Ca2+ signalling is crucial to modulate sperm motility and changes in the intracellular calcium concentration [Ca2+]i may underlie ‘switching’ of sperm swimming behaviour in human spermatozoa, which is important for sperm progression in the female tract. I investigated the mechanism by which progesterone induces [Ca2+]i oscillations and also investigated the role of [Ca2+]i oscillations for sperm swimming behaviour and penetration ability into mucus. Sperm were loaded with Ca2+-indicator fluo4/AM, treated with different agonists and analysed in single live-cell imaging and functional assays were also performed in free-swimming sperm. Approximately 25% of the human sperm population exhibit [Ca2+]i oscillations, independently of sperm capacitation. [Ca2+]i rise first in flagellum then spread actively to the head, apparently triggering CICR. Membrane potential (Vm), Slo3 channels and CatSper channels contribute to [Ca2+]i oscillations generation. [Ca2+]i oscillations occur in free-swimming spermatozoa and are associated with switching of sperm swimming behaviour in both low and high viscosity environments: drives velocity, sperm turning, hyperactivated motility and regulates attachment/detachment from substrate. Consistent with the involvement of CatSper in generation of [Ca2+]i oscillations, regulation of sperm swimming velocity and cell progression are dependent on CatSper channels. Additionally, sperm ability to penetrate into mucus environment is also dependent on CatSper activity
