Daily exercise attenuates the sympathetic component of the arterial baroreflex control of heart rate

The influence of daily spontaneous running (DSR) on the sympathetic (SC) and parasympathetic components of the arterial baroreflex control of heart rate (HR) was examined in 16 female Long Evans rats 8 sedentary (SED) and 8 DSR. After 8-9 wk of SED control or DSR, animals were chronically instrumented with arterial and venous catheters. DSR resulted in an increased heart weight-to-body weight ratio (2.71 +/- 0.11 vs. 3.09 +/- 0.09 g/kg) and a resting bradycardia (378 +/- 6 vs. 330 +/- 5 beats/min). Arterial baroreflex function was examined during ramp infusions of phenylephrine and sodium nitroprusside under the following three experimental conditions: 1) control, 2) after beta 1-adrenergic receptor blockade (beta 1-X), and 3) after muscarinic-cholinergic receptor blockade (M-X). Arterial baroreflex function parameters were compared between SED and DSR rats. In the control condition, DSR attenuated the range (182 +/- 15 vs. 124 +/- 18 beats/min), maximum HR (464 +/- 9 vs. 394 +/- 15 beats/min), and maximal gain (Gmax; 5.57 +/- 0.42 vs. 3.2 +/- 0.45 beats.min-1.mmHg-1). Similarly, after M-X, DSR attenuated the range (84 +/- 5 vs. 62 +/- 8 beats/min), maximum HR (449 +/- 11 vs. 412 +/- 11 beats/min), and Gmax (2.73 +/- 0.37 vs. 1.57 +/- 0.32 beats.min-1.mmHg-1). In contrast, after beta 1-X, DSR did not alter the range (61 +/- 13 vs. 70 +/- 7 beats/min), maximum HR (326 +/- 9 vs. 313 +/- 7 beats/min), or Gmax (3.04 +/- 0.54 vs. 3.75 +/- 0.52 beats.min-1.mmHg-1). Results demonstrate that DSR attenuated the arterial baroreflex control of HR by reducing the SC.