Key result
Muskrat exercise tachycardia involves sympathetic activation in air but relies solely on parasympathetic reduction underwater.
Why the study?
The neural control of cardiac responses to exercise in air and underwater in muskrats was not fully understood, particularly the roles of sympathetic and parasympathetic systems.
The study demonstrates that the autonomic control of heart rate in muskrats differs significantly between terrestrial and underwater exercise, with parasympathetic tone dominating during diving.
Dual autonomic blockade alters cardiac responses in exercising and diving muskrats; leaves open translation to human physiology.
Neural control of the cardiac responses to exercise in air (running) and under water (diving) was studied in the muskrat (Ondatra zibethicus) by means of acute pharmacological blockade with the muscarinic blocker atropine and the beta-adrenergic blocker nadolol. Saline injection was used as a control. Controls running on a treadmill showed a marked increase in heart rate with exercise. Atropine-treated animals had a higher resting heart rate than controls, but heart rate still increased with running. Nadolol-treated animals had a lower resting heart rate than controls and displayed a less pronounced increase in heart rate with running than controls. Animals treated with a combination of atropine and nadolol had a resting heart rate similar to that of controls but their heart rate was unaffected by running. Thus, exercise tachycardia in muskrats is due to activation of the sympathetic system and also to a reduction in parasympathetic tone. Heart rate decreased markedly during voluntary submergence in controls but rose as muskrats swam submerged against increasing water flows. Nevertheless, diving bradycardia was still present. Free-diving bradycardia and the relative increase in heart rate with underwater exercise were abolished by atropine and unaffected by nadolol. Hence, unlike the cardiac response to exercise in air, the cardiac response to underwater exercise is due only to a reduction in parasympathetic tone. Injection of the beta-adrenergic agonist isoproterenol markedly increased heart rate in air but had little effect during voluntary and forced dives, indicating a marked decrease in the sensitivity of cardiac cells to adrenergic stimulation during submergence. These results strongly suggest that accentuated antagonism between the two branches of the autonomic nervous system occurs during diving so that parasympathetic influences on the heart predominate and inhibit any chronotropic response to adrenergic stimulation.
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Signore et al. (1996) studied this question. Pharmacological blockade (atropine, nadolol, isoproterenol) vs. Saline injection was evaluated on Heart rate response to exercise in air and under water. In muskrats, exercise tachycardia in air is driven by sympathetic activation and parasympathetic reduction, whereas the cardiac response to underwater exercise is solely due to a reduction in parasympathetic tone.
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