| Summary: | While diving, harbour seals (Phoca vitulina) manage their oxygen stores through
cardiovascular adjustments including bradycardia, a concurrent reduction in cardiac
output, and peripheral vasoconstriction. At the surface, post-dive tachycardia facilitates
rapid reloading of these oxygen stores. Although harbour seals can tolerate over 20 min
of submergence, the majority of their natural dives are only 2 to 6 min and are usually
followed by surface intervals that are less than 1 min, so they spend about 80 % of their
time at sea submerged. Given that harbour seals meet their ecological needs through
repetitive short aerobic dives, I was interested in the functional role, if any, of the
cardiovascular dive response during these short dives. During voluntary diving in an 11
m deep tank, the cardiovascular responses to submergence of five captive harbour seals
were manipulated using specific pharmacological antagonists, and the effects on diving
behaviour were observed. Effects of pharmacological blockade on mean heart rate and on
heart rate variability were also examined in order to assess the autonomic control of heart
rate during voluntary diving. Heart rate was recorded using subcutaneous electrodes and
a custom data logger while diving behaviour was monitored using a video camera. The
muscarinic blocker methoctramine blocked diving bradycardia; the α-adrenergic blocker
prazosin blocked diving vasoconstriction; and the β-adrenergic blocker metoprolol
blocked post-dive tachycardia. Mean heart rate analysis revealed that diving bradycardia
is primarily modulated by the vagus, and post-dive tachycardia is due to increased
sympathetic stimulation of the heart as well as parasympathetic withdrawal. Results of
power spectral analysis of heart rate variability supported the conclusion that the
parasympathetic nervous system predominates during diving but not during surface
intervals. None of the pharmacological blockers had any effect on average dive or surface
interval duration. Mean dive duration ranged from 2.3±0.47 to 2.9±0.10 min
(mean±S.E.M.) for all treatments including controls, and mean surface interval duration
ranged from 23 to 29 sec. Seals maintained a high percent dive time (83 to 87 %) in all
treatments including controls. Thus, harbour seals do not need the dive response during
short dives in order to maintain an efficient dive strategy.
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