Dual effects of tetrandrine on cytosolic calcium in human leukaemic HL‐60 cells: intracellular calcium release and calcium entry blockade

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Abstract
Tetrandrine (TET, a Ca2+ antagonist of Chinese herbal origin) and thapsigargin (TSG, an endoplasmic reticulum Ca2+ pump inhibitor) concentration‐dependently mobilized Ca2+ from intracellular stores of HL‐60 cells, with EC50 values of 20 μm and 0.8 nm, respectively. After intracellular Ca2+ release by 30 nm TSG, there was no more discharge of Ca2+ by TET (100 μm), and vice versa. Pretreatments with 100 nm rauwolscine (α2‐adrenoceptor antagonist), 100 nm prazosin (α1‐adreno‐ceptor antagonist), 10 nm phorbol myristate acetate (PMA, a protein kinase C activator) or 100 nm staurosporine (a protein kinase C inhibitor) had no effect on 100 μm TET‐induced intracellular Ca2+ release. After intracellular Ca2+ release by 30 nm TSG in Ca2+‐free medium, readmission of Ca2+ caused a substantial and sustained extracellular Ca2+ entry. The latter was almost completely inhibited by 100 μm TET (IC50 of 20 μm) added just before Ca2+ readmission. In Ca2+‐containing medium, 30 nm TSG caused a sustained phase of cytosolic Ca2+ elevation, which could be abolished by 100 μm TET. TET was also demonstrated to retard basal entry of extracellular Mn2+ and completely inhibit TSG‐stimulated extracellular Mn2+ entry. TSG‐induced extracellular Ca2+ entry was insensitive to the L‐type Ca2+ channel blocker, nifedipine (1 μm), but was completely inhibited by the non‐selective Ca2+ channel blocker La3+ (300 μm). Depolarization with 100 mm KCl did not raise the cytosolic Ca2+ level. These data suggest that (a) TET and TSG mobilized the same Ca2+ pool and TET‐induced intracellular Ca2+ release was independent of protein kinase C activity and a‐adrenoceptor activation, and (b) TET blocked the voltage‐insensitive Ca2+ entry pathway activated by TSG. These dual effects on HL‐60 cells were also observed with hernandezine (HER), a TET‐like compound and in another cell type, murine B lymphoma M12.4 cells.