Bicarbonate: carbon-dioxide regulation of sperm capacitation, hyperactivated motility, and acrosome reactions

The bicarbonate: CO2 (HCO3-:CO2) concentration dependencies of hamster sperm motility, spontaneous acrosome reactions, and zona penetration (used to assay the zona-induced acrosome reaction) were examined. A cross-over experimental design was used to segregate effects on early stages of capacitation, spanning the first 5 h of incubation, from those on acrosome reactions and zona penetration during the last 1 h. After 5 h, HCO3-:CO2 concentrations were increased, decreased, or kept the same for 1 h. Compared to no HCO3-:CO2, as little as 2.9 mM: 0.6% HCO3-:CO2 increased the sperm motility index (MI) by 2.7-3.6 times. When HCO3-:CO2 was continuously present, both progressive and hyperactivated motility were stimulated by HCO3-:CO2 in a dose-dependent manner by 3-4 h, well before completion of capacitation. Stimulation of acrosome reactions or zona penetration, by addition of HCO3-:CO2 to sperm for 1 h late in capacitation, depended mainly on levels of HCO3-:CO2 present earlier in capacitation. When 25 mM: 5% HCO3-:CO2 was added only at 5 h, responses were significantly lower than with sperm treated continuously with the same concentration of HCO3-:CO2, being 2.5 times lower for MI, 2 times lower for acrosome reactions, and 6.3 times lower for zona penetration. In contrast, decreasing HCO3-:CO2 to suboptimal levels after 5 h did not decrease any 6-h sperm responses significantly. The average maximal and one-half maximal preincubation HCO3- concentrations for all responses were 34.2 +/- 1.0 and 9.2 +/- 0.3 mM, respectively. Zona penetration and hyperactivation were highly correlated.(ABSTRACT TRUNCATED AT 250 WORDS)