) effects on CO2 responses of 17 intrapulmonary chemoreceptors (IPC) were investigated in 9 anesthetized (pentobarbital; 30 mg/kg) and unidirectionally ventilated tegu lizards (Tupinambis nigropunctatus). At 30 degrees C, all IPC (n = 15) had a stable discharge pattern. At 20 degrees C, IPC discharge (n = 14) was stable at high PCO2 but irregular at low PCO2 and often (10/14) consisted of bursts of activity separated by one or more seconds of quiescence. Responses of IPC to static and dynamic changes in PCO2 were quantified at both Tb and the discharge rate vs PCO2 response curves were compared. Static discharge frequency (fSTAT) decreased as PCO2 increased at both Tb. At 20 degrees C: (1) fSTAT was diminished at all PCO2 levels relative to 30 degrees C; and (2) the slope of the fSTAT vs PCO2 relationship was markedly attenuated. The Q10 was 3.7 +/- 0.5 and was independent of PCO2. The peak discharge associated with a step decrease in PCO2 (dynamic response; fDYN) also decreased as PCO2 increased. At 20 degrees C: (1) fDYN was diminished at all PCO2 levels relative to 30 degrees C; but (2) the slope of the fDYN vs PCO2 relationship was similar at both Tb. The Q10 was 2.6 +/- 0.3 and was significantly less than the Q10 of fSTAT (P less than 0.05). Acute changes in Tb exert large effects on the CO2 response and discharge pattern of IPC; these effects on IPC may be important in ventilatory control at different Tb in lizards.

Minimum anesthetic dose and cardiopulmonary dose response for halothane in chickens

The minimum anesthetic dose (MAD) and the cardiopulmonary dose-response for halothane were determined in male chickens. The MAD for halothane was 0.85 +/- 0.09% (mean +/- SD), with a range of 0.75% to 0.98%. There was a significant (P less than 0.002) positive correlation between increasing concentrations of halothane and PaCO2, and significant negative correlations of halothane concentration with respiratory rate (P less than 0.04), arterial blood pH (P less than 0.008), and mean arterial blood pressure P less than 0.008). A significant correlation was not found between halothane concentration and heart rate or arterial blood bicarbonate concentration. It was concluded that the MAD for halothane in chickens is similar to values for minimum alveolar concentrations of halothane in mammalian species, and that there is substantial dose-dependent depression of cardiopulmonary function in chickens.

Ventilatory responses to lung inflation and arterial CO2 in halothane-anesthetized dogs

Hypercapnia attenuates the effects of static airway pressure (Paw) on phrenic burst frequency (f) and the expiratory duration (TE) in chloralose-urethan-anesthetized dogs. Surgical removal of the carotid bodies abolishes this interaction. Since halothane anesthesia in hyperoxia greatly impairs peripheral chemoreflexes, experiments were conducted to determine whether hypercapnia would attenuate the effects of Paw on f and TE in halothane-anesthetized dogs (approximately 1.5 minimum alveolar concentration). Integrated activity of the phrenic nerve was monitored as a function of Paw (2-12 cmH2O) in a vascularly isolated left lung at varied levels of arterial PCO2 (PaCO2; 38-80 Torr) controlled by inspired gas concentrations ventilating the denervated but perfused right lung. Halothane was administered only to the right lung. The results were as follows: 1) integrated phrenic amplitude increased with PaCO2 but was unaffected by Paw; 2) f decreased as Paw increased but was not affected by PaCO2; 3) the inspiratory duration (TI) increased as PaCO2 increased but was unaffected by Paw; 4) TE increased as Paw increased but was unaffected by PaCO2; and 5) there was no phrenic response to intravenous sodium cyanide (50-100 micrograms/kg). Thus, unlike chloralose-urethan-anesthetized dogs, hypercapnia does not attenuate the effect of lung inflation on f or TE in halothane-anesthetized dogs. Furthermore, hypercapnia increases TI during halothane anesthesia, an effect found after carotid denervation but not found in intact chloralose-urethan-anesthetized dogs. It is suggested that these differences between chloralose-urethan- and halothane-anesthetized dogs may be due to functional carotid chemoreceptor denervation by halothane.

