Abstract 316: The Obligatory Role of Intrinsic Cardiac Adrenergic Cells in δ-Opioid Receptor-Initiated Protection against Myocardial Ischemia
Background: Both catecholamines and δ-opioid receptor (DOR) agonists exert infarct-size-limiting effect against ischemia via the same final signaling pathways. We hypothesized that DOR-initiated cardioprotection is dependent on adrenergic activation via intrinsic cardiac adrenergic (ICA) cells, a newly identified cardiac neuroendocrine system.
Methods and Results: Using immunofluorescent double labeling coupled with in situ hybridization, we have detected tyrosine hydroxylase (TH) mRNA, the gene for rate-limiting catecholamine-forming enzyme in human ventricular ICA cells. We have colocalized the immunoreactivity of TH and DOR to the ICA cells in human and rat hearts. No TH mRNA or DOR immunoreactivity was identified in ventricular myocytes or sympathetic nerve endings. The physiological significance of DOR expression was examined by determining changes of cytosolic [Ca2+]i transients in fura-2-loaded isolated rat ICA cells using a fluorescence spectrophotometer. Application of a DOR agonist DPDPE to ICA cells increased [Ca2+]i transients in a concentration-dependent manner. DPDPE (100 nM) failed to enhance the [Ca2+]i transients in the presence of the DOR antagonist naltrindole (1 μM), protein kinase A inhibitor Rp-cAMP or Ca2+ channel blocker nifedipine (1 μM). HPLC detection demonstrated a 2.4-fold (n=4) increase in epinephrine release from ICA cells following application of DPDPE. Significance of epinephrine derived from ICA cells in DOR-initiated cardioprotection was demonstrated in a rat myocardial infarct model (30 min ischemia and 4h reperfusion, 6 rats in each group). The infract size was standardized as % infarct zone/area at risk of LV. Infusion of DPDPE (100 μg/kg) 30 min prior to the coronary artery occlusion reduced infarct size by 53% (p<0.01) compared to the control. However, treatment of rats with labetalol (2 mg/kg) before the DPDPE infusion nearly abolished DPDPE’s infarct-size-limiting effect.
Conclusions: DOR stimulation of ICA cells increases Ca2+ influx resulting in enhanced epinephrine liberation that is essential for DOR-initiated infarct size reduction. Our study demonstrates a novel DOR signaling and its preconditioning mechanism that is dependent on enhanced neuroendocrine function of ICA cells.