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1 Department of Physiology, University of Cambridge, Cambridge, UK2 Victor Chang Cardiac Research Institute, Darlinghurst, NSW 2010, Australia
G-protein-coupled receptor signalling has been suggested to be voltage dependent in a number of cell types; however, the limits of sensitivity of this potentially important phenomenon are unknown. Using the non-excitable rat megakaryocyte as a model system, we now show that P2Y receptor-evoked Ca2+ mobilization is controlled by membrane voltage in a graded and bipolar manner without evidence for a discrete threshold potential. Throughout the range of potentials studied, the peak increase in intracellular Ca2+ concentration ([Ca2+]i) in response to depolarization was always larger than the maximal reduction in [Ca2+]i following an equivalent amplitude hyperpolarization. Significant [Ca2+]i increases were observed in response to small amplitude (<5 mV, 5 s duration) or short duration (25 ms, 135 mV) depolarizations. Individual cardiac action potential waveforms were also able to repeatedly potentiate P2Y receptor-evoked Ca2+ release and the response to trains of normally paced stimuli fused to generate prolonged [Ca2+]i increases. Furthermore, elevation of the temperature to physiological levels (36°C) resulted in a more sustained depolarization-evoked Ca2+ increase compared with more transient or oscillatory responses at 20–24°C. The ability of signalling via a G-protein-coupled receptor to be potentiated by action potential waveforms and small amplitude depolarizations has broad implications in excitable and non-excitable tissues.
(Received 14 October 2003;
accepted after revision 25 November 2003;
first published online 28 November 2003)
Corresponding author M. P. Mahaut-Smith: Department of Physiology, University of Cambridge, Downing Street, Cambridge CB2 3EG, UK. Email: mpm11{at}cam.ac.uk
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