MOR1K activation stimulates cAMP and Ca2+. COS1 (A,B) or BE2C (C) cells were transiently transfected with MOR1K, MOR1, or empty vector control expressing constructs. (A) Forskolin (FSK, 10 μM) was used to increase cAMP levels prior to morphine treatment. Following morphine treatment, cells expressing MOR1 exhibited reduced cAMP levels, while those expressing MOR1K did not. In fact, cells expressing MOR1K exhibited a trend towards an increase in cAMP levels. (B) Following morphine treatment, COS1 cells expressing MOR1K exhibited substantial increases in Ca2+ levels, while those expressing MOR1 did not. (C) Stimulation of MOR1K with morphine produced a robust dose-dependent increase in Ca2+ levels in Be2C cells. This increase was significantly different from the moderate increase in morphine-evoked Ca2+ levels observed in Be2C cells transfected with MOR1 or empty vector, which were likely due to the high endogenous expression of MOR1K in Be2C cells. For all panels, both MOR1 and MOR1K, morphine-dependent effects were antagonized in the presence of naloxone (0.1 μM). Data are presented as mean ± S.E.M from at least 6 experiments. $P < 0.05 different from control and *P < 0.05 different from control and MOR1.