ATTENUATION OF ISCHEMIA-INDUCED ACTIVATION OF CARDIAC SYMPATHETIC AFFERENTS FOLLOWING BRIEF MYOCARDIAL-ISCHEMIA IN CATS

Objective: We have previously shown that brief myocardial ischemia imp airs neural conduction in cardiac sympathetic efferent fibers. However , attenuation of the activity of afferent sympathetic nerves, which ma y contribute to impaired ischemic nociception and reflex hemodynamic r esponses, is not well understood. Therefore, we studied the electrophy siological effects of brief myocardial ischemia on the mechano-, chemo - and ischemia-sensitive properties of cardiac sympathetic afferent fi bers. Methods: Single unit activity of sympathetic afferent A delta an d C fibers innervating the left ventricle (LV) was recorded from the t horacic chain or rami communicantes in 16 anesthetized cats. We tested the response of impulse activity to (1) mechanical LV probing, (2) ep icardial application of bradykinin (10 mu g), H2O2 (1.5%) or adenosine (500 mu g), and (3) 1 min of coronary occlusion. Repeat tests were pe rformed in 11 of 16 fibers after 15 min of coronary occlusion and 15 m in of reperfusion. Control responses were measured in five fibers befo re and after a 30-min interval without ischemia. Results: Afferent fib ers responded with increased activity to LV probing (16/16 fibers), br adykinin (13/16 fibers), H2O2 (7/11 fibers), adenosine (1/11 fibers), and to a 60 s period of coronary occlusion (11/16 fibers). The unit im pulse activity to 1 min of coronary occlusion was markedly attenuated after 15 min of myocardial ischemia and 15 min of reperfusion (P < 0.0 5). This attenuation was associated with reduced sensitivity to mechan ical and chemical stimuli, while in separate time-control studies (n = 5) no attenuation was observed in absence of ischemia. Conclusion: A brief period of myocardial ischemia is capable of attenuating mechano- , chemo- and ischemia-sensitive activity of cardiac afferent sympathet ic nerves. This may have important implications for the mechanism of s ilent myocardial ischemia. (C) 1998 Elsevier Science B.V. All rights r eserved.