Ventricular reflex interactions during transmural myocardial ischemia

The role of left ventricular receptors with sympathetic afferent fibers in the reflex response to myocardial ischemia is controversial, particularly in the canine model. Previous experiments have shown that reflex excitatory responses mediated by left ventricular sympathetic afferents can be detected in sinoaortic denervated and vagotomized dogs during transmural myocardial ischemia. The purpose of these experiments was to determine if reflex excitatory responses occur in dogs with intact left ventricular vagal afferents. Experiments were performed in 27 chloralose-anesthetized dogs with sinoaortic denervation. Responses of efferent renal sympathetic nerve activity, arterial, and left atrial pressures to transmural and non-transmural inferoposterior myocardial ischemia were measured before and after interruption of left ventricular sympathetic afferents by stellectomy. The adequacy of sympathetic deafferentation was assessed by measurement of renal nerve responses to epicardial bradykinin. Prior to stellectomy, excitatory responses were observed in 10 animals and inhibitory responses in 9 animals. The remaining animals had no responses and were excluded from analysis. In the excitatory group, reflex increases in renal nerve activity during both transmural and non-transmural inferoposterior ischemia were abolished by stellectomy and not replaced by inhibitory responses. In the inhibitory group, non-transmural inferoposterior ischemia elicited greater reflex decreases in renal nerve activity when left ventricular sympathetic afferents were intact. After stellectomy, transmural ischemia elicited greater reflex inhibition of renal nerve activity. Renal nerve responses to epicardial bradykinin were abolished by stellectomy. These results indicate that reflex excitatory responses mediated by left ventricular receptors with sympathetic afferent fibers can be elicited in dogs with intact vagal afferents. These excitatory responses are most apparent during transmural myocardial ischemia. In dogs with inhibitory responses to coronary occlusion, activation of sympathetic afferents during transmural ischemia appears to attenuate reflex inhibitory responses mediated by left ventricular vagal afferents.