Activation of cardiac afferents by arachidonic acid: relative contributions of metabolic pathways

Abstract

Arachidonic acid (AA) is metabolized via cyclooxygenase (COX), lipoxygenase (LOX), and cytochrome P-450 (CP450) pathways to a variety of bioactive products. The sensitivity of cardiac afferent endings to AA and its metabolites, especially those derived from LOX and CP450 pathways, is currently unclear. We examined AA-induced activation of cardiac vagal chemosensitive afferents in non- and postischemic hearts in rats and evaluated the relative contributions of the three metabolic pathways to the effects. Epicardial application of AA activated the cardiac afferents dose dependently in both nonischemic and postischemic hearts, with afferent responses greater in the latter condition. In nonischemic hearts, the afferent response to AA was abolished only after simultaneous administration of indomethacin and 17-octadecynoic acid (COX and CP450 inhibitors, respectively). Nordihydroguaiaretic acid (a LOX inhibitor) had no effect on the afferent response to AA. In postischemic hearts, abolition of the afferent response to AA required simultaneous blockade of all three pathways. None of the AA metabolic inhibitors affected resting activity of cardiac afferents in nonischemic hearts, but each suppressed afferent activity during ischemia-reperfusion. Most COX metabolites, CP450 metabolites, and 5-LOX metabolites tested were capable of activating cardiac afferents. The 12-LOX metabolites and 15-LOX metabolites had no effect on afferent activity. These data indicate that in the nonischemic heart, basal AA metabolism does not contribute to resting afferent activity, but AA is capable of activating cardiac afferents via COX and CP450 but not LOX pathways. During ischemia-reperfusion, all three metabolic pathways contribute to activation of cardiac vagal afferents with an enhanced responsiveness to AA. Our results suggest that induction of the 5-LOX pathway contributes to the enhanced sensitivity of cardiac vagal afferents to AA in the ischemic condition.