Coronary chemoreflex evoked by intrapericardial nicotine has a somatic component.

Stimulation of vagally innervated cardiac or pulmonary receptors reflexly evokes depressor responses called the coronary chemoreflex and pulmonary depressor reflex, respectively. The efferent arm of the pulmonary depressor reflex contains a somatic component wherein monosynaptic and polysynaptic muscle reflexes are attenuated. The purpose of the present investigation was to determine whether the efferent arm of the coronary chemoreflex also attenuates the monosynaptic knee-jerk reflex. Cardiac receptors were stimulated by injection of nicotine and bradykinin into the pericardial sac of 15 chloralose-anesthetized (50 mg/kg) cats. The knee-jerk reflex was elicited by tapping the patellar tendon intermittently. Intrapericardial nicotine attenuated the knee-jerk reflex by -26.2 +/- 6.5%. The attenuation was mediated by the vagus nerves, because vagotomy abolished the nicotine-induced attenuation. Stimulation of cardiac receptors evoked the attenuation, because blockade using intrapericardial procaine abolished the nicotine-induced attenuation. On the other hand, intrapericardial bradykinin augmented the knee-jerk reflex by 17.5 +/- 3.3%. The effect was not mediated by the vagus nerves; vagotomy did not abolish the bradykinin-induced augmentation. Changes in the knee-jerk reflex were not correlated with changes in blood pressure or heart rate evoked by intrapericardial nicotine or bradykinin. These findings demonstrate that reflex somatomotor responses accompany the reflex autonomic responses elicited by cardiac receptors. The somatic component of the vagally mediated coronary chemoreflex could contribute to the fainting reaction associated with exertional syncope of aortic stenosis and vasovagal syncope.