CA2+ SPIKE INITIATION FROM SENSITIZED INOSITOL 1,4,5-TRISPHOSPHATE-SENSITIVE CA2+ STORES IN MEGAKARYOCYTES

Ca2+-mediated Ca2+ spikes were analysed in fura-2-loaded megakaryocytes. Direct Ca2+ loading using whole-cell dialysis induced an all-or-none Ca2+ spike on top of a tonic increase in cellular Ca2+ concentration ([Ca2+](i)) with a latency of 3-7 s. The latency decreased with increasingly higher concentrations of Ca2+ in the dialysing solution. Spike size and its initiation did not correlate with the tonic level of [Ca2+](i). Thapsigargin completely abolished the Ca2+-induced spike initiation, suggesting that Ca2+ spikes originate from thapsigargin-sensitive Ca2+ pools. An inhibitor of phosphatidylinositide-specific phospholipase C (PLC), 2-nitro-4-carboxyphenyl-N,N-diphenyl-carbamate prolonged the latency without changes of spike size in most cases (6/9 cells), but abolished the spike initiation in the other cells (3/9). The results suggest that an increase in [Ca2+](i) charges up the inositol-1,4,5-trisphosphate-(InsP(3))- and thapsigargin-sensitive Ca2+ pools which progressively sensitize to low or slightly elevated levels of InsP(3) by the action of Ca2+-dependent PLC until a critical Ca2+ content is reached, and then the Ca2+ spike is triggered. Thus, the limiting step of Ca2+ spike triggering is the initial filling process and the level of InsP(3) in megakaryocytes.