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 microM and 0.8 nM, respectively. After intracellular Ca2+ release by 30 nM TSG, there was no more discharge of Ca2+ by TET (100 microM), and vice versa. 2. Pretreatments with 100 nM rauwolscine (alpha 2-adrenoceptor antagonist), 100 nM prazosin (alpha 1-adrenoceptor 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 microM TET-induced intracellular Ca2+ release. 3. After intracellular Ca2+ release by 30 nM TSG in Ca(2+)-free medium, readmission of Ca2+ caused a substantial and sustained extracellular Ca2+ entry. The latter was almost completely inhibited by 100 microM TET (IC50 of 20 microM) added just before Ca2+ readmission. In Ca(2+)-containing medium, 30 nM TSG caused a sustained phase of cytosolic Ca2+ elevation, which could be abolished by 100 microM TET. TET was also demonstrated to retard basal entry of extracellular Mn2+ and completely inhibit TSG-stimulated extracellular Mn2+ entry. 4. TSG-induced extracellular Ca2+ entry was insensitive to the L-type Ca2+ channel blocker, nifedipine (1 microM), but was completely inhibited by the non-selective Ca2+ channel blocker La3+ (300 microM). Depolarization with 100 mM KCl did not raise the cytosolic Ca2+ level. 5. 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 ox-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.
