Role of ions and ion channels in capacitation and acrosome reaction of spermatozoa.

Capacitation and acrosome reaction are important prerequisites of the fertilization process. Capacitation is a highly complex phenomenon occurring in the female genital tract, rendering the spermatozoa capable of binding and fusion with the oocyte. During capacitation various biochemical and biophysical changes occur in the spermatozoa and the spermatozoal membranes. Ions and ion channels also play important roles in governing the process of capacitation by changing the fluxes of different ions which in turn controls various characteristics of capacitated spermatozoa. Along with the mobilization of ions the generation of free radicals and efflux of cholesterol also plays an important role in the capacitation state of the spermatozoa. The generation of free radical and efflux of cholesterol change the mechanodynamic properties of the membrane by oxidation of the polyunsaturated lipids and by generating the cholesterol free patches. The process of capacitation renders the spermatozoa responsive to the inducers of the acrosome reaction. The glycoprotein zona pellucida 3 (ZP3) of the egg coat zona pellucida is the potent physiological stimulator of the acrosome reaction; progesterone, a major component of the follicular fluid, is also an inducer of the acrosome reaction. The inducers of the acrosome reaction cause the activation of the various ion-channels leading to high influxes of calcium, sodium and bicarbonate. The efflux of cholesterol during the process of capacitation alters the permeability of the membrane to the ions and generate areas which are prone to fusion and vesiculation process during the acrosome reaction. This review focuses mainly on effects of the ion and ion-channels, free radicals, and membrane fluidity changes during the process of capacitation and acrosome reaction.