Evidence for Ca2+-dependent ATPase activity,stimulated by decapacitation factor and calmodulin,in mouse sperm

Membrane preparations from mouse sperm heads and tails were used in a γ³²P-ATP hydrolysis assay to investigate Ca²⁺-dependent ATPase activity. In membranes from sperm heads, but not tails, a Ca²⁺-dependent ATPase that was further stimulated by calmodulin was detected. The addition of partially purified mouse sperm decapacitation factor (DF) to head membrane preparations significantly stimulated Ca²⁺-ATPase activity, this effect being further increased in the presence of DF plus calmodulin; in contrast, no response was observed when the same treatment was applied to tail membranes. Sperm preincubated in the presence of trifluoperazine (TFP), a calmodulin antagonist, were significantly more fertile than cells from the same males incubated in the absence of TFP, indicating that inhibition of calmodulin accelerates capacitation. When sperm cells were preincubated briefly, then gently centrifuged to remove DF and resuspended in medium containing ⁴⁵Ca²⁺ ± DF, their ability to accumulate ⁴⁵Ca²⁺ was significantly lower in the early stages after resuspension in the presence of DF than in its absence. These data correlated with chlortetracycline analysis of the sperm functional state. When cells were centrifuged and resuspended in medium only, there was a noticeable shift from the F pattern (characteristic of uncapacitated cells) to the B pattern (characteristic of capacitated cells), but the reintroduction of DF caused a significant reversion to the F pattern. Finally, using a monoclonal antibody to somatic cell Ca²⁺-ATPase, we have obtained evidence that the enzyme is particularly localized to the postacrosomal region of the mouse sperm head; specific binding was observed only in permeabilized cells, indicating that the epitope involved in the binding has an intracellular location. Based on these various pieces of evidence, we propose that when present on mouse sperm, DF stimulates calmodulin-sensitive Ca²⁺-ATPase activity and thus ensures maintenance of a low intracellular Ca²⁺ concentration. As capacitation proceeds, DF is lost and Ca²⁺-ATPase activity declines, allowing intracellular Ca²⁺ to rise and promoting capacitation-related changes. The fact that inhibitors of Ca²⁺-ATPase and calmodulin appear to accelerate capacitation in several mammalian species, as determined by chlortetracycline analysis, suggests that Ca²⁺-ATPase activity may play an important role in modulating capacitation in many or even all mammals. © 1996 Wiley-Liss, Inc