Passive Hindlimb Cycling Enhances Tolerance of Cardiac Electrical Conduction in Rats with Spinal Cord Injuries.

High-level spinal cord injury (SCI) often disrupts supraspinal control of sympathetic input to the heart. The resulting imbalance in the autonomic nervous system increases the risk of developing cardiac arrhythmias. It was previously demonstrated that passive hindlimb cycling (PHLC) effectively maintains or improves bodily function including cardiovascular performance following SCI. However, it remains unclear whether the exercise can affect cardiac electrical disorders. To address this specific question, we complemented a complete SCI at a high-thoracic level in rats and then performed PHLC for 5 or 10 weeks. Naive rats or those receiving injury alone served as controls. Subsequently, a telemetric transmitter was implanted to record blood pressure and electrocardiogram. In 24-h resting recordings, cycling training did not influence SCI-induced hypotension but significantly reduced the events of spontaneous autonomic dysreflexia. When colorectal distension was employed to artificially trigger autonomic dysreflexia, a fewer number of severe arrhythmias (e.g., atrioventricular block, premature ventricular contraction single, and sinus pause) were found in animals with 10-week PHLC compared with injury controls. As a stress test, a series of increasing concentrations of dobutamine was administered to stimulate cardiac sympathetic activity. Consequently, various types of arrhythmias occurred in animals with SCI alone, whereas very few were detected in animals obtaining exercise training for 10 weeks. Furthermore, pharmacological intervention disclosed that exercise appeared to reduce unopposed parasympathetic tone that arose post to injury. Thus, the results suggest that activity-based training for the long term improves autonomic balance to enhance tolerance of cardiac electrical conduction following SCI.