A. Berkenbosch

The effect of low-dose acetazolamide on the ventilatory CO2 response curve in the anaesthetized cat

1. The effect of 4 mg kg‐1 acetazolamide (I.V.) on the slope (S) and intercept on the Pa,CO2 axis (B) of the ventilatory CO2 response curve of anaesthetized cats with intact or denervated carotid bodies was studied using the technique of dynamic end‐tidal forcing. 2. This dose did not induce an arterial‐to‐end‐tidal PCO2 (P(a‐ET),CO2) gradient, indicating that erythrocytic carbonic anhydrase was not completely inhibited. Within the first 2 h after administration, this small dose caused only a slight decrease in mean standard bicarbonate of 1.8 and 1.7 mmol l‐1 in intact (n = 7) and denervated animals (n = 7), respectively. Doses of acetazolamide larger than 4 mg kg‐1 (up to 32 mg kg‐1) caused a significant increase in the P(a‐ET),CO2 gradient. 3. In carotid body‐denervated cats, 4 mg kg‐1 acetazolamide caused a decrease in the CO2 sensitivity of the central chemoreflex loop (Sc) from 1.52 +/‐ 0.42 to 0.96 +/‐ 0.32 l min‐1 kPa‐1 (mean +/‐ S.D.) while the intercept on the Pa,CO2 axis (B) decreased from 4.5 +/‐ 0.5 to 4.2 +/‐ 0.7 kPa. 4. In carotid body‐intact animals, 4 mg kg‐1 acetazolamide caused a decrease in the CO2 sensitivity of the peripheral chemoreflex loop (Sp) from 0.28 +/‐ 0.18 to 0.19 +/‐ 0.12 l min‐1 kPa‐1. Se and B decreased from 1.52 +/‐ 0.55 to 0.84 +/‐ 0.21 l min‐1 kPa‐1, and from 4.0 +/‐ 0.5 to 3.0 +/‐ 0.6 kPa, respectively, not significantly different from the changes encountered in the denervated animals. 5. It is argued that the effect of acetazolamide on the CO2 sensitivity of the peripheral chemoreflex loop in intact cats may be caused by a direct effect on the carotid bodies. Both in intact and in denervated animals the effects of the drug on Sc and B may not be due to a direct action on the central nervous system, but rather to an effect on cerebral vessels resulting in an altered relationship between brain blood flow and brain tissue PCO2.

Carbonic anhydrase and control of breathing: different effects of benzolamide and methazolamide in the anaesthetized cat.

1. The effect of inhibition of erythrocyte carbonic anhydrase on the ventilatory response to CO2 was studied by administering benzolamide (70 mg kg‐1, i.v.), an inhibitor which does not cross the blood‐brain barrier, to carotid body denervated cats which were anaesthetized with chloralose‐urethane. 2. In the same animals the effect on the ventilatory response to CO2 of subsequent inhibition of central nervous system (CNS) carbonic anhydrase was studied by infusing methazolamide (20 mg kg‐1), an inhibitor which rapidly penetrates into brain tissue. 3. The results show that inhibition of erythrocyte carbonic anhydrase by benzolamide leads to a decrease in the slope of the normoxic CO2 response curve, and a decrease of the extrapolated arterial PCO2 at zero ventilation. 4. Inhibition of CNS carbonic anhydrase by methazolamide results in an increase in slope and alpha‐intercept of the ventilatory CO2 response curve. 5. Using a mass balance equation for CO2 of a brain compartment, it is argued that inhibition of erythrocyte carbonic anhydrase results in a decrease in slope of the in vivo CO2 dissociation curve, which can explain the effects of benzolamide. 6. The changes in slope and intercept induced by methazolamide are discussed in relation to effects on neurones containing carbonic anhydrase, which may include central chemoreceptors.

Influence of hypoxic duration and posthypoxic inspired O2 concentration on short term potentiation of breathing in humans.