Effects of hypercapnia on phrenic and stretch receptor responses to lung inflation

To determine if hypercapnia and reflex bronchoconstriction attenuate lung inflation effects on ventilatory activity by indirect effects on intrapulmonary stretch receptors (PSR), phrenic nerve activity and single unit PSR were monitored at controlled levels of static airway pressure (Paw) and arterial PCO2 in 15 anesthetized dogs. Paw in a vascularly isolated lung was varied between 2 and 14 cm H2O at levels of PaCO2 between 35 and 85 mm Hg. PSR activity (n = 38) in fine strands dissected from an otherwise intact vagus nerve and the integrated phrenic neurogram were recorded. The response to Paw varied from one PSR to another, but was consistent in a given unit; PaCO2 had no consistent effect on individual responses. Selected PSR (n = 15) were averaged to yield a population response to Paw; the selection criteria were: phrenic activity responded briskly to Paw and measurements were made at three levels of PaCO2. Average PSR discharge increased linearly with Paw but was unaffected by PaCO2. On the other hand, phrenic burst frequency decreased as Paw increased and hypercapnia attenuated the slope of this relationship. These results suggest that effects on the relationship between PSR activity and Paw cannot account for attenuation of the relationship between phrenic frequency and Paw in hypercapnia. The effect of PaCO2 on the phrenic frequency vs Paw relationship probably arises from integrative mechanisms in the central nervous system.

Effects of carotid denervation on interactions between lung inflation and PaCO2 in modulating phrenic activity

Hypercapnia attenuates the effects of static airway pressure (Paw) on phrenic burst frequency (f) and the expiratory duration. We examined the role of carotid chemoreceptors in this response using an experimental preparation that allowed independent control of lung inflation and CO2 reflexes. Experiments were conducted in intact (n = 6) and carotid denervated (CBX; n = 12) chloralose/urethane anesthetized dogs. Integrated phrenic amplitude (Phr), f, and the inspiratory (TI) and expiratory durations (TE) were measured as a function of Paw (2-12 cm H2O) at levels of PaCO2 between 30 and 80 mm Hg. In intact dogs: (1) f decreased as Paw increased, and elevated PaCO2 decreased the slope of this relationship; (2) neither PaCO2 nor Paw affected TI; and (3) TE increased hyperbolically with Paw, and elevated PaCO2 attenuated this relationship. In CBX dogs: (1) f decreased as Paw increased, but this relationship was not affected by PaCO2; (2) TI increased as PaCO2 increased but was unaffected by Paw; and (3) TE increased as Paw increased but was unaffected by PaCO2. The results indicate that carotid chemoreceptors are necessary in the mechanism whereby hypercapnia attenuates the effects of Paw on f and TE. Furthermore, carotid denervation reveals an effect of hypercapnia on TI, an effect that is not evident in dogs with functional carotid chemoreceptors.

Effects of hypoxemia on phrenic nerve responses to static lung inflation in anesthetized dogs

To study interactions between hypoxemia and lung stretch in modulating ventilatory activity, an experimental preparation was used that allows independent control of static airway pressure (Paw) and arterial PO2 in anesthetized dogs. Phrenic burst frequency (f) and integrated amplitude (Phr) were monitored while Paw was varied between 2 and 12 cm H2O at levels of PaO2 between 30 and 200 mm Hg. Experiments were repeated in intact (n = 8) and carotid denervated dogs (CBX; n = 7). In intact dogs, f decreased with increasing Paw through an effect on the expiratory duration (TE). Hypoxia increased f by decreasing both the inspiratory duration (TI) and TE. Hypoxia had no effect on the slope of the f vs Paw relationship, but attenuated the effect of Paw on TE. Phr was increased by hypoxia, but Paw had little effect. After CBX, f was still inhibited by Paw, but PaO2 had no consistent effect on f, TI or TE at any level of Paw. Phr was inhibited by hypoxia after CBX, but Paw had no effect. The results indicate that Paw and PaO2 exert additive effects on f in anesthetized dogs. Hypoxia attenuates the effect of Paw on TE, which alone would attenuate the slope of the f vs Paw relationship. However, the effect of hypoxia on TI enhances the slope of the f vs Paw relationship, restoring a parallel shift. These effects are abolished by carotid denervation.

