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- Heart rate regulation in diving sea lions: The vagus nerve rules
- Export Date: 8 May 2017, Review, Recent publications have emphasized the potential generation of morbid cardiac arrhythmias secondary to autonomic conflict in diving marine mammals. Such conflict, as typified by cardiovascular responses to cold water immersion in humans, has been proposed to result from exercise-related activation of cardiac sympathetic fibers to increase heart rate, combined with depth-related changes in parasympathetic tone to decrease heart rate. After reviewing the marine mammal literature and evaluating heart rate profiles of diving California sea lions (Zalophus californianus), we present an alternative interpretation of heart rate regulation that de-emphasizes the concept of autonomic conflict and the risk of morbid arrhythmias in marine mammals. We hypothesize that: (1) both the sympathetic cardiac accelerator fibers and the peripheral sympathetic vasomotor fibers are activated during dives even without exercise, and their activities are elevated at the lowest heart rates in a dive when vasoconstriction is maximal, (2) in diving animals, parasympathetic cardiac tone via the vagus nerve dominates over sympathetic cardiac tone during all phases of the dive, thus producing the bradycardia, (3) adjustment in vagal activity, which may be affected by many inputs, including exercise, is the primary regulator of heart rate and heart rate fluctuations during diving, and (4) heart beat fluctuations (benign arrhythmias) are common in marine mammals. Consistent with the literature and with these hypotheses, we believe that the generation of morbid arrhythmias because of exercise or stress during dives is unlikely in marine mammals
- Ponganis, McDonald, Tift, Williams
- Flipper stroke rate and venous oxygen levels in free-ranging California sea lions
- The depletion rate of the blood oxygen store, development of hypoxemia, and dive capacity are dependent on the distribution and rate of blood oxygen delivery to tissues while diving. Although blood oxygen extraction by working muscle would increase the blood oxygen depletion rate in a swimming animal, there is little information on the relationship between muscle workload and blood oxygen depletion during dives. Therefore, we examined flipper stroke rate, a proxy of muscle workload, and posterior vena cava oxygen profiles in four adult female California sea lions (Zalophus californianus) during foraging trips at sea. Flipper stroke rate analysis revealed that sea lions minimized muscle metabolism with a stroke-glide strategy when diving, and exhibited prolonged glides during the descent of deeper dives (> 100 m). During the descent phase of these deep dives, 55±21% of descent was spent gliding with the longest glides lasting over 160 s and covering a vertical distance of 340 m. Animals also consistently glided to the surface from 15-25 m depth during these deeper dives. Venous hemoglobin saturation (SvO2) profiles were highly variable throughout dives, with values occasionally increasing during shallow dives. The relationship between SvO2 and flipper stroke rate was weak during deeper dives, while this relationship was stronger during shallow dives. We conclude that 1) the depletion of oxygen in the posterior vena cava in deep diving sea lions is not dependent on stroke effort, and 2) stroke-glide patterns during dives contribute to a reduction of muscle metabolic rate.
- Tift, Hückstädt, McDonald, Thorson, Ponganis