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

Martyn P. Mahaut-Smith, Jamila F. Hussain and Michael J. Mason

The effect of membrane potential on [Ca²⁺]_i in rat megakaryocytes was studied using simultaneous whole-cell patch clamp and fura-2 fluorescence recordings.

Depolarization from -75 to 0 mV had no effect on [Ca²⁺]_i in unstimulated cells, but evoked one or more spikes of Ca²⁺ increase (peak increase: 714 ± 95 nM) during activation of metabotropic purinoceptors by 1 µM ADP.

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.

Intracellular dialysis with heparin blocked the depolarization-evoked Ca²⁺ increase, indicating a role for functional IP₃ receptors.

Under current clamp, ADP caused the membrane potential to fluctuate between -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 exposure to ADP.

We conclude that during stimulation of metabotropic purinoceptors, membrane depolarization over the physiological range can stimulate Ca²⁺ release from intracellular stores in the rat megakaryocyte, a non-excitable cell type. This may represent an important mechanism by which electrogenic influences can control patterns of [Ca²⁺]_i increase.

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