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Minimum and maximum extracellular Ca2+ requirements during mouse sperm capacitation and fertilization in vitro

1987; Bioscientifica; Volume: 81; Issue: 1 Linguagem: Inglês

10.1530/jrf.0.0810077

1741-7899

L. R. Fraser,

Healthcare and Venom Research

Summary. The minimum and maximum extracellular Ca2+ concentrations required to promote capacitation, the acrosome reaction, hyperactivated motility, zona penetration and gamete fusion in the mouse have been established. The traces of free calcium in Ca2+-deficient medium were shown not to enhance capacitation since the inclusion of EGTA to chelate free ions during a 120 min preincubation failed to alter the kinetics of capacitation from those observed in the absence of EGTA; 1 h after addition of 1·80 mm-Ca2+, both suspensions were highly fertile. Complete capacitation, when suspensions were immediately functional upon the addition of 1·80 mm-Ca2+, required the presence of ≥90 μm-Ca2. Considerably higher concentrations were required to initiate optimal sperm responses: acrosome reaction, 900 μm; gamete fusion, 900 μm; hyperactivated motility, 1·80 mm; zona penetration, 1·80 mm. None of these changes was effected when Ca2+ was <450 μm. The responses to elevated Ca2+ were dependent on the length of incubation, being initially positive and then negative. A short (30 min) exposure to 3·40 mm-Ca2+ (× 2 the standard) accelerated capacitation, as evidenced by significantly increased acrosome loss, precocious expression of hyperactivated motility and enhanced fertilizing ability when Ca2+ was reduced to 1·80 mm. However, extended (120 min) preincubation irreversibly damaged sperm function. In the presence of 7·20 mm-Ca2+ (× 4), fertilizing ability was inhibited at both 30 and 120 min, despite a high incidence of acrosome loss. The primary deleterious effect appeared to be on motility which was judged to be more erratic than in 1·80 mm-Ca2+, possibly due to elevated intracellular Ca2+. Because of the considerable difference in threshold Ca2+ concentrations, it is now possible to dissociate the Ca2+-dependent events of capacitation from those of the acrosome reaction and motility changes.