Avian arterial chemoreceptor responses to steps of CO2 and O2

Intranasal butorphanol and midazolam administered prior to intramuscular alfaxalone provides safe and effective sedation in Quaker parrots (Myiopsitta monachus)

OBJECTIVE

To evaluate 2 doses of alfaxalone on cardiopulmonary parameters, temperature, sedation, endotracheal intubation, the incidence of muscle tremors, and radiographic positioning in Quaker parrots previously administered intranasal midazolam and butorphanol.

ANIMALS

10 healthy adult Quaker parrots (male = 5; female = 5).

PROCEDURES

A randomized, masked, crossover study was conducted where birds received midazolam (2 mg/kg) and butorphanol (2 mg/kg) intranasally 15 minutes prior to a low- or high-dose of intramuscular alfaxalone: 2 mg/kg (LDA) or 5 mg/kg (HDA), respectively. Heart (HR) and respiratory rate (RR), cloacal temperature, sedation quality, and ability to position for radiographs were recorded over time. The incidence of muscle tremors and the ability to intubate were recorded. Data were compared to baseline values and between treatments where appropriate. Significance was set at P < .05.

RESULTS

There were no significant differences in HR, RR, cloacal temperature, and sedation scores between treatments at any time point. Duration of time from midazolam-butorphanol administration to complete recovery from treatment administration was significantly shorter for LDA when compared to HDA (90 [60 to 195] vs 127.5 [90 to 10] minutes, respectively). Compared to baseline, sedation scores were significantly higher from T = 15 to 60 for LDA and from T = 15 to 75 for HDA. The incidence of muscle tremors was greater in HDA (9/10) than in LDA (7/10). All birds were successfully intubated and positioned for radiographs.

CLINICAL RELEVANCE

The combination of intranasal midazolam-butorphanol and intramuscular alfaxalone at the doses examined was a safe and effective method for sedating Quaker parrots. LDA produced adequate sedation with a shorter time to recovery and with fewer muscle fasciculations when compared to HDA.

Hypercarbic cardiorespiratory reflexes in the facultative air-breathing fish jeju (Hoplerythrinus unitaeniatus): the role of branchial CO2 chemoreceptors

The aim of the present study was to determine the roles that externally versus internally oriented CO2/H+-sensitive chemoreceptors might play in promoting cardiorespiratory responses to environmental hypercarbia in the air-breathing fish, Hoplerythrinus unitaeniatus (jeju). Fish were exposed to graded hypercarbia (1, 2.5, 5, 10 and 20% CO2) and also to graded levels of environmental acidosis (pH ~7.0, 6.0, 5.8, 5.6, 5.3 and 4.7) equal to the pH levels of the hypercarbic water to distinguish the relative roles of CO2versus H+. We also injected boluses of CO2-equilibrated solutions (5, 10 and 20% CO2) and acid solutions equilibrated to the same pH as the CO2 boluses into the caudal vein (internal) and buccal cavity (external) to distinguish between internal and external stimuli. The putative location of the chemoreceptors was determined by bilateral denervation of branches of cranial nerves IX (glossopharyngeal) and X (vagus) to the gills. The data indicate that the chemoreceptors eliciting bradycardia, hypertension and gill ventilatory responses (increased frequency and amplitude) to hypercarbia are exclusively branchial, externally oriented and respond specifically to changes in CO2 and not H+. Those involved in producing the cardiovascular responses appeared to be distributed across all gill arches while those involved in the gill ventilatory responses were located primarily on the first gill arch. Higher levels of aquatic CO2 depressed gill ventilation and stimulated air breathing. The chemoreceptors involved in producing air breathing in response to hypercarbia also appeared to be branchial, distributed across all gill arches and responded specifically to changes in aquatic CO2. This would suggest that chemoreceptor groups with different orientations (blood versus water) are involved in eliciting air-breathing responses to hypercarbia in jeju.

Control of breathing and adaptation to high altitude in the bar-headed goose

The bar-headed goose flies over the Himalayan mountains on its migratory route between South and Central Asia, reaching altitudes of up to 9,000 m. We compared control of breathing in this species with that of low-altitude waterfowl by exposing birds to step decreases in inspired O(2) under both poikilocapnic and isocapnic conditions. Bar-headed geese breathed substantially more than both greylag geese and pekin ducks during severe environmental (poikilocapnic) hypoxia (5% inspired O(2)). This was entirely due to an enhanced tidal volume response to hypoxia, which would have further improved parabronchial (effective) ventilation. Consequently, O(2) loading into the blood and arterial Po(2) were substantially improved. Because air convection requirements were similar between species at 5% inspired O(2), it was the enhanced tidal volume response (not total ventilation per se) that improved O(2) loading in bar-headed geese. Other observations suggest that bar-headed geese depress metabolism less than low-altitude birds during hypoxia and also may be capable of generating higher inspiratory airflows. There were no differences between species in ventilatory sensitivities to isocapnic hypoxia, the hypoxia-induced changes in blood CO(2) tensions or pH, or hypercapnic ventilatory sensitivities. Overall, our results suggest that evolutionary changes in the respiratory control system of bar-headed geese enhance O(2) loading into the blood and may contribute to this species' exceptional ability to fly high.

