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

Abstract

Whilst fertilizing capacity depends upon a K+ conductance (GK) that allows the spermatozoon membrane potential (Vm) 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 Vm was dependent upon a hyperpolarizing K+ current that flowed via channels that displayed only weak voltage dependence and limited (∼7-fold) K+ versus Na+ selectivity. This conductance was blocked by quinidine (0.3 mM), bupivacaine (3 mM) and clofilium (50 µM), NNC55-0396 (2 µM) and mibefradil (30 µM), 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’ (ITail) that flowed via a second population of ion channels with poor (∼3-fold) K+ versus Na+ selectivity. The activity of these channels was increased by quinidine, 4-AP and progesterone. Vm 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 µM progesterone had no effect upon these channels, this hormone did activate the pharmacologically distinct channels that mediate ITail. In conclusion, this study reveals three functionally and pharmacologically distinct cation channels: Ik, ITail, ICatSper.

Exposure to Cadmium Impairs Sperm Functions by Reducing CatSper in Mice

“…The CatSper channel, which is specifically located on the mammalian sperm flagellum, mediates a series of sperm functions [22, 26, 27, 33]. In view of the channel’s vital role, we tried to further observe whether cadmium treatment caused functional impairment via the CatSper abnormality.…”

“…Previous studies proposed that the CatSper channel was composed of four different pore-forming subunits (CatSper1-4) [22, 34], and any mutation of these subunits would abolish the CatSper current [33]. Therefore, it is necessary to clarify whether the decrease of the CatSper current in the cadmium-treated mice results from gene expression changes of these pore-forming subunits.…”

CFTR/ENaC-dependent regulation of membrane potential during human sperm capacitation is initiated by bicarbonate uptake through NBC

“…A previous report have postulated that SLO1 is the principal K + channel of human sperm (22) while others have claimed that the K + current of human sperm is mediated by SLO3 (23). Our group has previously studied the role of K + channels SLO1 and SLO3 in capacitation (55). By using a combination of K + channels inhibitors, membrane hyperpolarization is abrogated, suggesting that both members of the SLO family may potentially be involved in the regulation of Em.…”

Flagellar ion channels of sperm: similarities and differences between species

“…From these biophysical properties as well as the pharmacological profile, human KSper seems to resemble the calcium-activated big conductance potassium channel Slo1 (KCNMA1) [20]. Recently other models were proposed suggesting that human capacitated spermatozoa possess a different type of Slo channel [145] or even a modified version of Slo3 that is calcium-sensitive and weakly pH-dependent [144], an unusual set of properties for this type of ion channel (Fig. 2).…”

“…Furthermore, the crystal structure of human Slo3 indicated the channel lacks a calcium-sensitive bowl [142]. One apparent difference is that both studies used slightly different human sperm cells: capacitated [144] versus noncapacitated cells [20], but another possible explanation could be that KSper consists of a heteromer of Slo1 and Slo3 subunits [145,149]. While Slo channels exist as tetrameric complexes of alpha subunits (Slo1 or Slo3) and auxiliary gamma subunits (LRCC26 or LRCC52), additional beta subunits can change their biophysical and pharmacological properties [140,150,151].…”