Phrenic nerve responses to hypoxia and CO2 in decerebrate dogs

Phrenic responses to isocapnic hypoxia and hypercapnia were studied using paralyzed vagotomized dogs (either decerebrate or chloralose-anesthetized). The hypoxia-induced increase in phrenic minute activity (PMA) was significantly greater in anesthetized dogs when compared with the response observed in decerebrate dogs. Phrenic responses to hypercapnia were also significantly different in the two groups of dogs. Increases in phrenic amplitude (AMP) and frequency (FREQ) were observed in anesthetized dogs, whereas decerebrate dogs responded to CO2 without a change in FREQ. Spontaneously breathing dogs (either decerebrate or anesthetized) were used for studying the effects of vagotomy on the integrated phrenic neurogram. Changes in phrenic pattern in response to vagotomy were qualitatively similar in anesthetized and decerebrate dogs. However, in decerebrate dogs, AMP was disproportionately increased relative to the decrease in FREQ such that PMA increased following vagal transection. Conversely, in anesthetized dogs, the increase in AMP and decrease in FREQ in response to vagotomy were proportional; PMA remained unchanged. These results suggest that mesencephalic decerebration disrupts neuronal circuits which participate in the chemical control of breathing. In addition, suprapontine structures may be involved in coupling FREQ and AMP (tidal volume) so that PMA (ventilation) is stabilized. Finally, these studies provide evidence for a vagally-independent frequency controller in dogs which is sensitive to hypoxia and hypercapnia, but appears to be highly dependent upon suprapontine structures.

Neural and humoral factors in control of tracheal caliber

To assess the contributions of neural (vagal) and humoral (blood borne) mechanisms in the tracheal constriction that occurs when pump ventilation is transiently withheld, experiments were conducted on decerebrate dogs. The dogs were paralyzed and thoracotomized, and each lung was independently ventilated. Pressure changes within an isolated tracheal segment (Ps) were monitored as an index of tracheal caliber. In series I, pump ventilation was withheld 20 s at the prevailing end-expiratory pressure (3-5 cmH2O) from both lungs or either lung alone when 1) both lungs were intact; or 2) the left lung was vascularly isolated (VIL) by occluding the pulmonary artery and the right gas exchange lung (GEL) was vagally denervated. In series II, steady-state pressure changes in the VIL were made with constant GEL ventilation. With both lungs intact, 20-s apnea elicited a 17.3 +/- 2.6 cmH2O increase in Ps; the left and right lungs contributed equally to this response. Following vagotomy and pulmonary arterial occlusion, a 7.8 +/- 2.6 cmH2O increase in Ps was elicited from two lungs; the VIL response was 60 +/- 11% and the GEL 41 +/- 15%. Onset and one-half response times were faster from the VIL than GEL. Prolonged maneuvers elicited progressively larger responses from the GEL, but the VIL response plateaued within 30 s and then adapted towards the control level. In series II, steady-state increases or decreases in VIL pressure elicited small decreases or increases in Ps, respectively, which showed nearly complete adaptation within several minutes.(ABSTRACT TRUNCATED AT 250 WORDS)

Acid-base balance during lactic acid infusion in the lizard Varanus salvator

Although reptiles rely heavily on anaerobic metabolism and lactic acid production during activity, little is known concerning their ventilatory response to the attendant metabolic acidosis. We measured arterial PCO2, H+ and lactate (L)ion concentrations, and the rates of CO2 (MCO2) and O2 (MO2) exchange in Varanus salvator (n = 9) during intravenous infusions of lactic acid (HL) or sodium lactate (NaL; 250 mM) at rest. Two protocols were used: (1) 15 min infusions of 0.42 ml/min at both 25 and 35 degrees C with measurements every 5 min; (2) 4.5 min infusions of 1.73 ml/min at 35 degrees C with measurements at 4.5 min. At 35 degrees C, control pH decreased from its value at 25 degrees C with a slope of -0.007/degrees C and PaCO2 increased. The results of HL infusion were: (1) [L]a increased, (2) MCO2 increased, (3) R (MCO2/MO2) increased, (4) pHa decreased, and (5) PaCO2 remained unchanged from control at both temperatures and at both infusion rates. The only significant changes in PaCO2 observed were following the termination of fast HL infusions, when PaCO2 decreased. In NaL infusions, only small changes were observed except in [L]a. The results indicate that: (1) delta pH/delta T in V. salvator is less than in other poikilothermic vertebrates, but consistent with other varanid lizards, (2) respiratory compensation is slight in response to acute metabolic acidosis in this species, and (3) ventilation follows changes in MCO2 rather closely, accounting for precise regulation of PaCO2 despite 4-fold increases in MCO2 elicited by bicarbonate buffering and increased metabolic rate.

