Albumin is required to support the acrosome reaction but not capacitation in mouse spermatozoa in vitro

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Abstract
Albumin was required specifically for penetration of the zona pellucida (< 10% of eggs fertilized in the absence of albumin), but was not required for capacitation. A similar rate of capacitation was observed in the presence of albumin at concentrations ranging from 30 to 1 mg/ml, while a slightly slower rate was observed in the presence of 0.25 and 0.1 mg albumin/ml. In the absence of albumin, capacitation occurred at a rate which lagged behind that of the albumin-incubated counterparts by .apprx. 30 min; once capacitated, the addition of albumin promoted rapid sperm penetration. In albumin-free media (.+-. the macromolecule PVA [polyvinyl alcohol]), sperm motility was frequently reduced, with fewer cells exhibiting hyperactivated motility but improvements were observed after introduction of albumin. Acrosome loss was significantly lower in the absence of albumin, but within 5 min of its addition at concentrations ranging from 30 to 0.1 mg/ml to capacitated sperm suspensions, acrosome loss was stimulated and reached levels similar to those seen in control samples. Therefore, albumin can trigger the acrosome reaction in capacitated spermatozoa. It appears to act by assisting in the removal of a surface-associated inhibitory component, the presence of which stabilizes the sperm membranes and inhibits the acrosome reaction.