A novel role for membrane potential in the modulation of intracellular Ca2+ oscillations in rat megakaryocytes.
1. The effect of membrane potential (Vm) on ADP-evoked [Ca2+]i oscillations was investigated in rat megakaryocytes, a non-excitable cell type recently shown to exhibit depolarisation-evoked Ca2+ release from intracellular stores during metabotropic purinoceptor stimulation. 2. Hyperpolarising voltage steps caused a transient fall in [Ca2+]i and either abolished Ca2+ oscillations or reduced the oscillation amplitude. These effects were observed in both the presence and absence of extracellular Ca2+ and also in Na+-free saline solutions, suggesting that hyperpolarisation leads to a reduction in the level of ADP-dependent Ca2+ release without a requirement for altered transmembrane Ca2+ fluxes. 3. In the presence of Ca2+ oscillations, depolarising voltage steps transiently enhanced the amplitude of Ca2+ oscillations. Following run-down of Ca2+ oscillations, depolarisation briefly restimulated oscillations. 4. Simultaneous [Ca2+]i and current-clamp recordings showed that Ca2+ and Vm oscillate in synchrony, with an average fluctuation of approximately 30-40 mV, due to activation and inactivation of Ca2+-dependent K+ channels. Application of a physiological oscillating Vm waveform to non-oscillating cells under voltage clamp stimulated [Ca2+]i oscillations. 5. Analysis of the relationship between [Ca2+]i and Vm showed a threshold for activation of hyperpolarisation at about 250-300 nM. The implications of this threshold in the interaction between Vm and Ca2+ release during oscillations are discussed. 6. We conclude that the ability of voltage to control release of endosomal Ca2+ in ADP-stimulated megakaryocytes is bipolar in nature. Our data suggest that Vm changes are active components of the feedback/feedforward mechanisms contributing to the generation of Ca2+ oscillations.
Mason
M J
MJ
Department of Physiology, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK. mjm39@cam.ac.uk
Hussain
J F
JF
Mahaut-Smith
M P
MP
D000244:Adenosine Diphosphate / Q000494:pharmacology; D000818:Animals; D020013:Calcium Signaling / Q000187:drug effects / Q000502:physiology*; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008533:Megakaryocytes / Q000187:drug effects / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D008856:Microscopy, Fluorescence; D018408:Patch-Clamp Techniques; D051381:Rats; D017208:Rats, Wistar; D011983:Receptors, Purinergic / Q000187:drug effects; D012964:Sodium / Q000378:metabolism
D011983:Receptors, Purinergic; D000244:Adenosine Diphosphate; D012964:Sodium