Development of avian intrapulmonary chemoreceptors

Although avian intrapulmonary chemoreceptors (IPC) have been studied extensively in adults, the maturation of IPC CO(2) sensitivity during development is completely unknown. To begin investigating IPC development we asked two fundamental questions: (1) Are IPC capable of sensing CO(2) during early development, and, if so, how early? And, (2) does IPC CO(2) sensitivity during early development exhibit postnatal maturation compared to IPC discharge characteristics in adult ducks? We addressed these questions by recording from single IPC Anas platyrhynchos ducklings beginning approximately 6 h prior to internal pipping through 4 days of postnatal development. We then compared mean IPC discharge characteristics during early development with mean IPC activity from adult ducks greater than 12 weeks old. In total, we recorded 28 individual IPC from 5 ducklings and 12 adult ducks. Results show that IPC were capable of responding to rapid step changes in CO(2) before hatching occurred, during the paranatal developmental period. We also found that mean IPC activity during early development had increased peak discharge frequencies, greater spike frequency adaptation, and less tonic CO(2) sensitivity when compared to adults (P< or =0.05). These results suggest that during early development phasic IPC CO(2) sensitivity is fully developed, yet tonic IPC CO(2) sensitivity exhibits postnatal maturation possibly associated with hatching. These results also suggest that the mechanisms that underlie phasic and tonic IPC action potential discharge, and therefore the degree of partial spike frequency adaptation, may be independent processes with different developmental trajectories.

Calcium and avian intrapulmonary chemoreceptor response to CO2

Intrapulmonary chemoreceptors (IPC) are highly responsive respiratory chemoreceptors that innervate the lungs of birds and diapsid reptiles. IPC are stimulated by low levels of lung Pco(2), inhibited by high levels of lung Pco(2), and their vagal afferents serve as a sensory limb for reflex adjustments of breathing depth and rate. Most IPC exhibit both phasic and tonic sensitivity to CO(2), and spike frequency adaptation (SFA) contributes to their phasic CO(2) responsiveness. To test whether CO(2) responsiveness and SFA in IPC is modulated by a Ca(2+)-linked mechanism, we quantified the role of transmembrane Ca(2+) fluxes and Ca(2+)-related channels on single-unit IPC function in response to phasic changes in inspired Pco(2). We found that 1) broad-spectrum blockade of Ca(2+) channels using cadmium or cobalt and blockade of L-type Ca(2+) channels using nifedipine increased IPC discharge; 2) activation of L-type Ca(2+) channels using BAY K 8644 reduced IPC discharge; 3) blockade of Ca(2+)-activated potassium channels using charybdotoxin (antagonist of large-conductance Ca(2+)-dependent K(+) channel) increased IPC discharge, but neither charybdotoxin nor apamin affected SFA; and 4) blockade of chloride channels, including Ca(2+)-activated chloride channels, with niflumic acid decreased IPC discharge at low Pco(2) and increased IPC discharge at high Pco(2), resulting in a net attenuation of the IPC CO(2) response. We conclude that Ca(2+) influx through L-type Ca(2+) channels has an inhibitory effect on IPC afferent discharge and CO(2) sensitivity, that spike frequency adaptation is not due to apamin- or charybdotoxin-sensitive Ca(2+)-activated K(+) channels in IPC, and that chloride channels blocked by niflumic acid help modulate IPC CO(2) responses.

Benzolamide, acetazolamide, and signal transduction in avian intrapulmonary chemoreceptors

Intrapulmonary chemoreceptors (IPC) are CO(2)-sensitive sensory neurons that innervate the lungs of birds, help control the rate and depth of breathing, and require carbonic anhydrase (CA) for normal function. We tested whether the CA enzyme is located intracellularly or extracellularly in IPC by comparing the effect of a CA inhibitor that is membrane permeable (iv acetazolamide) with one that is relatively membrane impermeable (iv benzolamide). Single cell extracellular recordings were made from vagal filaments in 16 anesthetized, unidirectionally ventilated mallards (Anas platyrhynchos). Without CA inhibition, action potential discharge rate was inversely proportional to inspired PCO(2) (-9.0 +/- 0.8 s(-1). lnTorr(-1); means +/- SE, n = 16) and exhibited phasic responses to rapid PCO(2) changes. Benzolamide (25 mg/kg iv) raised the discharge rate but did not alter tonic IPC PCO(2) response (-9.8 +/- 1.6 s(-1). lnTorr(-1), n = 8), and it modestly attenuated phasic responses. Acetazolamide (10 mg/kg iv) raised IPC discharge, significantly reduced tonic IPC PCO(2) response to -3.5 +/- 3.6 s(-1). lnTorr(-1) (n = 6), and severely attenuated phasic responses. Results were consistent with an intracellular site for CA that is less accessible to benzolamide. A model of IPC CO(2) transduction is proposed.

Measuring ventilatory acclimatization to hypoxia: comparative aspects

Acclimatization to hypoxia increases the hypoxic ventilatory response (HVR) in mammals. The literature on humans shows that several protocols can quantify this increase in HVR if isocapnia is maintained, regardless of the exact level of Pa(CO(2)). In rats, the isocapnic HVR also increases with chronic hypoxia and this cannot be explained by a non-specific effect of increased ventilatory drive on the HVR. Changes in arterial pH are predicted to increase the HVR during chronic hypoxia in rats but this has not been quantified. Limitations in determining mechanisms of change in the HVR from reflex experiments are discussed. Chronic hypoxia changes some, but not all, indices of ventilatory motor output that are useful for normalization between experiments on anesthetized rats. Finally, ducks also show time-dependent increases in ventilation during chronic hypoxia and birds provide a good experimental model to study reflex interactions. However, reflexes from intrapulmonary CO(2) chemoreceptors can complicate the measurement of changes in the isocapnic HVR during chronic hypoxia in birds.