No evidence for pericardial restraint in the snapping turtle (Chelydra serpentina) following pharmacologically induced bradycardia at rest or during exercise

Abstract

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.

Keywords: Testudines; activity; cardiovascular; ectotherm; reptile.

Figure 1.

Effects of ivabradine (7.57 mg·L⁻¹; 15 µmol·L⁻¹) on twitch force in in vitro myocardial preparations from common snapping turtles (Chelydra serpentina) from atrium (A) or ventricle (B). Each preparation, either treated with ivabradine (red) just before the protocol or left untreated (black) as a control, underwent a force-frequency trial (increasing stimulation frequency in 0.3 Hz steps) before and after being treated with epinephrine (10 µmol·L⁻¹). Mixed-effect models were used to analyze the effects of stimulation frequency, epinephrine treatment and ivabradine treatment. n = 8 female snapping turtles for each treatment. Data are means ± SE.

Figure 2.

Effect of ivabradine on heart rate (A), pulmonary blood flow (C), and pulmonary stroke volume (B) in common snapping turtles (Chelydra serpentina). Turtles either remained with pericardium intact (circle symbols, plain bars) or underwent surgical opening of the pericardium (square symbols, hatched bars). Resting values were measured when animals were stationary, and exercise (swim) values were taken during or immediately after an enforced 5-min swim. Animals were then treated with ivabradine (1 mg·kg⁻¹ intra-arterial injection), and the swim protocol was repeated. Mixed-effect models were used to analyze the effects of ivabradine, pericardium opening, and exercise. n = 8 female snapping turtles for each bar. Bars show means ± SE and symbols are individual values.

Figure 3.

Effect of ivabradine on heart rate (A), systemic blood flow (C), and systemic stroke volume (B) in common snapping turtles (Chelydra serpentina). Turtles either remained with pericardium intact (circle symbols, plain bars) or underwent surgical opening of the pericardium (square symbols, hatched bars). Resting values were measured when animals were stationary, and exercise (swim) values were taken during or immediately after an enforced 5-min swim. Animals were then treated with ivabradine (1 mg·kg⁻¹ intra-arterial injection), and the swim protocol was repeated. Mixed-effect models were used to analyze the effects of ivabradine, pericardium opening, and exercise. n = 8 female snapping turtles for each bar except pericardium cut open after ivabradine (both at rest and swimming) where n = 7. Bars show means ± SE and symbols are individual values.

Figure 4.

Effect of ivabradine on heart rate (A), total (pulmonary + systemic) blood flow (C), and total stroke volume (B) in common snapping turtles (Chelydra serpentina). Turtles either remained with pericardium intact (circle symbols, plain bars) or underwent surgical opening of the pericardium (square symbols, hatched bars). Resting values were measured when animals were stationary, and exercise (swim) values were taken during or immediately after an enforced 5-min swim. Animals were then treated with ivabradine (1 mg·kg⁻¹ intra-arterial injection), and the swim protocol was repeated. Mixed-effect models were used to analyze the effects of ivabradine, pericardium opening, and exercise. n = 6 female snapping turtles for each bar except pericardium cut open after ivabradine (both at rest and swimming) where n = 5. Bars show means ± SE and symbols are individual values.