RT Journal Article SR Electronic T1 Differential Effects of μ-Opioid Receptor Ligands on Ca2+ Signaling JF Journal of Pharmacology and Experimental Therapeutics JO J Pharmacol Exp Ther FD American Society for Pharmacology and Experimental Therapeutics SP 1002 OP 1012 DO 10.1124/jpet.302.3.1002 VO 302 IS 3 A1 J. Mark Quillan A1 Kurt W. Carlson A1 Chunyan Song A1 Danxin Wang A1 Wolfgang Sadée YR 2002 UL http://jpet.aspetjournals.org/content/302/3/1002.abstract AB Activation of μ-opioid receptors (MORs) transfected into human embryonic kidney 293 cells, caused a multiphasic increase in cytosolic free Ca2+ levels (Ca2+i). The first Ca2+i maximum (peak 1) between 5 and 7 s depended on the presence of extracellular Ca2+ (Ca2+e). The second phase peaking at ∼15 s (peak 2) was independent of Ca2+e and thus represents Ca2+ release from intracellular stores. A decrease in temperature from 37 to 25°C also caused reduction of peak 1 but not peak 2, suggesting that the two responses arise from mechanistically distinct pathways. A delayed Ca2+e-dependent third response phase is thought to represent capacitative Ca2+e influx evoked after release of Ca2+ from internal stores. Agonists and antagonists of two major classes of opioid ligands, oxymorphinans (morphine and naloxone) and oripavines (etorphine and diprenorphine), had differential effects on Ca2+ currents. Although morphine activated both phases with equal potency, etorphine was 20-fold less potent at stimulating peak 1 over peak 2. Similarly, the antagonists, naloxone and diprenorphine, blocked the Ca2+ response to each agonist with greatly varying potencies. Specifically, concomitant injection of diprenorphine failed to affect peak 1 (thought to represent rapid Ca2+e influx) stimulated by morphine while fully blocking peak 2 (intracellular Ca2+ release). However, diprenorphine potently inhibited peak 1 as well when added to the cells before morphine, indicating limited or slow access of diprenorphine to these morphine binding sites. The existence of multiple, functionally distinct binding site conformations could account for these findings. In conclusion, different opioid ligands can differentially affect Ca2+ response patterns resulting from MOR activation. The American Society for Pharmacology and Experimental Therapeutics