Figure 15.

Overview of cation-permeable channels in the membranes of human spermatozoa. Electrophysiological studies of human sperm have now identified (i) a poorly selective K⁺ conductance that appears to set the resting membrane potential by allowing hyperpolarizing K⁺ current (IK) to flow across the membranes of cells depolarized past approximately −30 mV. (ii) An unidentified channel that underlies the transient inward ‘tail’ current (I_Tail) that is seen upon repolarization, although the function of this conductance is unknown, the fact that it is activated by progesterone raises the possibility that it may form part of the mechanism that allows spermatozoa to respond to this female hormone. (iii) The spermatozoon cation channel (CatSper) appears to be Ca²⁺-selective under physiological conditions, although it is freely permeable to Na⁺, K⁺ and Cs⁺ in the absence of divalent cations. The effects of a range of pharmacological agents are summarized in the lower part of the figure. Each identified conductance has a characteristic pharmacological signature indicating that they must be associated with different ion channels.