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).
Cougnon
M
M
INSERM, Faculté Necker-Enfants Malades, Université Paris, France.
Bouyer
P
P
Hulin
P
P
Anagnostopoulos
T
T
Planelles
G
G
D000818:Animals; D001464:Barium / Q000494:pharmacology; D001692:Biological Transport / Q000187:drug effects; D002462:Cell Membrane / Q000502:physiology; D004159:Diphenylamine / Q000031:analogs & derivatives / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D009865:Oocytes / Q000166:cytology* / Q000378:metabolism*; D026902:Potassium Channel Blockers; D000644:Quaternary Ammonium Compounds / Q000378:metabolism* / Q000494:pharmacology*; D011803:Quinine / Q000494:pharmacology; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D014982:Xenopus laevis
D026902:Potassium Channel Blockers; D000644:Quaternary Ammonium Compounds; D013757:Tetraethylammonium Compounds; C059537:3',5-dichlorodiphenylamine-2-carboxylic acid; D001464:Barium; D019789:Tetraethylammonium; D004159:Diphenylamine; D011803:Quinine