Stimulus interaction between CO2 and almitrine in the cat carotid chemoreceptors

Doxapram stimulates the carotid body via a different mechanism than hypoxic chemotransduction

To determine if doxapram stimulates the carotid body through the same mechanism as hypoxia, we compared the effects of doxapram and hypoxia on isolated-perfused carotid bodies in rabbits. Doxapram stimulated the carotid body in a dose-dependent manner. In Ca(2+)-free solution, neither doxapram nor hypoxia stimulated the carotid body. Although, doxapram had an additive effect on the carotid body chemosensory response to hypercapnia, a synergistic effect was not observed. Also, we investigated the various K(+) channel activators on the response to doxapram and hypoxia: pinacidil and levcromakalim as ATP-sensitive K(+) channel activators; NS-1619 as a Ca(2+)-sensitive K(+) channel activator; and halothane as a TASK-like background K(+) channel activator. The hypoxic response was partially reduced by halothane only, while pinacidil, levcromakalim and NS-1619 had no effect. Interestingly, the effect of doxapram was partially inhibited by NS-1619. Neither pinacidil nor levcromakalim affected the stimulatory effect of doxapram. We conclude that doxapram stimulates the carotid body via a different mechanism than hypoxic chemotransduction.

Reciprocal modulation of O2 and CO2 cardiorespiratory chemoreflexes in the tambaqui

This study examined the effect of acute hypoxic and hypercapnic cardiorespiratory stimuli, superimposed on existing cardiorespiratory disturbances in tambaqui. In their natural habitat, these fish often encounter periods of hypoxic hypercapnia that can be acutely exacerbated by water turnover. Tambaqui were exposed to periods of normoxia, hypoxia, hyperoxia and hypercapnia during which, externally oriented O2 and CO2 chemoreceptors were further stimulated, by administration into the inspired water of sodium cyanide and CO2-equilibrated water, respectively. Hyperoxic water increased the sensitivity of the NaCN-evoked increase in breathing frequency (f(R)) and decrease in heart rate. Hypoxia and hypercapnia attenuated the increase in f(R) but, aside from blood pressure, did not influence the magnitude of NaCN-evoked cardiovascular changes. Water PO2 influenced the magnitude of the CO2-evoked cardiorespiratory changes and the sensitivity of CO2-evoked changes in heart rate and blood flow. The results indicate that existing respiratory disturbances modulate cardiorespiratory responses to further respiratory challenges reflecting both changes in chemosensitivity and the capacity for further change.

Increased propensity for apnea via dopamine-induced carotid body inhibition in sleeping dogs

We determined the effects of specific carotid body chemoreceptor inhibition on the propensity for apnea during sleep. We reduced the responsiveness of the carotid body chemoreceptors using intravenous dopamine infusions during non-rapid eye movement sleep in six dogs. Then we quantified the difference in end-tidal Pco2 (PetCO2) between eupnea and the apneic threshold, the “CO2 reserve,” by gradually reducing PetCO2 transiently with pressure support ventilation at progressively increased tidal volume until apnea occurred. Dopamine infusions decreased steady-state eupneic ventilation by 15 ± 6%, causing a mean CO2 retention of 3.9 ± 1.9 mmHg and a brief period of ventilatory instability. The apneic threshold PetCO2 rose 5.1 ± 1.9 Torr; thus the CO2 reserve was narrowed from −3.9 ± 0.62 Torr in control to −2.7 ± 0.78 Torr with dopamine. This decrease in the CO2 reserve with dopamine resulted solely from the 20.5 ± 11.3% increase in plant gain; the slope of the ventilatory response to CO2 below eupnea was unchanged from normal. We conclude that specific carotid chemoreceptor inhibition with dopamine increases the propensity for apnea during sleep by narrowing the CO2 reserve below eupnea. This narrowing is due solely to an increase in plant gain as the slope of the ventilatory response to CO2 below eupnea was unchanged from normal control. These findings have implications for the role of chemoreceptor inhibition/stimulation in the genesis of apnea and breathing periodicity during sleep.

