Effects of pH on the Genesis of Membrane Potential Changes at Fertilization in the Egg of the Frog Rana cameranoi


Abstract: The apparent change in the egg membrane potential at fertilization gives rise to fertilization potential (FP) . FP is the initial, transient electrical block to polyspermy as shown in most species especially those exhibiting external fertilization, and it protects the egg from a second sperm entry until the permanent, mechanical block is set up. Polyspermy is lethal in most species, and for a successful fertilization resting membrane potential (RMP) and FP of the egg have to be held in optimum ranges. Shifts in environmental pH may interfere with a successful fertilization by affecting these bioelectrical potentials. In the present study, we investigated how pH alterations affected the RMP and FP parameters of the egg in the frog, Rana cameranoi. Egg membrane potentials were recorded by the conventional microelectrode technique. RMP and FP parameters were evaluated. Ten percent Ringer solution (pH=7.80) was used for the control group. Acidic (Acd) and alkaline (Alk) experimental groups were kept in the Ringer solutions at pH 6.50 and 9.00, respectively. RMP was found to be significantly more negative in the Acd group (p<0.05), but the difference was not significant in the Alk group when compared with the control. The peak FP value was significantly more positive in the Alk group, and less positive in the Acd group. FP duration was significantly prolonged in the Alk group, and shortened in the Acd group, but the latter change was not significant. Fertilized egg membrane potential in the Acd group was significantly more negative when compared with the Alk group, but did not differ significantly when compared with the control. Overall evaluation of these results led to the conclusion that moderate shifts in pH affected FP parameters significantly, but did not prevent the genesis of the effective electrical block. However, greater changes in pH may prevent the electrical block, leading to increased polyspermy risk. Alterations in environmental pH, which may interfere with successful fertilization in externally fertilizing species such as frogs, may be due to industrial pollution and consequently be hazardous for the ecologic equilibrium.

Keywords: Resting membrane potential, fertilization potential, pH, frog egg.

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