THE PATHWAY FOR REFILLING INTRACELLULAR CA2-CELLS( STORES PASSES THROUGH THE CYTOSOL IN HUMAN LEUKEMIA)

The pathway for refilling the intracellular Ca2+ stores of HL60 and U9 37 human leukaemia cells loaded with fura-2 has been investigated. On addition of external Ca2+ to cells with empty stores there was an incr ease in the cytosolic Ca2+ concentration ([Ca2+]i) which preceded the refilling of the stores. The increase in [Ca2+]i was faster than the r efilling, by 3- to 15-fold, depending on the cell type. In measurement s in single HL60 cells we found that the refilling of the stores corre lated with the extent of the [Ca2+]i increase on addition of external Ca2+. The cells showing no [Ca2+]i increase were unable to refill thei r stores. The addition of Ni2+ to the extracellular medium prevented b oth the [Ca2+]i increase and the refilling of the stores. These result s indicate that the limiting step for store refilling is the entry of Ca2+ from the extracellular medium to the cytosol. Hence, we conclude that extracellular Ca2+ cannot gain access directly to the intracellul ar Ca2+ stores in these cells, but must first enter the cytosol and be taken up from there into the stores.