The effect of C-terminal truncation of the recombinant δ-opioid receptor on Ca(i)²⁺ signaling

We have previously shown a stimulatory coupling of the recombinant δ-opioid receptor to phospholipase C leading to production of inositol (1,4,5) triphosphate [Ins(1,4,5)P₃] that is affected by truncation of the C-terminus of the receptor. Using a C-terminal mutant of the δ-opioid receptor lacking the final 37 amino acids (CHOδ37), we examined its coupling to intracellular calcium ion concentration ([Ca²⁺](i)) compared to the full length wild type receptor (CHOδWT) in transfected Chinese hamster ovary (CHO) cells. D-[Pen^2,5]enkephalin (DPDPE) mediated increases in [Ca²⁺](i) were measured fluorimetrically in fura-2 loaded whole cell suspensions. DPDPE produced time- and concentration-dependent increases in [Ca²⁺](i) in CHOδWT and CHOδ37. In both cell types the DPDPE simulated increase in [Ca²⁺](i) was naloxone reversible and pertussis toxin and thapsigargin sensitive. Removal of the C-terminus resulted in a rightward shift of the Ca²⁺ release concentration-response curve [pEC₅₀=8.43±0.13 and 6.08±0.25 for CHOδWT and CHOδ37, respectively]. These data indicate that the C-terminus of the recombinant δ-opioid receptor is important in [Ca²⁺](i) coupling and may be attributed to the effect of C-terminus truncation on phospholipase C coupling reported previously. Copyright (C) 1999 Elsevier Science B.V.