Multiple activation currents can be evoked in Xenopus laevis eggs when cortical granule exocytosis is inhibited by weak bases

Further investigation of ionic diffusive properties and of NH4+ pathways in Xenopus laevis oocyte cell membrane

To study the ionic diffusive properties and the NH4+ pathways in the Xenopus laevis oocyte cell membrane, we recorded the effects of various inhibitors on membrane potential (Vm) and membrane resistance (Rm); intracellular acidification was taken as an index of NH4+ influx from the bath to the cytoplasm. The following results were obtained: in the control state, barium and quinine (Q) ions depolarized Vm and raised Rm, consistent with inhibition of K+ conductance(s). Diphenylamine-2-carboxylic acid (DPC), 3',5'-dichlorodiphenylamine-2-carboxylic acid (DCDPC) and gadolinium ions hyperpolarized Vm and increased Rm, suggesting the inhibition of nonselective cationic conductance(s). In the presence of 20 mmol/l NH4Cl, Vm depolarized, Rm fell, and intracellular pH (pHi) decreased, consistent with an NH4+ influx. In the presence of DPC, the same manoeuvre induced a biphasic Vm change (i.e. a spike depolarization followed by a membrane hyperpolarization) and a fall of Rm; in most oocytes, intracellular acidification persisted and was reversible upon adding ouabain (Oua). These results indicate that a DPC-sensitive conductance is not the unique NH4+ pathway and that Na, K-ATPase may also mediate NH4+ influx. However, Oua did not prevent the Rm decrease, suggesting that ouabain-insensitive rheogenic pathway(s) are activated. Thus, we investigated the Vm change induced by NH4Cl addition in the presence of DPC: the spike depolarization followed by secondary hyperpolarization became a plateau depolarization when Q was added, suggesting involvement of Q-sensitive pathway(s) in the above described biphasic Vm change. In the presence of DPC, Q and Oua, intracellular acidification upon adding NH4Cl persisted consistent with further NH4+ influx through quinine-, DPC- and Oua-insensitive pathway(s).

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.