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Fig. 9. Tentative model for intracellular Ca²⁺ storage and generation of [Ca²⁺]_i signals and cellular responses in progesterone-stimulated human spermatozoa. The initial [Ca²⁺]_i transient that occurs immediately upon application of progesterone is mediated primarily by Ca²⁺-influx and induces acrosome reaction in a proportion of cells (grey arrow). Ca²⁺ is removed from the cytoplasm primarily by pumping at the plasma membrane (PMCA and Na⁺-Ca²⁺ exchanger, blue). Acrosome reaction may involve mobilisation of Ca²⁺ stored in the acrosome itself (De Blas et al., 2002; Herrick et al., 2005). Sustained elevation of [Ca²⁺]_i, which follows the transient (or can be induced by a progesterone concentration gradient to simulate approach to the oocyte, a procedure that induces [Ca²⁺]_i oscillations without a preceding [Ca²⁺]_i transient) (Harper et al., 2004), acts on RyRs (red) on a Ca²⁺ store in the caudal part of the head or midpiece, causing Ca²⁺-induced Ca²⁺ release. Cyclic release and re-uptake occur as a result of refilling of this store by bis-phenol-sensitive SPCA1 (green), generating slow [Ca²⁺]_i oscillations at the base of the flagellum and causing alternation of flagellar beat pattern (grey arrow).

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