Cytosolic calcium pre-elevation amplifies agonist-induced calcium release in human leukaemic HL-60 cells

Histamine, ATP, and two microsomal Ca(2+)-pump inhibitors, thapsigargin (TG) and cyclopiazonic acid (CPA), were able to release intracellular Ca2+ in human leukaemic HL-60 cells. The relationships between the agonist-, TG- and CPA-sensitive Ca2+ pools were investigated with optimal concentrations of these agents in Ca(2+)-free medium. CPA failed to release Ca2+ after the Ca2+ stores of the cells had been discharged by TG, and vice versa, suggesting that the TG- and CPA-sensitive pools exactly overlap. Using this protocol, it was further demonstrated that (a) histamine and ATP utilized the same agonist-sensitive pool, and (b) the CPA- or TG-sensitive pool was much larger than, and encompassed, the agonist-sensitive pool. Although optimal (30 microM) CPA treatment for 5 min totally emptied the agonist-sensitive pool, a brief exposure (1.5 min) to a sub-optimal concentration (3 microM) of CPA, which only slightly raised cytosolic free Ca2+ concentration ([Ca2+]i), substantially enhanced subsequent agonist-induced Ca2+ release. Brief pretreatments with sub-optimal concentrations of TG or ionomycin, which caused moderate [Ca2+]i elevation, also caused such enhancement. However, sub-optimal CPA pretreatment had no prominent effect on Ca2+ release, which was InsP3-independent: it did not enhance TG-induced Ca2+ release, and only relatively weakly augmented ionomycin-induced Ca2+ release. Our results represent a novel observation showing that low concentrations of CPA, TG and ionomycin can potentiate subsequent agonist-induced Ca2+ release, and suggest that a 'priming' moderate [Ca2+]i elevation can amplify subsequent InsP3-dependent Ca2+ release in HL-60 cells.