Bimodal δ-ipioid receptors regulate vagal bradycardia in canine sinoatrial node.

Methionine-enkephalin-arginine-phenylalanine (MEAP) introduced into the interstitium of the canine sinoatrial (SA) node by microdialysis interrupts vagal bradycardia. In contrast, raising endogenous MEAP by occluding the SA node artery improves vagal bradycardia. Both are blocked by the same δ-selective antagonist, naltrindole. We tested the hypothesis that vagal responses to intranodal enkephalin are bimodal and that the polarity of the response is both doseand opioid receptor subtype dependent. Ultralow doses of MEAP were introduced into the canine SA node by microdialysis. Heart rate frequency responses were constructed by stimulating the right vagus nerve at 1, 2, and 3 Hz. Ultralow MEAP infusions produced a 50-100% increase in bradycardia during vagal stimulation. Maximal improvement was observed at a dose rate of 500 fmol/min with an EDso near 50 fmol/min. Vagal improvement was returned to control when MEAP was combined with the δ-antagonist naltrindole. The dose of naltrindole (500 fmol/min) was previously determined as ineffective vs. the vagolytic effect of higher dose MEAP. When MEAP was later reintroduced in the same animals at nanomoles per minute, a clear vagolytic response was observed. The δ[sub 1]-selective antagonist 7-benzylidenenaltrexone (BNTX) reversed the vagal improvement with an EDso near 1 × 10[sup -21] mol/min, whereas the δ[sub 2]-antagonist naltriben had no effect through 10[sup -9] mol/min. Finally, the improved vagal bradycardia previously associated with nodal artery occlusion and endogenous MEAP was blocked by the selective δ[sub 1]-antagonist BNTX. These data support the hypothesis that opioid effects within the SA node are bimodal in character, that low doses are vagotonic, acting on δ[sub 1]-receptors, and that higher doses are vagolytic, acting on δ°2-receptors. [ABSTRACT FROM AUTHOR]