Low cost of pulmonary ventilation in American alligators (Alligator mississippiensis) stimulated with doxapram

Abstract

To determine the costs of pulmonary ventilation without imposing severe oxygen limitations or acidosis that normally accompany exposures to hypoxia or hypercapnia, we opted to pharmacologically stimulate ventilation with doxapram (5 and 10 mg kg −1 ) in alligators. Doxapram is used clinically to alleviate ventilatory depression in response to anaesthesia and acts primarily on the peripheral oxygen-sensitive chemoreceptors. Using this approach, we investigated the hypothesis that pulmonary ventilation is relatively modest in comparison to resting metabolic rate in crocodilians and equipped seven juvenile alligators with masks for concurrent determination of ventilation and oxygen uptake. Doxapram elicited a dose-dependent and up to fourfold rise in ventilation, primarily by increasing ventilatory frequency. The accompanying rise in oxygen uptake was very small; ventilation in resting animals constitutes no more than 5% of resting metabolic rate. The conclusion that pulmonary ventilation is energetically cheap is consistent with earlier studies on alligators where ventilation was stimulated by hypoxia or hypercapnia.

Respiratory mechanics of eleven avian species resident at high and low altitude

“…The metabolic cost of breathing for vertebrates at rest has been estimated to be 1-10% of BMR (Otis et al, 1950;Steffensen and Lomholt, 1983;Aaron et al, 1992;Skovgaard et al, 2016;Lee and Milsom, 2016). It has been hypothesized that cost of breathing would be higher in birds because of the heavy weight (on average 17% of body mass) of the pectoral flight muscles attached to the sternum (Markley and Carrier, 2010;Greenewalt, 1962).…”

Control of breathing and ventilatory acclimatization to hypoxia in deer mice native to high altitudes

“…Whether this is also true at the lower air densities at high altitudes is unclear, because the influence of air density on breathing mechanics is poorly understood. Nevertheless, it is foreseeable that improvements in respiratory O 2 uptake could more than outweigh the small effect of changes in breathing pattern on the metabolic cost of breathing (which is believed to be <10% of basal metabolic rate) …”

“…Nevertheless, it is foreseeable that improvements in respiratory O 2 uptake could more than outweigh the small effect of changes in breathing pattern on the metabolic cost of breathing (which is believed to be <10% of basal metabolic rate). 41,42 The population differences in breathing pattern in normoxia likely affected arterial O 2 saturation, but the appreciable improvement in Sa O2 in deep hypoxia in highlanders compared to lowlanders (Fig. 4a,b) likely arose in large part from the heightened blood-O 2 affinity of high-altitude deer mice.…”

Scaling of axial muscle architecture in juvenile Alligator mississippiensis reveals an enhanced performance capacity of accessory breathing mechanisms

“…The metabolic cost of breathing (ml O 2 min −1 kg −1 ) is reported to be low in crocodilians (~5% resting metabolic rate), on a par with birds and mammals (Skovgaard et al, 2016 ; Wang & Warburton, 1995 ). It is often considered in relation to the forces that must be generated to overcome elastic recoil of the lung, resistive forces in compressing the body wall and elastic and non‐elastic forces in expanding the body wall (Perry and Dunker 1978; Milsom & Vitalis, 1984 ).…”

“…However, in studies on A . mississippiensis , where changes in breathing frequency and/or tidal volume were induced to increase ventilation rate, the opposite was found, where the metabolic cost of breathing increased with frequency, but not volume (Skovgaard et al, 2016 ; Wang & Warburton, 1995 ). Similarly, following vagotomy the duration of inspiration increases, leading to a lower cost breathing (Skovgaard & Wang, 2007 ).…”