Most animals elevate cardiac output during exercise through a rise in heart rate ( f H ), whereas stroke volume (V S ) remains relatively unchanged. Cardiac pacing reveals that elevating f H alone does not alter cardiac output, which is instead largely regulated by the peripheral vasculature. In terms of myocardial oxygen demand, an increase in f H is more costly than that which would incur if V S instead were to increase. We hypothesized that f H must increase because any substantial rise in V S would be constrained by the pericardium. To investigate this hypothesis, we explored the effects of pharmacologically induced bradycardia, with ivabradine treatment, on V S at rest and during exercise in the common snapping turtle ( Chelydra serpentina) with intact or opened pericardium. We first showed that, in isolated myocardial preparations, ivabradine exerted a pronounced positive inotropic effect on atrial tissue but only minor effects on ventricle. Ivabradine reduced f H in vivo, such that exercise tachycardia was attenuated. Pulmonary and systemic V S rose in response to ivabradine. The rise in pulmonary V S largely compensated for the bradycardia at rest, leaving total pulmonary flow unchanged by ivabradine, although ivabradine reduced pulmonary blood flow during swimming (exercise × ivabradine interaction, P < 0.05). Although systemic V S increased, systemic blood flow was reduced by ivabradine both at rest and during exercise, despite ivabradine’s potential to increase cardiac contractility. Opening the pericardium had no effect on f H , V S , or blood flows before or after ivabradine, indicating that the pericardium does not constrain VS in turtles, even during pharmacologically induced bradycardia.
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Drivers of the dive response in trained harbour porpoises (Phocoena phocoena )
Pronounced dive responses through peripheral vasoconstriction and bradycardia enables prolonged apnoea in marine mammals. For most vertebrates, the dive response is initiated upon face immersion, but little is known about the physical drivers of diving and surfacing heart rate in cetaceans whose faces are always mostly submerged. Using two trained harbour porpoises instrumented with an ECG-measuring DTAG-3, we investigate the initiation and progression of bradycardia and tachycardia during apnoea and eupnoea for varying levels of immersion. We show that paranasal wetting drives bradycardia initiation and progression, whereas apnoea leads to dive-level bradycardia eventually, but not instantly. At the end of dives, heart rate accelerates independently of lung expansion, perhaps in anticipation of surfacing; however, full tachycardia is only engaged upon inhalation. We conclude that breathing drives surface tachycardia, whereas blowhole wetting is an important driver of bradycardia; although, anticipatory/volitional modulation can overrule such responses to sensory inputs.
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- Award ID(s):
- 1159123
- NSF-PAR ID:
- 10278958
- Date Published:
- Journal Name:
- Journal of Experimental Biology
- Volume:
- 222
- Issue:
- 19
- ISSN:
- 0022-0949
- Page Range / eLocation ID:
- jeb208637
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1242/jeb.208637
