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Deo SH, Johnson-Davis S, Barlow MA, Yoshishige D, Caffrey JL. Repeated δ1-opioid receptor stimulation reduces δ2-opioid receptor responses in the SA node. Am J Physiol Heart Circ Physiol 2006; 291:H2246-54. [PMID: 16782849 DOI: 10.1152/ajpheart.00122.2006] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [What about the content of this article? (0)] [Affiliation(s)] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Ultra-low-dose methionine-enkephalin-arginine-phenylalanine improves vagal transmission (vagotonic) and decreases heart rate via δ1-opioid receptors within the sinoatrial (SA) node. Higher doses activate δ2-opioid receptors, interrupt vagal transmission (vagolytic), and reduce the bradycardia. Preconditioning-like occlusion of the nodal artery produced a vagotonic response that was reversed by the δ1-antagonist 7-benzylidenaltrexone (BNTX). The following study tested the hypothesis that extended δ1-opioid receptor stimulation reduces subsequent δ2-receptor responses. The δ2-agonist deltorphin II was introduced in the SA node by microdialysis to evaluate δ2 responses before and after infusion of the δ1-agonist TAN-67. TAN-67 reduced the vagolytic effect of deltorphin by two-thirds. When the δ1-antagonist BNTX was combined with TAN-67, the deltorphin response was preserved, suggesting that attrition of the prior response was mediated by δ1 activity. When TAN-67 was omitted in time control studies, some loss of δ2 responses was apparent in the absence of the δ1 treatment. This loss was also eliminated by BNTX, suggesting that the attenuation of the response after deltorphin alone was also the result of δ1 activity. Additional studies tested TAN-67 alone in the absence of prior deltorphin. When time controls were conducted without the initial deltorphin treatment, a robust vagolytic response was observed. When TAN-67 preceded the delayed deltorphin, the vagolytic response was eroded, indicating an independent effect of TAN-67. BNTX infused afterward was unable to restore the δ2 response. These data support the conclusion that the loss of the δ2 response resulted from reduced δ2 activity mediated by continued δ1-receptor stimulation and not the arithmetic consequence of increased competition from that same δ1 receptor.
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MESH Headings
- Analgesics/pharmacology
- Analgesics, Opioid/pharmacology
- Animals
- Benzylidene Compounds/pharmacology
- Bradycardia/drug therapy
- Bradycardia/physiopathology
- Dogs
- Dose-Response Relationship, Drug
- Enkephalin, Methionine/analogs & derivatives
- Enkephalin, Methionine/pharmacology
- Female
- Male
- Microdialysis
- Naltrexone/analogs & derivatives
- Naltrexone/pharmacology
- Narcotic Antagonists/pharmacology
- Oligopeptides/pharmacology
- Quinolines/pharmacology
- Receptors, Opioid, delta/classification
- Receptors, Opioid, delta/drug effects
- Receptors, Opioid, delta/metabolism
- Sinoatrial Node/drug effects
- Sinoatrial Node/innervation
- Sinoatrial Node/physiology
- Stimulation, Chemical
- Vagus Nerve/drug effects
- Vagus Nerve/physiology
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Affiliation(s)
- S H Deo
- Dept. of Integrative Physiology, Univ. of North Texas Health Science Center, 3500 Camp Bowie Blvd., Fort Worth TX 76107, USA.
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