An increase in the concentration of intracellular free Ca2 and in the phosphotyrosine content of specific proteins characterizes human sperm capacitation. Whether tyrosine phosphorylation regulates the intracellular free Ca2 concentration through modulation of Ca2 -ATPase activity or the phosphotyrosine content is under Ca2 regulation was investigated using Ca2 -ATPase modulators and tyrosine kinase inhibitors. The presence of the Ca2 -ATPase-inhibitor thapsigargin during human sperm capacitation caused an increase in the cytoplasmic free Ca2 concentration and was associated with an increase in the phosphotyrosine content of specific sperm proteins. Conversely, a decrease in protein tyrosine phosphorylation was observed when gingerol, a Ca2 -ATPase activator, was present during the incubation period. On the other hand, thapsigargin had no effect on the phosphotyrosine content or the cytoplasmic Ca2 concentration when spermatozoa were incubated in the presence of the phosphodiesterase-inhibitor 3-isobutyl-1-methylxanthine (IBMX). However, the effect of IBMX on phosphotyrosine-containing proteins appears to be a Ca2 -dependent phenomenon, because it was partly inhibited in spermatozoa pretreated with 1,2-bis-(o-aminophenoxy)-ethane-N,N,N,N-tetraacetic acid tetra-(acetoxymethyl)-ester (BAPTA-AM) even though, by itself, BAPTA-AM caused an increase in sperm protein phosphotyrosine content. Tyrosine kinase inhibitors prevented the increase in the phosphotyrosine content without affecting the cytoplasmic free Ca2 concentration. Based on these findings, the present study suggests that Ca2 -ATPases are involved in the filling of internal Ca2 stores, such as the acrosome, and are inhibited later during capacitation. Their inhibition allows an increase in cytoplasmic free Ca2 , which is involved in the subsequent increase in the phosphotyrosine content of specific sperm proteins.
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