δ-Opioid receptors are more efficiently coupled to adenylyl cyclase than to L-type Ca²⁺ channels in transfected rat pituitary cells

Opioid receptors often couple to multiple effectors within the same cell. To examine potential mechanisms that contribute to the specificity by which δ-receptors couple to distinct intracellular effectors, we stably transfected rat pituitary GH₃ cells with cDNAs encoding for δ-opioid receptors. In cells transfected with a relatively low δ-receptor density of 0.55 pmol/mg of protein (GH₃DOR), activation of δ-receptors produced inhibition of adenylyl cyclase activity but was unable to alter L-type Ca²⁺ current. In contrast, activation of δ-receptors in a clone that contained a higher density of δ-receptors (2.45 pmol/mg of protein) and was also coexpressed with μ-opioid receptors (GH₃MORDOR), resulted in not only the expected inhibition of adenylyl cyclase activity but also produced inhibition of L-type Ca²⁺ current. The purpose of the present study was to determine whether these observations resulted from differences in δ-opioid receptor density between clones or interaction between δ- and μ-opioid receptors to allow the activation of different G proteins and signaling to Ca²⁺ channels. Using the δ-opioid receptor alkylating agent SUPERFIT, reduction of available δ-opioid receptors in GH₃MORDOR cells to a density similar to that of δ-opioid receptors in the GH₃DOR clone resulted in abolishment of coupling to Ca²⁺ channels, but not to adenylyl cyclase. Furthermore, although significantly greater amounts of all G proteins were activated by δ-opioid receptors in GH₃MORDOR cells, δ-opioid receptor activation in GH₃DOR cells resulted in coupling to the identical pattern of G proteins seen in GH₃MORDOR cells. These findings suggest that different threshold densities of δ-opioid receptors are required to activate critical amounts of G proteins needed to produce coupling to specific effectors and that δ-opioid receptors couple more efficiently to adenylyl cyclase than to L-type Ca²⁺ channels.