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Effect of arousal state and increased respiratory drive on cardio-respiratory variables in the harbour seal (Phoca vitulina richardsi) Skinner, Lisa Ann

Abstract

In this study, I examined the cardio-respiratory patterns of Pacific harbour seal pups (Phoca vitulina richardsi) under normoxic/normocarbic (air), hypoxic/normocarbic (15.22% O₂ ± 0.47 SEM, 11.99% O₂ ± 0.29 SEM, and 8.63% O₂ ± 0.22 SEM in air) and normoxic/hypercarbic (2.04% CO₂ ± 0.06 SEM, 4.16 %CO₂ ± 0.06 SEM, and 5.66% CO₂ ± 0.07 SEM in air) conditions while awake and sleeping on land. Animals were chronically instrumented to record electroencephalogram (EEG), electromyogram (EMG), and electrocardiogram (EKG) signals, and recordings of all variables, plus respiration (whole body plethysmography) and metabolic rate (O₂ consumption and CO₂ production), were made on each gas for two to four hours on separate days. My results show that, on air, metabolic rate was lower in sleep (7.71 ml O₂ min⁻¹ kg⁻¹ ± 0.93 SEM) than wakefulness (WAKE) (8.79 ml O₂ min⁻¹ kg⁻¹ ± 0.70 SEM), with the fall in metabolism accompanied by a decrease in breathing frequency (18.76 breaths/minute ± 0.23 SEM in WAKE to 10.70 breaths/minute ± 0.49 SEM in sleep) and an increase in the incidence of periods of apnea. The maximum apnea length recorded (~ 3 minutes) was similar in length to the average dive time of harbour seals (-3.2 minutes). Breathing was rarely seen in REM sleep, appearing in only one or two animals, during one or two REM sleep episodes. Tachypnea was present at all levels of increased respiratory drive, both hypoxia and hypercarbia, however hypoxia induced a dramatic bradycardia (up to 50% decrease in heart rate [f[sub H]]) on 8.63% O₂ ± 0.22 SEM regardless of arousal state, while hypercarbia produced a small tachycardia in slow-wave sleep (SWS) only. Metabolic rate (V[sub O]₂) was relatively unchanged in both hypoxia and hypercarbia, while total ventilation (V[sub E]) increased significantly in both WAKE and SWS. The hypoxic and hypercarbic chemosensitivities of the harbour seal pups, therefore, are similar to those of terrestrial mammals. Unlike the situation seen in terrestrial mammals however, where hypoxic and hypercarbic sensitivities are often reduced in SWS, the sensitivity of harbour seal pups to both gases appears unchanged during the change in arousal state from WAKE to sleep.

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