Depolarization-evoked Ca2+ release in a non-excitable cell, the rat megakaryocyte

1. The effect of membrane potential on [Ca2+](i) in rat megakaryocytes was studied using simultaneous whole-cell patch clamp and fura-2 fluorescence r ecordings. 2. Depolarization from -75 to 0 mV had no effect on [Ca2+](i) in unstimulat ed cells, but evoked one or more spikes of Ca2+ increase (peak increase: 71 4 +/- 95 nM) during activation of metabotropic purinoceptors by I mu M ADP. 3. The depolarization-evoked Ca2+ increase was present in Ca2+-free medium and also following removal of Na+. Thus depolarization mobilizes Ca2+ from an intracellular store without a requirement for altered Na+-Ca2+ exchange activity. 4. Intracellular dialysis with heparin blocked the depolarization-evoked Ca 2+ increase, indicating a role for functional IP3 receptors. 5. Under current clamp, ADP caused the membrane potential to fluctuate betw een -43 +/- 1 and -76 +/- 1. mV. Under voltage clamp, depolarization from - 75 to -45 mV evoked a transient [Ca2+](i) increase (398 +/- 91 nM) during e xposure to ADP. 6. We conclude that during stimulation of metabotropic purinoceptors, membr ane depolarization over the physiological range can stimulate Ca2+ release from intracellular stores in the rat megakaryocyte, a non-excitable cell ty pe. This may represent an important mechanism by which electrogenic influen ces can control patterns of [Ca2+](i) increase.