Crossing the apnoeic threshold: causes and consequences

This brief review addresses the characteristics, lability and the mechanisms underlying the hypocapnic-induced apnoeic threshold which is unmasked during NREM sleep. The role of carotid chemoreceptors as fast, sensitive detectors of dynamic changes in CO2 is emphasized and placed in historical context of the long-held debate over central vs. peripheral contributions to CO2 sensing and to apnoea. Finally, evidence is presented which points to a significant role for unstable, central respiratory motor output as a significant contributor to upper airway narrowing and obstruction during sleep.

The ventilatory responsiveness to CO(2) below eupnoea as a determinant of ventilatory stability in sleep

Sleep unmasks a highly sensitive hypocapnia-induced apnoeic threshold, whereby apnoea is initiated by small transient reductions in arterial CO(2) pressure (P(aCO(2))) below eupnoea and respiratory rhythm is not restored until P(aCO(2)) has risen significantly above eupnoeic levels. We propose that the 'CO(2) reserve' (i.e. the difference in P(aCO(2)) between eupnoea and the apnoeic threshold (AT)), when combined with 'plant gain' (or the ventilatory increase required for a given reduction in P(aCO(2))) and 'controller gain' (ventilatory responsiveness to CO(2) above eupnoea) are the key determinants of breathing instability in sleep. The CO(2) reserve varies inversely with both plant gain and the slope of the ventilatory response to reduced CO(2) below eupnoea; it is highly labile in non-random eye movement (NREM) sleep. With many types of increases or decreases in background ventilatory drive and P(aCO(2)), the slope of the ventilatory response to reduced P(aCO(2)) below eupnoea remains unchanged from control. Thus, the CO(2) reserve varies inversely with plant gain, i.e. it is widened with hyperventilation and narrowed with hypoventilation, regardless of the stimulus and whether it acts primarily at the peripheral or central chemoreceptors. However, there are notable exceptions, such as hypoxia, heart failure, or increased pulmonary vascular pressures, which all increase the slope of the CO(2) response below eupnoea and narrow the CO(2) reserve despite an accompanying hyperventilation and reduced plant gain. Finally, we review growing evidence that chemoreceptor-induced instability in respiratory motor output during sleep contributes significantly to the major clinical problem of cyclical obstructive sleep apnoea.

Effect of ventilatory drive on carbon dioxide sensitivity below eupnea during sleep

We determined the effects of changing ventilatory stimuli on the hypocapnia-induced apneic and hypopneic thresholds in sleeping dogs. End-tidal carbon dioxide pressure (PET(CO2)) was gradually reduced during non-rapid eye movement sleep by increasing tidal volume with pressure support mechanical ventilation, causing a reduction in diaphragm electromyogram amplitude until apnea/periodic breathing occurred. We used the reduction in PET(CO2) below spontaneous breathing required to produce apnea (DeltaPET(CO2)) as an index of the susceptibility to apnea. DeltaPET(CO2) was -5 mm Hg in control animals and changed in proportion to background ventilatory drive, increasing with metabolic acidosis (-6.7 mm Hg) and nonhypoxic peripheral chemoreceptor stimulation (almitrine; -5.9 mm Hg) and decreasing with metabolic alkalosis (-3.7 mm Hg). Hypoxia was the exception; DeltaPET(CO2) narrowed (-4.1 mm Hg) despite the accompanying hyperventilation. Thus, hyperventilation and hypocapnia, per se, widened the DeltaPET(CO2) thereby protecting against apnea and hypopnea, whereas reduced ventilatory drive and hypoventilation narrowed the DeltaPET(CO2) and increased the susceptibility to apnea. Hypoxia sensitized the ventilatory responsiveness to CO2 below eupnea and narrowed the DeltaPET(CO2); this effect of hypoxia was not attributable to an imbalance between peripheral and central chemoreceptor stimulation, per se. We conclude that the DeltaPET(CO2) and the ventilatory sensitivity to CO2 between eupnea and the apneic threshold are changeable in the face of variations in the magnitude, direction, and/or type of ventilatory stimulus, thereby altering the susceptibility for apnea, hypopnea, and periodic breathing in sleep.

