Exp Physiol 95.11 pp 1049–1060 1049 Experimental Physiology – G. L. Brown Prize Lecture Seeing is believing! Imaging Ca2+-signalling events in living cells J. Graham McGeown Centre for Vision and Vascular Sciences, Queen’s University of Belfast, Institute of Clinical Sciences, Grosvenor Road, Royal Victoria Hospital, Belfast BT12 6BA, UK Ever since it was shown that maintenance of muscle contraction required the presence of extracellular Ca2+, evidence has accumulated that Ca2+ plays a crucial role in excitation– contraction coupling. This culminated in the use of the photoprotein aequorin to demonstrate that [Ca2+]i increased after depolarization but before contraction in barnacle muscle. Green ﬂuorescent protein was extracted from the same jellyﬁsh as aequorin, so this work also has important historical links to the use of ﬂuorescent proteins as markers in living cells. The subsequent development of cell-permeant Ca2+ indicators resulted in a dramatic increase in related research, revealing Ca2+ to be a ubiquitous cell signal. High-speed, confocal Ca2+ imaging has now revealed subcellular detail not previously apparent, with the identiﬁcation of Ca2+ sparks. These act as building blocks for larger transients during excitation–contraction coupling in cardiac muscle, but their function in smooth muscle appears more diverse, with evidence suggesting both ‘excitatory’ and ‘inhibitory’ roles. Sparks can activate Ca2+-sensitive Cl− and K+ currents, which exert positive and negative feedback, respectively, on global Ca2+ signalling, through changes in membrane potential and activation of voltage-operated Ca2+ channels. Calcium imaging has also demonstrated that agonists that appear to evoke relatively tonic increases in average [Ca2+]i at the whole tissue level often stimulate much higher frequency phasic Ca2+ oscillations at the cellular level. These ﬁndings may require re-evaluation of some of our models of Ca2+ signalling to account for newly revealed cellular and subcellular detail. Future research in the ﬁeld is likely to make increasing use of genetically coded Ca2+ indicators expressed in an organelle- or tissue-speciﬁc manner. (Received 23 June 2010; accepted after revision 2 August 2010; ﬁrst published online 9 August 2010) Corresponding author J. G. McGeown: Centre for Vision and Vascular Sciences, Queen’s University of Belfast, Institute of Clinical Sciences, Grosvenor Road, Royal Victoria Hospital, Belfast BT12 6BA, UK. Email: g.mcgeown@qub.ac.uk In 2010 the G. L. Brown Prize Lecture was given by Dunville Professor of Physiology J. G. McGeown at the Universities of Cambridge, Edinburgh, Glasgow, Manchester, Liverpool, Leeds and Bristol and at Imperial College London. Live cell imaging: a terrible beauty Imaging biological processes in living cells has had and will continue to have great impact on our scientiﬁc understanding of biological function. I believe it also has an amazing and largely untapped potential to stir the imagination of society at large by presenting the world of cell biology in an accessible and aesthetically stimulating way. Just as the amazing images from the Hubble telescope make cosmologists of us all, so too advances in imaging technology have provided us with pictures dramatic enough to engage and enthuse even those with the most limited interest in biology. The invisible has been made visible, revealing unsuspected beauty and endless movement. This is dramatically exempliﬁed by the Ca2+ waves seen in Xenopus oocytes expressing muscarinic acetylcholine receptors (Fig. 1; Lechleiter et al. 1991). These spiral across the cell in endlessly evolving patterns, never repeating the same sequence twice but always obeying simple rules of propagation and annihilation. I will let this example stand for now not just as a great piece of science but also as a thing of wonder in its own right, easily appreciated by anyone who has ever stood delighted by a Catherine-wheel’s ﬁery corkscrewing against the night sky. C 2010 The Author. Journal compilation C 2010 The Physiological Society DOI: 10.1113/expphysiol.2010.052456