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2-opioid receptors in sinoatrial node
Department of Integrative Physiology, University of North Texas Health Science Center, Fort Worth, Texas 76107
Local cardiac opioids appear to be
important in determining the quality of vagal control of heart rate.
Introduction of the endogenous opioid
methionine-enkephalin-arginine-phenylalanine (MEAP) into the
interstitium of the canine sinoatrial node by microdialysis attenuates
vagally mediated bradycardia through a
-opioid receptor mechanism.
The following studies were conducted to test the hypothesis that a
2-opiate receptor subtype mediates the interruption of
vagal transmission. Twenty mongrel dogs were anesthetized and
instrumented with microdialysis probes inserted into the sinoatrial
node. Vagal frequency responses were performed at 1, 2, and 3 Hz during
vehicle infusion and during treatment with the native agonist MEAP, the
1-opioids
2-methyl-4aa-(3-hydroxyphenyl)-1,2,3,4,4a,5,12,12a
-octahydroquinolino[2,3,3- g]isoquinoline
(TAN-67) and [D-pen2,5]-enkephalin (DPDPE),
and the
2 opioid deltorphin II. The vagolytic effects of
intranodal MEAP and deltorphin were then challenged with the
1- and
2-opioid receptor
antagonists 7-benzylidenenaltrexone (BNTX) and naltriben,
respectively. Although the positive control deltorphin II was clearly
vagolytic in each experimental group, TAN-67 and DPDPE were
vagolytically ineffective in the same animals. In contrast, TAN-67
improved vagal bradycardia by 30-35%. Naltriben completely
reversed the vagolytic effects of MEAP and deltorphin. BNTX was
ineffective in this regard but did reverse the vagal improvement
observed with TAN-67. These data support the hypothesis that the
vagolytic effect of the endogenous opioid MEAP was mediated by
2-opioid receptors located in the sinoatrial node. These
data also support the existence of vagotonic
1-opioid
receptors also in the sinoatrial node.
TAN-67; heart rate
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