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Cardiac pacing reveals that elevating f<jats:sub>H<\/jats:sub> 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<jats:sub>H<\/jats:sub> is more costly than that which would incur if V<jats:sub>S<\/jats:sub> instead were to increase. We hypothesized that f<jats:sub>H<\/jats:sub> must increase because any substantial rise in V<jats:sub>S<\/jats:sub> would be constrained by the pericardium. To investigate this hypothesis, we explored the effects of pharmacologically induced bradycardia, with ivabradine treatment, on V<jats:sub>S<\/jats:sub> 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<jats:sub>H<\/jats:sub> in vivo, such that exercise tachycardia was attenuated. Pulmonary and systemic V<jats:sub>S<\/jats:sub> rose in response to ivabradine. The rise in pulmonary V<jats:sub>S<\/jats:sub> largely compensated for the bradycardia at rest, leaving total pulmonary flow unchanged by ivabradine, although ivabradine reduced pulmonary blood flow during swimming (exercise \u00d7 ivabradine interaction, P &lt; 0.05). Although systemic V<jats:sub>S<\/jats:sub> increased, systemic blood flow was reduced by ivabradine both at rest and during exercise, despite ivabradine\u2019s potential to increase cardiac contractility. Opening the pericardium had no effect on f<jats:sub>H<\/jats:sub>, V<jats:sub>S<\/jats:sub>, or blood flows before or after ivabradine, indicating that the pericardium does not constrain VS in turtles, even during pharmacologically induced bradycardia. <\/jats:p>","DOI":"10.1152\/ajpregu.00004.2022","type":"journal-article","created":{"date-parts":[[2022,2,24]],"date-time":"2022-02-24T15:00:55Z","timestamp":1645714855000},"page":"R389-R399","update-policy":"https:\/\/doi.org\/10.1152\/aps-crossmark-policy","source":"Crossref","is-referenced-by-count":3,"title":["No evidence for pericardial restraint in the snapping turtle (<i>Chelydra serpentina<\/i>) following pharmacologically induced bradycardia at rest or during exercise"],"prefix":"10.1152","volume":"322","author":[{"given":"Brandt","family":"Smith","sequence":"first","affiliation":[{"name":"Department of Biological Sciences, University of North Texas, Denton, 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