| Sumario: | Objective: To determine extracellular calcium (Ca2+) requirements for the maintenance of human sperm function in vitro. Design: Prospective study. Setting: Basic research laboratory. Patient(s): Normozoospermic volunteers provided fresh semen samples; follicular fluid (human FF) and oocytes were collected from women undergoing IVF-ET. Intervention(s): Spermatozoa were incubated for ≤18 hours in media containing different CaCl2 concentrations (maximum, 2.5 mM [control]). Main outcome measure(s): 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). Result(s): Cells maintained for 18 hours in medium containing ≥0.1 mM of Ca2+ were able to undergo the AR when exposed to human FF in the presence of 2.5 mM of Ca2+. Calcium concentrations of ≥0.22 mM were sufficient to reach protein tyrosine phosphorylation levels and hyperactivated motility values similar to those of controls. Higher Ca2+ 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. Conclusion(s): Different steps of the fertilization process have distinctive Ca2+ requirements. Whereas 0.22 mM of Ca2+ 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. © 2003 by American Society for Reproductive Medicine.
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