Spermatozoa within epididymis are immotile and unable to fertilize the ovum but, once deposited in the female reproductive tract the cells become motile and acquire fertilizing capacity. Since these poorly characterized changes, which are known collectively as capacitation, appear to involve alterations to the conductive properties of the membrane, we have now characterized the conductive properties of motile human spermatozoa by analyzing membrane currents recorded from human spermatozoa held under voltage clamp in the whole cell recording configuration.

Under quasi physiological ionic conditions ([K⁺]_o = 5 mM), depolarizing voltage ramps from –98 mV to +62 mV (0.25 s) evoked outward currents that became apparent once the cell was depolarized past ~–40 mV. The peak outward current flowing at 62 mV (I_{62 mV}) was normally 43.1 ± 5.2 pA / pF, and this current was virtually abolished by replacing internal K⁺ with Cs⁺ (I_{62 mV} = 2.57 ± 0.5 pA / pF, n = 8, P < 0.002). Whilst it is therefore clear that this current is carried by K⁺, regression analysis showed that the resting membrane potential (V_m) was –42.2 ± 3.58 mV, and this value differed significantly (P < 0.05) from E_K (–82 mV); other ionic conductances must therefore contribute to V_m. Further experiments showed that this voltage-induced K⁺ current was inhibited (~85%) by 3 mM bupivicaine (n = 3) but not by 2 mM 4-amino pyridine (n = 4). Raising [K⁺]_o to 130 mM, and thus shifting E_K to 3 mV, depolarized V_m to –4.48 ± 4.0 mV (P < 0.001) but, despite this clear and consistent finding, we were unable to discern any inward K⁺ current under these conditions. The K⁺ conductance in human spermatozoa therefore appears to display strong outward rectification and, to explore the biophysical basis of this effect, further experiments characterized the currents evoked by step depolarizations to 62 mV. These studies showed that the voltage-induced K⁺ currents described above normally activated over a period of ~20 ms. Raising bath K⁺ had little effect upon the development of this outward current but, under these conditions, repolarization to –98 mV evoked a tail current that displayed rapid (~5 ms) inactivation.

Human spermatozoa therefore express an outwardly rectifying K⁺ conductance and, whilst the physiological role of this conductance is unknown, it does allow depolarizing stimuli to evoke hyperpolarizing K⁺ currents and it may thus play a role in the regulation of V_m.