Calcium requirements for human sperm function in vitro

To determine extracellular calcium (Ca 2+) requirements for the maintenance of human sperm function in vitro. Prospective study. Basic research laboratory. Normozoospermic volunteers provided fresh semen samples; follicular fluid (human FF) and oocytes were collected from women undergoing IVF-ET. Spermatozoa were incubated for ≤18 hours in media containing different CaCl 2 concentrations (maximum, 2.5 mM [control]). Protein tyrosine phosphorylation patterns, development of hyperactivated motility, induction of the acrosome reaction (AR) in response to human FF, and sperm interaction with homologous zona pellucida (ZP). Cells maintained for 18 hours in medium containing ≥0.1 mM of Ca 2+ were able to undergo the AR when exposed to human FF in the presence of 2.5 mM of Ca 2+. Calcium concentrations of ≥0.22 mM were sufficient to reach protein tyrosine phosphorylation levels and hyperactivated motility values similar to those of controls. Higher Ca 2+ concentrations (≥0.58 mM) were required to produce maximum human FF–induced AR in previously capacitated cells and to obtain an adequate sperm–ZP binding. Different steps of the fertilization process have distinctive Ca 2+ requirements. Whereas 0.22 mM of Ca 2+ is sufficient for the development of some capacitation-related events, human FF–induced AR and sperm–ZP interaction require 0.58 mM of this cation.