Calcium regulates motility and protein phosphorylation by changing cAMP and ATP concentrations in boar sperm in vitro

Considering the importance of calcium (Ca2+) in regulating sperm capacitation, hyperactivation and acrosome reaction, little is known about the molecular mechanism of action of this ion in this process. In the present study, assessment of the molecular mechanism from the perspective of energy metabolism occurred. Sperm motility variables were determined using computer-assisted sperm analysis (CASA) and the phosphorylation of PKA substrates, tyrosine residues and AMP-activated protein kinase (AMPK) were analyzed by Western blot. Moreover, intracellular sperm-specific glyceraldehyde 3-phosphatedehydrogenase (GAPDH) activity, 3′-5′-cyclic adenosine monophosphate (cAMP) and adenosine 5′-triphosphate (ATP) concentrations were assessed in boar sperm treated with Ca2+. Results of the present study indicated that, under greater extracellular Ca2+concentrations (≥3.0 mM), sperm motility and protein phosphorylation were inhibited. Interestingly, these changes were correlated with that of GAPDH activity, AMPK phosphorylation, cAMP and ATP concentrations. The negative effects of Ca2+ on these intracellular processes were attenuated by addition of the calmodulin (CaM) inhibitor W7 and the inhibitor of calmodulin-dependent protein kinase (CaMK), KN-93. In the presence of greater extracellular Ca2+, however, the phosphorylation pathway was suppressed by H-89. Taken together, these results suggested that Ca2+ had a dual role in regulating boar sperm motility and protein phosphorylation due to the changes of cAMP and ATP concentrations, in response to cAMP-mediated signal transduction and the Ca2+ signaling cascade. The present study provided some novel insights into the molecular mechanism underlying the effects of Ca2+ on boar sperm as well as the involvement of energy metabolism in this mechanism.