Patch clamp studies of human sperm under physiological ionic conditions reveal three functionally and pharmacologically distinct cation channels

Whilst fertilizing capacity depends upon a K+ conductance (G(K)) that allows the spermatozoon membrane potential (V-m) to be held at a negative value, the characteristics of this conductance in human sperm are virtually unknown. We therefore studied the biophysical/pharmacological properties of the K+ conductance in spermatozoa from normal donors held under voltage/current clamp in the whole cell recording configuration. Our standard recording conditions were designed to maintain quasi-physiological, Na+, K+ and Cl- gradients. Experiments that explored the effects of ionic substitution/ion channel blockers upon membrane current/potential showed that resting V-m was dependent upon a hyperpolarizing K+ current that flowed via channels that displayed only weak voltage dependence and limited (similar to 7-fold) K+ versus Na+ selectivity. This conductance was blocked by quinidine (0.3 mM), bupivacaine (3 mM) and clofilium (50 A mu M), NNC55-0396 (2 A mu M) and mibefradil (30 A mu M), but not by 4-aminopyridine (2 mM, 4-AP). Progesterone had no effect upon the hyperpolarizing K+ current. Repolarization after a test depolarization consistently evoked a transient inward 'tail current' (I-Tail) that flowed via a second population of ion channels with poor (similar to 3-fold) K+ versus Na+ selectivity. The activity of these channels was increased by quinidine, 4-AP and progesterone. V-m in human sperm is therefore dependent upon a hyperpolarizing K+ current that flows via channels that most closely resemble those encoded by Slo3. Although 0.5 A mu M progesterone had no effect upon these channels, this hormone did activate the pharmacologically distinct channels that mediate I-Tail. In conclusion, this study reveals three functionally and pharmacologically distinct cation channels: Ik, I-Tail, I-CatSper.