Weak bases mimic the fertilization-associated chloride conductance increase and induce morphological changes in the cortex of Xenopus laevis eggs.

Ammonia and other weak bases (methylamine, trimethylamine) including certain local anesthetics (benzocaine, procaine, lidocaine) depolarized the plasma membrane of unactivated Xenopus eggs. Exposure to different [Cl-]o or to the other halides (F-, Br-, I-) revealed a similar anion dependency of the weak base-induced conductance change to that found for the normal fertilization potential. Although transient, the induced depolarization was slower and of longer duration than the fertilization potential. Furthermore, [NH4+] greater than 40 mM elicited depolarizations of greater amplitude than the latter. In fertilized eggs, the amplitude of the induced depolarization decreased as the interval between fertilization potential and time of application of the NH4+ solution was increased. Treatment of unactivated eggs with NH4+ or procaine produced dramatic changes in the configuration of surface microvilli and thickening of the peripheral layer of cytoplasm. These effects were obtained only if the pH of the bathing solution was close to the pKa of the amine. This implicates intracellular pH, which would be expected to rise due to entry of the uncharged amine.