Ion channels in sperm motility and capacitation

Abstract

Spermatozoa depend upon ion channels to rapidly exchange information with the outside world and to fertilise the egg. These efficient ion transporters participate in many of the most important sperm processes, such as motility and capacitation. It is well known that sperm swimming is regulated by [Ca2+]i. In the sea urchin sperm speract, a decapeptide isolated from egg outer envelope, induces changes in intracellular Ca2+ ([Ca2+]i), Na+, cAMP and cGMP, membrane potential (Em) and pH (pHi). Photoactivation of a speract analogue induces Ca2+ fluctuations that generate turns that are followed by straighter swimming paths. A fast component of the [Ca2+], increase that most likely occurs through voltage dependent Ca2+ channels (Ca(v)s) is essential for these turns. The Ca(v)s involved are modulated by the Em changes triggered by speract. On the other hand, mammalian sperm gain the ability to fertilise the egg after undergoing a series of physiological changes in the female tract. This maturational process, known as capacitation, encompasses increases in [Ca2+]i and pHi, as well as an Em hyperpolarization in mouse sperm. Our electrophysiological, immunological and molecular-biological experiments indicate that inwardly rectifying K+ channels regulated by ATP (KATP channels) and epithelial Na+ channels (ENaCs) are functionally present in mouse spermatogenic cells and sperm. Notably, pharmacological experiments indicate that the opening of KATP channels and closure of ENaCs may contribute to the hyperpolarization that accompanies mouse sperm capacitation. Remarkably, both in the sea urchin sperm speract response and in the mouse sperm capacitation, Em hyperpolarization seems necessary to remove inactivation from Ca(v) channels so they can then open.