Electrophysiology of l-lysine entry across the brush-border membrane of Necturus intestine

Abstract Microelectrode measurements of apical membrane potentials ( V a ) in absorptive cells of isolated Necturus intestine showed that, in the presence or absence of external Na + , 10 mM lysine added to the mucosal medium caused rapid depolarization followed by slower repolarization of V a . In Na + -free media the effects of 10 mM lysine on V a were abolished by 10 mM leucine which alone had no effect on V a under these conditions. This indicates that uncoupled electrodiffusion of lysine plays little or no role in lysine entry across the brush-border membrane. When external Na + was > 10 mM the maximum depolarization of V a ( ΔV ′ a ) induced by [Lys] ranging from 5 to 30 mM was a simple saturable function of [Lys]. In Na + -free media, the relationship between ΔV ′ a and [Lys] was biphasic. At first, ΔV ′ a increased with increasing [Lys] reaching a maximum at 10 mM lysine. When [Lys] was further increased, ΔV ′ a declined progressively to reach zero or near zero values. A single transport pathway model is proposed to account for rheogenic lysine entry across the brush-border membrane in the presence and absence of Na + . This postulates an amino acid transporter in the membrane with two binding sites. One is an amino acid site specific for the α-amino-α-carboxyl group. The other is a Na + site. Neutral amino acids (e.g. leucine) compete with lysine for the amino acid site. The Na + site has some affinity for the e-amino group of lysine. When external Na + is high the Na + site is essentially ‘saturated’ with Na + and formation of a mobile complex between an amino acid and the transporter depends in a saturable fashion on amino acid concentration. In Na + -free media or in media containing low [Na + ]; (i) at low external [Lys] the e-amino group of a lysine molecule (simultaneously attached to the amino acid site) interacts with the Na + site to form a mobile complex, (ii) as external [Lys] is increased, attachment of different lysine molecules to each site of an increasing number of transporters to form nontransported or poorly transported complexes results in substrate inhibition of the rheogenic lysine transport process.