Intrapericardial algogenic chemicals evoke cardiac-somatic motor reflexes in rats

Many patients suffer from secondary muscle hyperalgesia after experiencing angina pectoris. Electrophysiological studies have also demonstrated electromyography (EMG) activities evoked during brief occlusion of the coronary artery in cats. Our objectives are: (1) to develop an animal model to study muscle changes resulting from cardiac dysfunction and (2) to investigate possible links between muscle hyperalgesia and EMG activities observed with ischemic heart diseases. Intrapericardial infusion of algogenic chemicals produced EMG activities in paraspinal muscles. Among these paraspinal muscles, the spinotrapezius produced the most consistent responses (38/53 animals). The evoked responses, in most cases, persisted longer than 90 s (29/53 muscles). The EMG activities consisted of three patterns of motor-unit discharge, namely single-unit, complex, and ventilation-related. The duration of the patterns, as well as the shape and amplitude of the motor-unit waveforms, are similar to the characteristics of muscle spasms. Since sensitization of muscle afferent fibers by noxious stimuli could produce angina-like pain, the spasm-like EMG activities might sensitize muscle afferent fibers that contribute to acute angina pain and secondary muscle hyperalgesia of cardiac patients.