Heterologous Desensitization of Opioid-stimulated Ca2+ Increase by Bradykinin or ATP in NG108-15 Cells

Abstract

Leucine-enkephalin (Leu-EK) dose-dependently elicited an increase in cytosolic Ca²⁺ concentration ([Ca²⁺]_i) with an EC₅₀ of 1.2 μM via the phosphoinositide cascade in NG108-15 cells. Chronic treatment of cells with [D-Ala²,D-Leu⁵]enkephalin caused time-dependent homologous desensitization. In the presence of extracellular Ca²⁺, ATP as well as bradykinin stimulated significantly higher increases in inositol 1,4,5-trisphosphate (IP³) generation than did Leu-EK; however, the magnitude of intracellular Ca²⁺ pools increased after ATP stimulation, whereas bradykinin depleted intracellular pools. Hence, cells lost their [Ca²⁺]_i response to Leu-EK if bradykinin was first added to induce a [Ca²⁺]_i increase, whereas the response was unchanged if Leu-EK was added after addition of ATP. When Leu-EK was added simultaneously with bradykinin or ATP, an additive response was observed in IP³ generation; however, the rise in [Ca²⁺]_i reached the same level as that induced by bradykinin or ATP alone. In the absence of extracellular Ca²⁺ in which the replenishment of intracellular pools was not possible, ATP displayed an inhibitory effect similar to that of bradykinin on the Leu-EK-induced [Ca²⁺]_i increase. Prior treatment of cells with Leu-EK slightly heterologously desensitized the action of bradykinin, but had no effect on the ATP response. Our results suggest that a shared intracellular Ca²⁺ pool is sensitive to the opioid, bradykinin and P²-purinoceptor agonists; however, a defined pool of phosphatidylinositol 4,5-bisphosphate or a specific phospholipase C is responsible for each receptor.