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Vagal tone dominates autonomic control of mouse heart rate at thermoneutrality

¹Department of Biology, Williams College, Williamstown, Massachusetts; ²Laboratory of Bioorganic Chemistry, National Institute of Diabetes and Digestive and Kidney Diseases, Bethesda, Maryland; ³Department of Biomedical Sciences, Florida State University, Tallahassee, Florida; and ⁴Department of Biology, Emmanuel College, Boston, Massachusetts

Submitted 29 August 2007 ; accepted in final form 26 January 2008

It is generally accepted that cardiac sympathetic tone dominates the control of heart rate (HR) in mice. However, we have recently challenged this notion given that HR in the mouse is responsive to ambient temperature (T_a) and that the housing T_a is typically 21–23°C, well below the thermoneutral zone (30°C) of this species. To specifically test the hypothesis that cardiac sympathetic tone is the primary mediator of HR control in the mouse, we first examined the metabolic and cardiovascular responses to rapid changes in T_a to demonstrate the sensitivity of the mouse cardiovascular system to T_a. We then determined HR in 1) mice deficient in cardiac sympathetic tone ("β-less" mice), 2) mice deficient in cardiac vagal tone [muscarinic M₂ receptor (M₂R^–/–) mice], and 3) littermate controls. At a T_a of 30°C, the HR of β-less mice was identical to that of wild-type mice (351 ± 11 and 363 ± 10 beats/min, respectively). However, the HR of M₂R^–/– mice was significantly greater (416 ± 7 beats/min), demonstrating that vagal tone predominates over HR control at this T_a. When these mice were calorically restricted to 70% of normal intake, HR fell equally in wild-type, β-less, and M₂R^–/– mice (HR = 73 ± 9, 76 ± 3, and 73 ± 7 beats/min, respectively), suggesting that the fall in intrinsic HR governs bradycardia of calorically restricted mice. Only when the T_a was relatively cool, at 23°C, did β-less mice exhibit a HR (442 ± 14 beats/min) that was different from that of littermate controls (604 ± 10 beats/min) and M₂R^–/– mice (602 ± 5 beats/min). These experiments conclusively demonstrate that in the absence of cold stress, regulation of vagal tone and modulation of intrinsic rate are important determinants of HR control in the mouse.

telemetry; intrinsic heart rate; cold stress; caloric restriction; sympathetic nervous system