1. Short term potentiation (STP) of breathing refers to respiratory activity at a higher level than expected just from the dynamics of the peripheral and central chemoreceptors. In humans STP is activated by hypoxic stimulation. 2. To investigate the effects of the duration of hypoxia and the posthypoxic inspired O2 concentration on STP, the ventilatory responses to 30 s and 1, 3 and 5 min of hypoxia (end‐tidal PO2, P(ET.O2) approximately 6.5 kPa) followed by normoxia (P(ET.O2) approximately 14.5 kPa) and hyperoxia (P(ET.O2) approximately 70 kPa) were studied in ten healthy subjects. End‐tidal PCO2 (P(ET.CO2)) was clamped during hypoxic and recovery periods at 5.7 kPa. 3. Steady‐state ventilation (VE) was 13.7 +/‐ 0.6 l min‐1 during normoxia and increased to 15.5 +/‐ 0.3 l min‐1 during hyperoxia (P < 0.05) due to the reduced Haldane effect and some decrease in cerebral blood flow (CBF). 4. The mean responses following hypoxia reached normoxic baseline after 69, 54, 12 and 12 s when 30 s and 1, 3 and 5 min of hypoxia, respectively, were followed by normoxia. An undershoot of 10 and 20% below hyperoxic baseline was observed when 3 and 5 min of hypoxia, respectively, were followed by hyperoxia. Hyperoxic VE reached hyperoxic baseline after 9, 15, 12 and 9 s at the termination of 30 s and 1, 3 and 5 min of hypoxia, respectively. 5. Normoxic recovery from 30 s and 1 min of hypoxia displayed a fast and subsequent slow decrease towards normoxic baseline. The fast component was attributed to the loss of the hypoxic drive at the site of the peripheral chemoreceptors, and the slow component to the decay of the STP that had been activated centrally by the stimulus. A slow decrease at the termination of 30 s and 1 min of hypoxia by hyperoxia was not observed since this component was cancelled by the increase in ventilatory output due to the reduced Haldane effect and some decrease of CBF. 6. Decay of the STP was not apparent in the normoxic recovery from 3 and 5 min of hypoxia as a slow component since it cancelled against the slow ventilatory increase related to the increase of brain tissue PCO2 due to the reduction of CBF at the relief of hypoxia. The undershoot observed when hyperoxia followed 3 and 5 min of hypoxia reflects the stimulatory effects of hyperoxia on VE. 7. The manifestation of the STP as a slow ventilatory decrease depends on the duration of hypoxia and the subsequent inspired oxygen concentration. We argue that STP is not abolished by the central depressive effects of hypoxia, although the manifestation of the STP may be overridden or counteracted by other mechanisms.

The effects of hypoxia on the ventilatory response to sudden changes in CO2 in newborn piglets.

1. The ventilatory response to square‐wave challenges in end‐tidal partial pressure of CO2 (PCO2) was investigated at three levels of arterial PO2 (Pa,O2) in nineteen anaesthetized 2‐ to 11‐day‐old piglets. 2. The ventilatory responses, measured on a breath‐to‐breath basis, were separated into a peripheral and a central component using a two‐compartment model. Both components were described by a CO2 sensitivity, a time constant, a time delay and a single offset. 3. Fifty‐six responses were analysed against a background of normoxaemia (Pa,O2 = 12.70 +/‐ 0.72 kPa, mean +/‐ S.D.), fifty‐three against a background of moderate hypoxaemia (Pa,O2 = 8.63 +/‐ 0.34 kPa) and fifty‐one against a background of severe hypoxaemia (Pa,O2 = 4.98 +/‐ 0.30 kPa). 4. The sensitivity of the peripheral chemoreceptors in mediating the response to CO2 increased from 38.3 +/‐ 17.0 ml min‐1 kPa‐1 kg‐1 during normoxaemia to 48.8 +/‐ 15.3 ml min‐1 kPa‐1 kg‐1 during moderate hypoxaemia and to 72.9 +/‐ 24.0 ml min‐1 kPa‐1 kg‐1 at severe hypoxaemia. 5. As compared with the central CO2 sensitivity during moderate hypoxaemia and normoxaemia (104.0 +/‐ 39.0 and 100.8 +/‐ 41.6 ml min‐1 kPa‐1 kg‐1, respectively) it decreased to 85.9 +/‐ 54.1 ml min‐1 kPa‐1 kg‐1 at severe hypoxaemia. 6. We conclude that in newborn piglets there is a positive interaction between hypoxia and hypercapnia at the level of the peripheral chemoreceptors while severe hypoxaemia reduced the CO2 sensitivity centrally.