Pulmonary control systems in exercise: update

We examined recent ideas and findings concerned with the regulation of ventilation and gas transport in moderate and heavy exercise. The primary mediation of exercise hyperpnea remains unknown and highly controversial, but two unique approaches to the problem have advanced our understanding of this neurohumoral regulatory scheme. On the one hand, experimental separation of the pulmonary and systemic circulations was used to reveal a vagally mediated ventilatory response that is clearly attributable to CO2 flow to the lung. This mechanism seems to be most effective as a homeostatic regulator of ventilatory control near resting levels of metabolic rate. On the other hand, a descending neurogenic drive to hyperpnea from the locomotor regions of the central nervous system was also demonstrated experimentally. The importance of regulatory feedback by conventional chemoreceptors in determining the precision of the hyperpneic response was emphasized in explaining the wide spectrum of arterial acid-base regulation during exercise in humans and non-human species. Two commonly accepted homeostatic regulators believed to be operative during heavy exercise were questioned, i.e., the compensatory hyperventilatory response and the maintenance of arterial oxygenation. For example, the hyperventilatory response was shown not to require metabolic acidosis; hyperventilation was not always observed at high work rates despite an abundance of chemical stimuli; and arterial hypoxemia occurred at very high metabolic rates in a significant number of highly fit athletes. These data implied that the capabilities of some aspects of even the healthy pulmonary system may be approached-or even exceeded-during heavy exercise.

Changes in the VI-VCO2 relationship during exercise in goats: role of carotid bodies

To assess the role of carotid bodies in modulating the ventilation-CO2 production relationship, steady-state responses to mild exercise were determined in goats following several experimental manipulations that led to chronic changes in resting ventilation and arterial blood gases. The experimental conditions were 1) control, 2) whole body serotonin depletion (induced by p-chlorophenylalanine, 100 mg/kg), 3) carotid body denervation (CBX), and 4) serotonin depletion with CBX. Resting values of arterial CO2 pressure (Pco2) ranged from 32 to 48 Torr. In each condition, arterial Pco2 was regulated to a similar degree in exercise due to changes in the slope of the ventilation-CO2 production relationship (delta Vi/delta Vco2) in accordance with the requirements of gas exchange. delta Vi/delta Vco2 increased with serotonin depletion both before and after CBX. The principal component of ventilation contributing to changes in delta Vi/delta Vco2 was tidal volume. These results suggest a basic property of the ventilatory control system whereby enhanced ventilatory activity at rest is associated with an increased ventilatory response to exercise via a mechanism that does not require peripheral chemoreceptors.

Central-peripheral chemoreceptor interaction in awake cerebrospinal fluid-perfused goats

We assessed the ventilatory interaction between central [central nervous system (CNS)] and peripheral chemoreceptor stimuli in five awake goats. CNS extracellular fluid (ECF) [H+] was altered with cisterna magna perfusion of mock CSF. Peripheral chemoreceptors were stimulated with three doses of NaCN given intravenously. The resulting dose-response curves were used to assess interaction of the central and peripheral stimuli. The observed interaction was hypoadditive; i.e., the average slope of the NaCN-inspired minute ventilation dose-response line was significantly greater during alkaline perfusion than during acidic perfusion. This correlation can be described by slope = -0.24 (CSF [H+]) + 30.7; r = 0.67 (P less than 0.01). Increased ventilatory responses were accompanied by increases in mean inspiratory flow, tidal volume, and breathing frequency and decreases in expiratory time in response to peripheral chemoreceptor stimulation. Unlike previous reports in anesthetized and denervated animals, in our awake intact goats the ventilatory and tidal volume responses showed no significant dependence on the level of control (pre-NaCN stimulus) inspired minute ventilation. We conclude that the level of [H+] in cerebral ECF exerts a significant reflex-mediated hypoadditive effect on the ventilatory responses to peripheral chemoreceptor stimulation.

Ventilatory response of goats to treadmill exercise: grade effects

The steady-state ventilatory responses of 7 goats to treadmill exercise were studied at several different combinations of speed (0-7.7 km per hr) and grade (0-15%). Carbon dioxide production (VCO2) increased as much as 6 times the resting value. The goats responded to exercise with hyperventilation and respiratory alkalosis, which was proportional to VCO2. The increased ventilation was due chiefly to increases in breathing frequency (f). When responses to increasing speed at 0% grade were compared to those at 15% grade, there were no differences in expired minute ventilation or PaCO2. There were differences in ventilatory pattern. At a given VCO2, f was higher and tidal volume (VT) lower at 0% grade than at 15% grade. We conclude that ventilatory pattern (at a given VCO2) is influenced by the grade used during treadmill exercise and therefore stimuli other than VCO2 alone must be involved in the generation of ventilatory pattern during treadmill exercise.

