Fig. 4. Opioid receptor antagonists and morphine effects on L-type Ca2+current (ICa.L). ICa.Lwas increased by the administration of 0.1 μm morphine, and the increase was partially reversed upon drug washout for 10 min. Pretreatment (5 min) with nor-binaltorphimine (nor-BNI, 5 μm, n = 7) and naltrindole (NTD, 5 μm, n = 8), but not CTOP (5 μm, n = 6), prevented morphine-induced increase of ICa.L. Data are expressed as mean ± sd. * P < 0.05 versus control.