The ventilatory response to CO2 of the peripheral and central chemoreflex loop before and after sustained hypoxia in man.

1. The ventilatory response to sustained hypoxia is characterized by a fast increase due to the peripheral chemoreceptors followed by a slow decline. The mechanism of this decline is unknown. 2. To investigate the characteristics of the ventilatory response to sustained hypoxia ten healthy subjects were exposed to two consecutive periods of isocapnic hypoxia (arterial saturation 78%) separated by a 5 min exposure to isocapnic normoxia. 3. The acute hypoxic response to the second exposure to hypoxia (mean increase in ventilation +/‐ S.E.M., 7.2 +/‐ 0.8 l min‐1) was significantly depressed (P = 0.04) compared to the first one (9.5 +/‐ 1.3 l min‐1). 4. To investigate whether this depression was due to central or peripheral effects or both we measured, in the same ten subjects, the normoxic ventilatory response to CO2 before and after a period of 25 min of hypoxia using the technique of dynamic end‐tidal forcing. 5. Each response was separated into a fast peripheral and slow central component characterized by a CO2 sensitivity, time constant, time delay and an off‐set. 6. A total of thirty‐six prehypoxic and thirty posthypoxic responses were analysed. The ventilatory CO2 sensitivities of the peripheral and central chemoreflex loops and the overall off‐set (apnoeic threshold) after 25 min of hypoxia were somewhat larger than their prehypoxic values, but this effect was not significant. 7. We argue that the hypoxic ventilatory decline in man is due to a change in the off‐set of the peripheral chemoreflex loop.

Effect on ventilation of papaverine administered to the brain stem of the anaesthetized cat.

1. To investigate whether cerebral vasodilatation by itself contributes to the decrease in ventilation as found during brain stem hypoxia the role of cerebral vasodilatation on minute ventilation was investigated in twelve cats anaesthetized with alpha‐chloralose‐urethane. 2. Cerebral vasodilatation in the medulla oblongata was produced by adding papaverine to the blood perfusing the brain stem. 3. Papaverine at concentrations of 10‐35 micrograms per millilitre of blood had an appreciable depressant effect on ventilation. At a concentration of 14.3 micrograms ml‐1 the depression in ventilation averaged 0.7 +/‐ 0.1 l min‐1. 4. The ventilatory response to stepwise changes in papaverine concentration could be adequately described with a single exponential function with a time delay. 5. The time constant of the ventilatory response following a step increase in papaverine concentration (134 +/‐ 15 s) was longer than that of the step decrease (105 +/‐ 10 s) in concentration (P = 0.034). The time delays of the ventilatory response (88 +/‐ 21 s and 53 +/‐ 8 s respectively) were not significantly different (P = 0.126). 6. The ventilatory response to stimulation of the peripheral chemoreceptors by hypoxia and of the central chemoreceptors by hypercapnia was not impaired by papaverine. 7. The results support the hypothesis that cerebral vasodilatation by itself contributes to the decrease in ventilation by brain stem hypoxia.

A pseudo-rebreathing technique for assessing the ventilatory response to carbon dioxide in cats.

1. The ventilatory sensitivity to CO2 obtained from a non‐steady‐state step‐ramp CO2 challenge (analogous to the Read rebreathing method) was compared with the one of the steady‐state method. 2. Experiments were performed during normoxia on twenty cats anaesthetized with chloralose‐urethane. In eight of these cats additional measurements were carried out during metabolic acidosis and alkalosis. 3. The slope of the non‐steady‐state ventilatory response curve to CO2 was not significantly different from the steady‐state one only if the ratio of the step‐wise increase in end‐tidal PCO2 (PET,CO2) (A) above its resting value and the subsequent rate of rise of the PET,CO2 (R) was equal to the time constant of the central chemoreflex pathway (tau c). This also held true during metabolic acidosis and alkalosis. 4. It is predicted that in human beings during hyperoxia the ventilatory response line obtained with Reads rebreathing method is to a fair approximation shifted to the right by a value of A with respect to the steady‐state response line, provided A/R = tau c. 5. We argue that Reads prescription that a PET,CO2 equilibrium should be established between mixed venous blood, arterial blood and end‐tidal gas has to be regarded as an experimental condition leading to stable‐experiments rather than dictated by physiological mechanisms.