Metabolic acids and [H+] regulation in brain tissue during acclimatization to chronic hypoxia

Ventilatory acclimatization, arterial acid-base status, brain stem and cortex metabolic acids, and high-energy phosphates were determined in rats during 2-h to 7-days exposure to two levels of hypoxia (O2 inspiratory pressure 75 and 58 Torr), and upon acute restoration of normoxia. Brain lactate concentrations increased during acute exposure to both moderate hypoxia (+52% in cortex and +61% in stem) and severe hypoxia (+211% in cortex and +163% in stem), and during chronic hypoxia remained unchanged or decreased, respectively, as hyperventilation progressed and arterial O2 content rose. Restoration of normoxia after chronic hypoxic exposure resulted in continued hyperventilation and elevated lactate concentrations. Brain intracellular pH was unchanged throughout moderate hypoxia and during severe hypoxia, became acid during acute exposure, but was completely compensated after 72 h of continued hypoxia. Preventing the hypocapnia (via increased inspiratory fraction of CO2) during acute hypoxia or restoring normocapnia during posthypoxic normoxia revealed the effects of hypocapnia independently of hypoxemia. Hypocapnia accounted for all of the changes in brain lactate concentration during moderate hypoxia but only 40-60% of the lactate changes during severe hypoxia. We speculate that the observed changes in plasma and brain tissue metabolic acids and in plasma [HCO-3] would acidify cerebral interstitial fluid (ISF) in chronic hypoxia; however, the time course of this change in ISF acid-base status would be quite different between the two levels of hypoxia and uncorrelated with the patterns of ventilatory acclimatization.

Effects of p-chlorophenylalanine on ventilatory control in goats

The effects of tryptophan hydroxylase inhibition with p-chlorophenylalanine (PCPA; 100 mg/kg iv) on ventilatory control were studied in awake goats. Ventilation, CO2 production, and blood gases were measured 16-24 h after PCPA at rest and during mild exercise in normoxia and at rest in hypoxia and hypercapnia. PCPA increased ventilation 36% at rest, predominantly through an effect on respiratory frequency, and decreased arterial PCO2 (PaCO2) 6.5 Torr. Ventilatory gain in exercise (delta VI/deltaVCO2) was increased 20% by PCPA thereby maintaining PaCO2 at its new resting value. Hypoxia (fractional inspired O2 concentration = 0.12) had little effect on ventilation or PaCO2 at rest, either on control or on PCPA test days. Ventilatory sensitivity to CO2 at rest (delta VI/delta PaCO2) was unaffected by PCPA. Bilateral carotid body denervation (CBX) was performed in the animals, and experiments were repeated 3 mo after the first administration of PCPA. CBX alone decreased ventilation 29% and increased PaCO2 9.4 Torr. Administration of PCPA increased ventilation 35%, decreased PaCO2 by 10.2 Torr at rest, and increased ventilatory gain in exercise 26%. Thus carotid bodies are not necessary for the ventilatory response to PCPA. Furthermore, the primary neural pathways associated with exercise or hypercapnia are not specifically affected by inhibition of serotonin metabolism via PCPA.

Interactions between lung stretch and PaCO2 in modulating ventilatory activity in dogs

The effects of changes in airway pressure (Paw) and arterial PCO2 (PaCO2) on ventilatory activity were studied in anesthetized thoracotomized dogs in which both lungs were ventilated separately. Pulmonary artery occlusion on one side and contralateral vagotomy allowed the reflex effects on ventilation of changes in Paw and PaCO2 to be elicited independently of each other. Ventilatory activity was assessed from integrated efferent phrenic activity, analyzed with respect to burst amplitude (Phr), burst frequency (f), and inspiratory TI) and expiratory duration (TE). While Phr increased linearly with PaCO2, it was independent of Paw. Both PaCO2 and Paw affected f in a complex nonadditive way; this response was entirely mediated by effects on TE, TI being unaffected by either stimulus. The analog of ventilation, estimated as Phr x f, increased linearly with PaCO2 and decreased linearly with Paw, but the effects of both stimuli appeared to be additive. It is concluded that the apparently simple effect of Paw and PaCO2 on ventilation results from more complex effects these stimuli exert on its components.

