Acidosis antagonizes intracellular calcium response to κ-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.