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Tift, MS, Huckstadt LA, Ponganis PJ.  2018.  Anterior vena caval oxygen profiles in a deep-diving California sea lion: arteriovenous shunts, a central venous oxygen store and oxygenation during lung collapse. Journal of Experimental Biology. 221   10.1242/jeb.163428   AbstractWebsite

Deep-diving California sea lions (Zalophus californianus) can maintain arterial hemoglobin saturation (S-O2) above 90% despite lung collapse (lack of gas exchange) and extremely low posterior vena caval S-O2 in the middle of the dive. We investigated anterior vena caval P-O2 and S-O2 during dives of an adult female sea lion to investigate two hypotheses: (1) posterior vena caval S-O2 is not representative of the entire venous oxygen store and (2) a well-oxygenated (arterialized) central venous oxygen reservoir might account for maintenance of arterial S-O2 during lung collapse. During deep dives, initial anterior vena caval S-O2 was elevated at 83.6 +/- 8.4% (n = 102), presumably owing to arteriovenous shunting. It remained high until the bottom phase of the dive and then decreased during ascent, whereas previously determined posterior vena caval S-O2 declined during descent and then often increased during ascent. These divergent patterns confirmed that posterior vena caval S-O2 was not representative of the entire venous oxygen store. Prior to and early during descent of deep dives, the high S-O2 values of both the anterior and posterior venae cavae may enhance arterialization of a central venous oxygen store. However, anterior vena caval S-O2 values at depths beyond lung collapse reached levels as low as 40%, making it unlikely that even a completely arterialized central venous oxygen store could account for maintenance of high arterial S-O2. These findings suggest that maintenance of high arterial S-O2 during deep dives is due to persistence of some gas exchange at depths beyond presumed lung collapse.

Ponganis, PJ, McDonald BI, Tift MS, Williams CL.  2017.  Heart rate regulation in diving sea lions: the vagus nerve rules. Journal of Experimental Biology. 220:1372-1381.   10.1242/jeb.146779   AbstractWebsite

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.

McDonald, BI, Ponganis PJ.  2014.  Deep-diving sea lions exhibit extreme bradycardia in long-duration dives. Journal of Experimental Biology. 217:1525-1534.   10.1242/jeb.098558   AbstractWebsite

Heart rate and peripheral blood flow distribution are the primary determinants of the rate and pattern of oxygen store utilisation and ultimately breath-hold duration in marine endotherms. Despite this, little is known about how otariids (sea lions and fur seals) regulate heart rate (f(H)) while diving. We investigated dive f(H) in five adult female California sea lions (Zalophus californianus) during foraging trips by instrumenting them with digital electrocardiogram (ECG) loggers and time depth recorders. In all dives, dive f(H) (number of beats/duration; 50 +/- 9 beats min(-1)) decreased compared with surface rates (113 +/- 5 beats min(-1)), with all dives exhibiting an instantaneous f(H) below resting (<54 beats min(-1)) at some point during the dive. Both dive f(H) and minimum instantaneous f(H) significantly decreased with increasing dive duration. Typical instantaneous f(H) profiles of deep dives (>100 m) consisted of: (1) an initial rapid decline in f(H) resulting in the lowest instantaneous f(H) of the dive at the end of descent, often below 10 beats min-1 in dives longer than 6 min in duration; (2) a slight increase in f(H) to similar to 10-40 beats min(-1) during the bottom portion of the dive; and (3) a gradual increase in f(H) during ascent with a rapid increase prior to surfacing. Thus, f(H) regulation in deep-diving sea lions is not simply a progressive bradycardia. Extreme bradycardia and the presumed associated reductions in pulmonary and peripheral blood flow during late descent of deep dives should (a) contribute to preservation of the lung oxygen store, (b) increase dependence of muscle on the myoglobin-bound oxygen store, (c) conserve the blood oxygen store and (d) help limit the absorption of nitrogen at depth. This f(H) profile during deep dives of sea lions may be characteristic of deep-diving marine endotherms that dive on inspiration as similar f(H) profiles have been recently documented in the emperor penguin, another deep diver that dives on inspiration.

Jobsis, PD, Ponganis PJ, Kooyman GL.  2001.  Effects of training on forced submersion responses in harbor seals. Journal of Experimental Biology. 204:3877-3885. AbstractWebsite

In several pinniped species, the heart rates observed during unrestrained dives are frequently higher than the severe bradycardias recorded during forced submersions. To examine other physiological components of the classic 'dive response' during such moderate bradycardias, a training protocol was developed to habituate harbor seals (Phoca vitulina) to short forced submersions. Significant changes were observed between physiological measurements made during naive and trained submersions (3-3.5min). Differences were found in measurements of heart rate during submersion (naive 18 +/-4.3 beats min(-1) versus trained 35 +/-3.4 beats min(-1)), muscle blood flow measured using laser-Doppler flowmetry (naive 1.8 +/-0.8 ml min(-1) 100 g(-1) versus trained 5.8 +/-3.9 ml min(-1) 100 g(-1)), change in venous P-O 2 (naive -0.44 +/-1.25 kPa versus trained -1.48 +/-0.76 kPa) and muscle deoxygenation rate (naive -0.67 +/-0.27 mvd s(-1) versus trained -0.51 +/-0.18 mvd s(-1), a relative measure of muscle oxygenation provided by the Vander Niroscope, where mvd are milli-vander units). In contrast to the naive situation, the post-submersion increase in plasma lactate levels was only rarely significant in trained seals. Resting eupneic (while breathing) heart rate and total oxygen consumption rates (measured in two seals) were not significantly different between the naive and trained states. This training protocol revealed that the higher heart rate and greater muscle blood flow in the trained seals were associated with a lower muscle deoxygenation rate, presumably secondary to greater extraction of blood O-2 during trained submersions. Supplementation of muscle oxygenation by blood O-2 delivery during diving would increase the rate of blood O-2 depletion but could prolong the duration of aerobic muscle metabolism during diving. This alteration of the dive response may increase the metabolic efficiency of diving.