Pulmonary oxygen transport during activity in lizards

Oxygen consumption (MO2), effective alveolar ventilation (Veff), arterial and alveolar PO2 (PaO2, PAO2) and the alveolar-arterial PO2 difference [(A--a)PO2] were determined in the lizards Varanus exanthematicus and Iguana iguana at rest and during treadmill exercise at 35 degrees C. In both species, Veff increased more rapidly than MO2 giving rise to an increased PAO2. In contrast, PaO2 remained unchanged through the highest levels of MO2 attained. As a result, the (A--a)PO2 increased with increasing MO2. We suggest that the observed increase in (A--a)PO2 may be due to a rather low pulmonary oxygen diffusing capacity (DLO2) and limited capacity to increase DLO2 during exercise. Arterial desaturation was prevented by a compensatory hyperventilation, thus enhancing the gradient for alveolar-capillary gas exchange. These results indicate that both lizard species increase pulmonary oxygen transport sufficiently so that it is not a limiting factor to aerobic scope under the conditions of this study.

Ventilation and acid-base balance during graded activity in lizards

Arterial PCO2, hydrogen ion ([H+]a), and lactate ([L]a) concentrations, rates of metabolic CO2 production (VCO2) and O2 consumption (VO2), and effective alveolar ventilation (Veff) were determined in the lizards Varanus exanthematicus and Iguana iguana at rest and during steady-state treadmill exercise at 35 degrees C. In Varanus, VCO2 increased ninefold and VO2 sixfold without detectable rise in [L]a at running speeds below 1.0 to 1.5 km x h-1. In this range, Veff increased 12-fold resulting in decreased levels of PaCO2 and [H+]a. At higher speeds [L]a rose. Increments of 5 mM [L]a were accompanied by hyperventilation, reducing PaCO2 and thus maintaining [H+]a near its resting level. When [L]a increased further, [H+]a increased. Sustainable running speeds (0.3-0.5 km x h-1 and below) were often associated with increased VO2, VCO2, and [L]a in Iguana. Sixfold increases in VCO2 and 9-mM increments in [L]a were accompanied by sufficient increase in Veff (9-fold) to maintain [H+]a at or below its control level. When [L]a increased further, [H+]a increased. These results indicate that both lizard species maintain blood acid-base homeostasis rather effectively via ventilatory adjustments at moderate exercise intensities.

Effects of intrapulmonary CO2 and airway pressure on phrenic activity and pulmonary stretch receptor discharge in dogs

Intrapulmonary CO2 and stretch sensitivity were studied in anesthetized, thoracotomized dogs in which both lungs were independently ventilated. The left pulmonary artery was occluded so that changes in left (Test) lung CO2 did not alter systemic arterial PCO2. Adequate gas exchange was maintained in the right lung, which was vagally denervated. In one series of experiments, neural activity in the C5 root of the phrenic nerve was integrated to assess ventilatory drive at various levels of Test lung CO2 and airway pressure both during cyclic ventilation and static lung inflation. In a second series, single unit activity in vagal afferents from pulmonary stretch receptors (PSR) was recorded. Both phrenic activity and PSR discharge were strongly affected by changes in airway pressure but not by changes in Test lung CO2 between 2 and 7%. However, when test lung CO2 was decreased below 2%, phrenic activity decreased and PSR activity increased. These changes were invariably accompanied by an increase in peak airway pressure during cyclic ventilation suggesting that lung mechanics had been altered. The results indicate that, in this preparation, the effects of lung stretch on ventilatory drive spans a wide range whereas intrapulmonary CO2 exerts an effect only at very low levels. It appears that the reflex effects of both stimuli can be accounted by an effect on PSR activity.

A comparison between carbon dioxide inhalation and increased dead space ventilation in chickens

We compared the ventilatory response to elevated PICO2 with the response to elevation of dead space volume (delta VD) in anesthetized, hyperoxic chickens. In the first experimental series (I), PICO2 was varied between 0 and 26 Torr. In a second series (II), delta VD was raised between 0 and 17 ml by placing lengths of tubing between the bird and the source of inspired gas. In series I, ventilation increased and PaCO2 remained constant at levels of PICO2 below 20 Torr. In contrast, PaCO2 increased with low levels of delta VD in Series II. When changes in PaCO2 and ventilation were expressed as a function of the change in CO2 load reaching the lungs (delta L), the change in ventilation was greater, and that in PaCO2 less in Series I than in Series II at all levels of delta L below 25 ml (STPD) . min-1. The differences in ventilatory response to PICO2 and delta VD may qualitatively be explained by the distinct time patterns of CO2 concentration in the lungs which result in different discharge frequencies of CO2-sensitive intrapulmonary chemoreceptors and, possibly, by effects on ventilation resulting from differences in the timing of receptor discharge. Thus, these data provide additional evidence that avian intrapulmonary chemoreceptors may play a significant role in the chemical control of ventilation and regulation of PaCO2.