Thoracic spinal neuron responses to repeated myocardial ischemia and epicardial bradykinin

Bradykinin has been strongly implicated as a mediator of cardiac nociception. During coronary artery occlusion, the content of bradykinin in coronary sinus blood increases. In non-cardiac tissues nociception to bradykinin exhibits tachyphylaxis, however, this phenomenon has not been rigorously studied in the heart. This raises the question that repeated coronary occlusions may also result in tachyphylaxis, thereby reducing cardiac sensation on subsequent ischemic stimulation. We therefore examined the hypothesis that repetitive episodes of myocardial ischemia and of epicardial application of bradykinin demonstrate tachyphylaxis. Mongrel cats were anesthetized with α-chloralose and heart rate, arterial pressure, and thoracic spinal neuron firing rate were recorded during 60 s of anterior descending coronary occlusion or local epicardial application of bradykinin (10 μM). Neurons were identified by cutaneous receptive fields in the left shoulder area. Sixty-one of 93 neurons tested responded with an increase in firing rate to coronary artery occlusion only ( n=24), bradykinin only ( n=19) or to both ( n=18). On repetitive coronary occlusion, 14 of 25 neurons demonstrated tachyphylaxis compared to 12 of 15 tested with bradykinin ( p