Ventilatory interaction between hypoxia and hypercapnia in piglets shortly after birth

In 12 piglets aged 0-1.5 days we assessed the relative contribution of the peripheral and central chemoreceptors in mediating the ventilatory response to CO2 at three levels of arterial O2 tension using the dynamic end-tidal forcing technique. With this technique the ventilatory response is separated into a peripheral and a central component using a two-compartment model. Each component is described by a CO2 sensitivity, a time constant, a transport time and a single apnoeic threshold. The results showed that the sensitivity of the peripheral chemoreceptors significantly (P < 0.01) increased from 25.0 +/- 23.6 ml.min-1.kPa-1.kg-1 (mean +/- SD) during normoxia (PaO2 = 12.8 +/- 0.3 kPa) to 42.5 +/- 29.4 ml.min-1.kPa-1.kg-1 during moderate hypoxia (PaO2 = 8.8 +/- 0.4 kPa) and to 80.2 +/- 44.4 ml.min-1.kPa-1.kg-1 at severe hypoxia (PaO2 = 5.1 +/- 0.3 kPa). There was no significant effect of the level of PaO2 on the other parameters. The results were compared with those obtained in a previous study in piglets aged 2-11 days. It showed that the interaction strength at the level of the peripheral chemoreceptors, defined as the negative ratio of the change in the peripheral CO2 sensitivity to the changes in PaO2 was greater in the younger piglets. From these results we conclude that in the newborn piglet the positive ventilatory interaction between hypoxia and hypercapnia at the level of the peripheral chemoreceptors is already developed shortly after birth and becomes smaller during development.

Ventilatory responses to respiratory and metabolic acid-base disturbances in cats

To determine the relative importance of the peripheral and central chemoreceptors in the ventilatory response to acute metabolic acid-base disturbances we measured the normoxic ventilatory response to acute respiratory and metabolic acidosis and alkalosis in 10 chloralose-urethane anesthetized cats using a technique of vertebral artery perfusion that allows one to independently manipulate the PaCO2, PaO2 and the H+ concentration of the blood in the systemic circulation (peripheral) and the blood perfusing the brain stem (central) (Berkenbosch et al., 1979). The ventilation could be satisfactorily described by a linear function of the peripheral and central arterial H+ concentration and the central PaCO2. Mean values (+/- SEM) found for the peripheral arterial H+ sensitivity and the isocapnic central arterial H+ sensitivity were 26.0 +/- 3.2 and 12.7 +/- 1.8 ml X min-1 X nM-1, respectively; the isohydric central arterial CO2 sensitivity was 545.9 +/- 96.7 ml X min-1 X kPa-1. We conclude that in the ventilatory response to an acute metabolic acid-base disturbance both the peripheral and central chemoreceptors play a role. However, the sensitivity of the peripheral chemoreceptors to isocapnic changes in the arterial H+ concentration is twice as large as the sensitivity of the central chemoreceptors. It is argued that in the adaptation of the ventilation to an acute metabolic acidosis the stimulatory effect of the peripheral chemoreceptors is counteracted by a diminished stimulation of the central chemoreceptors.

Influence of CSF calcium concentration on the ventilatory response to CO2 and O2

SummaryIn experiments on anaesthetized cats, changes in the calcium concentration of cerebrospinal fluid were induced by means of the ventriculocisternal perfusion technique. A study was performed to evaluate the effects of these changes in calcium concentration on the ventilatory responses to hypercapnia and hypoxia.An increase of the calcium concentration in the perfusion fluid produces a decrease of ventilation and a decrease of the slope of the