Influence of calcium on the true acrosome reaction and capacity of bull spermatozoa to penetrate oocytes in vitro.

A series of experiments were conducted to determine the role of Ca in several physiological functions of bovine spermatozoa. For spermatozoa incubated in the absence of Ca for up to 24 h, motility was not different from those incubated with Ca. For spermatozoa incubated in the continuous presence of Ca, true acrosome reaction values were 0% at 0 h, 1.5% at 6 h, and 6.0% at 12 h. Spermatozoa incubated in vitro for up to 12 h in the absence of Ca did not undergo a true acrosome reaction; however, when Ca was added during incubation, a synchronous true acrosome reaction was induced within 10 min (0% at 0 h, 8.5% at 6 h, and 8.5% at 12 h). When spermatozoa were preincubated in the presence or absence of Ca for 6 h, then added to zona-intact dead bovine oocytes and incubation continued with and without Ca for 18 h, the number of spermatozoa binding to and penetrating each oocyte was greater when Ca was present. Also, the percentage of oocytes being penetrated was greater when Ca was present. These results indicate that: 1) Ca is not necessary for maintenance of spermatozoan motility; 2) Ca is required for the induction of a true acrosome reaction among a population of spermatozoa; 3) Ca is able to induce the synchronous true acrosome reaction in a low percentage of spermatozoa; and 4) Ca is important in spermatozoan binding and initiation of penetration of oocytes.