Acquisition of the potential for sperm motility in steelhead (Oncorhynchus mykiss): effect of pH on dynein ATPase

Salmonid sperm pre-incubated at extracellular pH (pH_e) values less than about 7.4 do not become motile upon water activation whereas sperm maintained above about pH 8.0 demonstrate maximal motility upon activation. The basis for this permissive effect of elevated pH_e on sperm motility is not known. Since it is conceivable that the pH sensitivity of dyneinATPase (the molecular motor that drives flagellar movement) could be the basis of, or contribute to this pH dependency, the pH sensitivity of this enzymatic activity was evaluated in membrane-permeabilized axonemes (isolated flagella) ofsteelhead sperm. DyneinATPase activity was found to be sensitive to pH. This activity in permeabilized axonemes was about 3.5-fold higher at pH 7.6 compared to 7.0. To determine whether the pH sensitivity ofATP regeneration might affect the interpretation of the effect of pH on dyneinATPase activity, the pH sensitivities of creatine kinase and adenylate kinase were established. The rates ofATP generation by these enzymes were insensitive to pH between 6.5 and 8.0. The results of these studies are consistent with the hypothesis that prior maintenance at pH_e, in part, controls the potential for sperm motility upon water activation via an influence on dyneinATPase activity. However, the potential for motility ofsteelhead sperm is particularly sensitive to prior maintenance at pH_e values between about 7.4 and 8.0 whereas the dyneinATPase activity of permeabilized axonemes was particularly sensitive to pH values between 7.0 and 7.6. Phosphorus NMR spectroscopy was used to determine that sperm intracellular pH (pH_i) increased with increasing pH_e between 7.0 and 8.5 and pH_i was, on average 0.4–0.5 pH units lower than pH_e. Therefore the pH_e sensitivity of the potential for motility appears to correspond to the pH_i sensitivity of dyneinATPase activity. The data indicate that pH_i is directly related to pH_e and that prior incubation at pH_e may, in part, control the sperm's potential for motility upon water activation via an influence on dyneinATPase activity.