Calcium signalling in human spermatozoa: a specialized 'toolkit' of channels, transporters and stores

Ca2+ is a ubiquitous intracellular messenger which encodes information by temporal and spatial patterns of concentration. In spermatozoa, several key functions, including acrosome reaction and motility, are regulated by cytoplasmic Ca2+ concentration. Despite the very small size and apparent structural simplicity of spermatozoa, evidence is accumulating that they possess sophisticated mechanisms for regulation of cytoplasmic Ca2+ concentration and generation of complex Ca2+ signals. In this review, we consider the various components of the Ca2+-signalling 'toolkit' that have been characterized in somatic cells and summarize the evidence for their presence and activity in spermatozoa. In particular, data accumulated over the last few years show that spermatozoa possess one (and probably two) Ca2+ stores as well as a range of plasma membrane pumps and channels. Selective regulation of the various components of the 'toolkit' by agonists probably allows spermatozoa to generate localized Ca2+ signals despite their very small cytoplasmic volume, permitting the discrete and selective activation of cell functions.