Effects of almitrine bismesylate on the ionic currents of chemoreceptor cells from the carotid body

Almitrine is a drug used in the treatment of hypoxemic chronic lung diseases such as bronchitis and emphysema because it is a potent stimulant of the carotid bodies in human and different animal species that produces a long-lasting enhancement of alveolar ventilation, ameliorating arterial blood gases. However, the mechanism of action of almitrine remains unknown. We investigated the effect of almitrine on ionic currents of chemoreceptor cells isolated from the carotid body of rat and rabbits by using the whole-cell and inside-out configurations of the patch-clamp technique. Almitrine at concentrations up to 10 microM did not affect whole-cell voltage-dependent K+, Ca2+, or Na+ currents in rat or rabbit cells. However, this concentration of almitrine significantly inhibited the Ca2+-dependent component of K+ currents in rat chemoreceptor cells. This effect of almitrine on the Ca2+-dependent component of K+ currents was investigated further at the single-channel level in excised patches in the inside-out configuration. In this preparation, almitrine inhibited the activity of a high-conductance (152 +/- 13 pS), Ca2+-dependent K+ channel by decreasing its open probability. The IC50 value of the effect was 0. 22 microM. The inhibitory effect of almitrine on Ca2+-dependent K+ channels also was observed in GH3 cells. We conclude that almitrine inhibits selectively the Ca2+-dependent K+ channel and that in rat chemoreceptor cells, this inhibition could represent an important mechanism of action underlying the therapeutic actions of the drug.

Decrease in cytosolic ATP/ADP ratio and activation of pyruvate kinase after in vitro addition of almitrine in hepatocytes isolated from fasted rats

Previously, we have shown in experiments with isolated mitochondria that almitrine, a drug used for patients with chronic lung disease, affects the H+/ATP stoichiometry of the F0F1-ATPase [Rigoulet, M., Fraisse, L., Ouhabi, R., Guérin, B., Fontaine, E. & Leverve, X. M. (1990) Biochim. Biophys. Acta 1018, 91-97]. In the present study, we have investigated the effect of almitrine on gluconeogenesis and oxygen consumption in isolated hepatocytes. Almitrine decreased both the cytosolic and mitochondrial ATP/ADP ratios but had no effect on oxygen consumption in cells incubated with and without octanoate. This must have been due to a double effect. On the one hand, a decrease in the ATP/ADP ratio decreases ATP utilization; on the other hand, in the presence of almitrine more oxygen is required to synthesize ATP. Almitrine did affect gluconeogenesis from various substrates (lactate + pyruvate, glycerone or fructose), but had no effect on glycerol or glutamine metabolism. The effect on gluconeogenesis from glycerone was due to an increase in glycolytic flux. The rate of lactate + pyruvate production increased whereas there was no effect on glycerone utilization. This effect was caused by an activation of pyruvate kinase. Our data indicate that this enzyme is an extremely sensitive sensor of the cytosolic ATP/ADP ratio. Hence, under our experimental conditions, the cytosolic ATP/ADP ratio decrease affects only the balance between glucose and lactate + pyruvate productions, and not the phosphorylation of glycerone, the first and controlling step of this pathway.

Effects of almitrine on the release of catecholamines from the rabbit carotid body in vitro

1. Almitrine increases ventilation by stimulating the carotid body (CB) arterial chemoreceptors but neither its intraglomic target nor its mechanism of action have been elucidated. 2. We have tested the hypothesis that chemoreceptor cells are targets for almitrine by studying its effects on the release of 3H-catecholamines in an in vitro rabbit CB preparation. 3. It was found that almitrine (0.3 and 1.5 x 10(-6) M; i.e. 0.2 and 1 mg ml-1) increases the resting release of 3H-catecholamines from CBs (previously loaded with [3H]-tyrosine) incubated in a balanced 95% O2/5% CO2-equilibrated solution. 4. Almitrine at a concentration of 3 x 10(-6) M (2 mg l-1) also augmented the release of 3H-catecholamines elicited by incubating the CBs in a hypoxic solution (equilibrated with 7% O2/5% CO2 in N2), by high external K+ (35 mM) and by veratridine (2 x 10(-5) M), but did not modify release induced by dinitrophenol (7.5 x 10(-5) M). 5. At the same concentration (3 x 10(-6) M), almitrine increased the rate of dopamine synthesis and was ineffective in modifying the cyclic AMP levels in either normoxic or hypoxic CBs. 6. It is concluded that chemoreceptor cells are the intraglomic targets for almitrine. The mechanisms of action of almitrine on chemoreceptor cells are discussed.