title: Acidosis antagonizes intracellular calcium response to kappa-opioid receptor stimulation in the rat heart.
abstract: To study the effects of kappa-opioid receptor stimulation on intracellular Ca2+ concentration ([Ca2+]i) homeostasis during extracellular acidosis, we determined the effects of kappa-opioid receptor stimulation on [Ca2+]i responses during extracellular acidosis in isolated single rat ventricular myocytes, by a spectrofluorometric method. U-50488H (10-30 microM), a selective kappa-opioid receptor agonist, dose dependently decreased the electrically induced [Ca2+]i transient, which results from the influx of Ca2+ and the subsequent mobilization of Ca2+ from the sarcoplasmic reticulum (SR). U-50488H (30 microM) also increased the resting [Ca2+]i and inhibited the [Ca2+]i transient induced by caffeine, which mobilizes Ca2+ from the SR, indicating that the effects of the kappa-opioid receptor agonist involved mobilization of Ca2+ from its intracellular pool into the cytoplasm. The Ca2+ responses to 30 microM U-50488H were abolished by 5 microM nor-binaltorphimine, a selective kappa-opioid receptor antagonist, indicating that the event was mediated by the kappa-opioid receptor. The effects of the agonist on [Ca2+]i and the electrically induced [Ca2+]i transient were significantly attenuated when the extracellular pH (pHe) was lowered to 6.8, which itself reduced intracellular pH (pHi) and increased [Ca2+]i. The inhibitory effects of U-50488H were restored during extracellular acidosis in the presence of 10 microM ethylisopropyl amiloride, a potent Na+/H+ exchange blocker, or 0.2 mM Ni2+, a putative Na+/Ca2+ exchange blocker. The observations indicate that acidosis may antagonize the effects of kappa-opioid receptor stimulation via Na+/H+ and Na+/Ca2+ exchanges. When glucose at 50 mM, known to activate the Na+/H+ exchange, was added, both the resting [Ca2+]i and pHi increased. Interestingly, the effects of U-50488H on [Ca2+]i and the electrically induced [Ca2+]i transient during superfusion with glucose were significantly attenuated; this mimicked the responses during extracellular acidosis. When a high-Ca2+ (3 mM) solution was superfused, the resting [Ca2+]i increased; the increase was abolished by 0.2 mM Ni2+, but the pHi remained unchanged. Like the responses to superfusion with high-concentration glucose and extracellular acidosis, the responses of the [Ca2+]i and electrically induced [Ca2+]i transients to 30 microM U-50488H were also significantly attenuated. Results from the present study demonstrated for the first time that extracellular acidosis antagonizes the effects of kappa-opioid receptor stimulation on the mobilization of Ca2+ from SR. Activation of both Na+/H+ and Na+/Ca2+ exchanges, leading to an elevation of [Ca2+]i, may be responsible for the antagonistic action of extracellular acidosis against kappa-opioid receptor stimulation.
authors.affiliation: Department of Physiology, and Institute of Cardiovascular Sciences and Medicine, Faculty of Medicine, The University of Hong Kong, Hong Kong, China.
authors.forename: J M
authors.initials: JM
authors.lastname: Pei
authors.forename: X C
authors.initials: XC
authors.lastname: Yu
authors.forename: J S
authors.initials: JS
authors.lastname: Bian
authors.forename: T M
authors.initials: TM
authors.lastname: Wong
mesh_terms: D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer / Q000494:pharmacology; D000138:Acidosis / Q000378:metabolism*; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D004558:Electric Stimulation; D005110:Extracellular Space / Q000187:drug effects / Q000378:metabolism; D005947:Glucose / Q000494:pharmacology; D006352:Heart Ventricles; D006863:Hydrogen-Ion Concentration; D007425:Intracellular Membranes / Q000378:metabolism*; D008297:Male; D009206:Myocardium / Q000166:cytology / Q000378:metabolism*; D009994:Osmolar Concentration; D051381:Rats; D017207:Rats, Sprague-Dawley; D017473:Receptors, Opioid, kappa / Q000819:agonists / Q000502:physiology*
chemical_list: D017473:Receptors, Opioid, kappa; D019900:3,4-Dichloro-N-methyl-N-(2-(1-pyrrolidinyl)-cyclohexyl)-benzeneacetamide, (trans)-Isomer; D005947:Glucose; D002118:Calcium