Electrophysiological investigation of the amino acid carrier selectivity in epithelial cells from Xenopus embryo.

The electrical responses induced by external applications of neutral amino acids were used to determine whether different carriers are expressed in the membrane of embryonic epithelial cells of Xenopus laevis. Competition experiments were performed under voltage-clamp conditions at constant membrane potential. Gly, L-Ala, L-Pro, L-Ser, L-Asn and L-Gln generate electrical responses with similar apparent kinetic constants and compete for the same carrier.They are [Na]o and voltage-dependent, insensitive to variations in [Cl]o and [HCO3]o, inhibited by pHo changes, by amiloride and, for a large fraction of the current, by MeAIB. The increase in [K]o at constant and negative membrane potential reduces the response, whereas lowering [K]o augments it. L-Leu, L-Phe and L-Pro appear to compete for another carrier. They generate electrogenic responses insensitive to amiloride and MeAIB, as well as to alterations of membrane potential, [Na]o and [K]o. Lowering [Cl]o decreases their size, whereas increasing [HCO3]o at neutral pHo increases it. It is concluded that at least two and possibly three transport systems (A, ASC and L) are expressed in the membrane of the embryonic cells studied. An unexpected electrogenic character of the L system is revealed by the present study and seems to be indirectly linked to the transport function. L-Pro seems to be transported by system A or ASC in the presence of Na and by system L in the absence of Na. MeAIB induces an inward current.