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 (V-m) and membrane resistance (R(m)); 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 V-m and raised R(m), consistent with inhibition of K+ conductance(s). Diphenylamine-2-carboxylic acid (DPC), 3',5'-dichlorodiphenylamine-2-carboxylic acid (DCDPC) and gadolinium ions hyperpolarized V-m and increased R(m), suggesting the inhibition of nonselective cationic conductance(s). In the presence of 20 mmol/l NH4Cl, V-m depolarized, R(m) fell, and intracellular pH (pH(i)) decreased, consistent with an NH4+ influx. In the presence of DPC, the same manoeuvre induced a biphasic V-m change (i.e. a spike depolarization followed by a membrane hyperpolarization) and a fall of R(m); 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 R(m) decrease, suggesting that ouabain-insensitive rheogenic pathway(s) are activated. Thus, we investigated the V-m 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 V-m 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).