delta-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. T o examine potential mechanisms that contribute to the specificity by which delta -receptors couple to distinct intracellular effectors, we stably tran sfected rat pituitary GH(3) cells with cDNAs encoding for delta -opioid rec eptors. In cells transfected with a relatively low delta -receptor density of 0.55 pmol/mg of protein (GH(3)DOR), activation of delta -receptors produ ced inhibition of adenylyl cyclase activity but was unable to alter L-type Ca2+ current. In contrast, activation of delta -receptors in a clone that c ontained a higher density of delta- receptors (2.45 pmol/ mg of protein) an d was also coexpressed with mu -opioid receptors (GH(3)MORDOR), resulted in not only the expected inhibition of adenylyl cyclase activity but also pro duced inhibition of L-type Ca2+ current. The purpose of the present study w as to determine whether these observations resulted from differences in del ta -opioid receptor density between clones or interaction between delta- an d mu -opioid receptors to allow the activation of different G proteins and signaling to Ca2+ channels. Using the delta -opioid receptor alkylating age nt SUPERFIT, reduction of available delta -opioid receptors in GH(3)MORDOR cells to a density similar to that of delta -opioid receptors in the GH(3)D OR clone resulted in abolishment of coupling to Ca2+ channels, but not to a denylyl cyclase. Furthermore, although significantly greater amounts of all G proteins were activated by delta -opioid receptors in GH(3)MORDOR cells, delta -opioid receptor activation in GH(3)DOR cells resulted in coupling t o the identical pattern of G proteins seen in GH(3)MORDOR cells. These find ings suggest that different threshold densities of delta -opioid receptors are required to activate critical amounts of G proteins needed to produce c oupling to specific effectors and that delta -opioid receptors couple more efficiently to adenylyl cyclase than to L-type Ca (2+) channels.