The role of calcium in maintaining motility in mouse spermatozoa

Maintenance of mouse sperm motility requires exogenous Ca+2 in most, but not all, samples of epididymal spermatozoa. In these samples, the loss of motility with time is the same in Tris/NaCl buffer containing 1.7 mM Ca+2 (medium TNC) as it is in complete culture medium used for in vitro fertilization (medium CM) over the first 2 hours; spermatozoa in medium TNC lose motility at more rapid rate thereafter. The cation specificity for maintenance of motility is unusual in that both Sr+2 and Mg+2 substitute for Ca+2, with Mg+2 being the more effective. In the absence of Ca+2, these samples of mouse epididymal spermatozoa in Tris/NaCl buffer (medium TN) lose motility in about 30 minutes. If Ca+2 is added after incubation in TN for 15 minutes, motility is maintained as well as it is in medium TNC. If Ca+2 is added at 30 minutes, motility is partially restored. But if Ca+2 is added after 60 minutes, there is no restoration of motility. Spermatozoa suspended in medium TNC lose motility rapidly on addition of EGTA in excess of the Ca+2 present. Attempts to show uptake of Ca+2 by spectrophotometric assay, by effects of Ca+2 on oxidative metabolism, and by electron probe X-ray microanalysis were unsuccessful; motility is not maintained by entry of Ca+2 into the cells. Our results are consistent with a Ca+2 site on the plasma membrane and suggest that these sites function as oligomers of ordered subunits whose structure requires Ca+2. In the absence of Ca+2, the ability to form the oligomer is lost with time, possibly due to an irreversible conformational change.