Characteristics of a cationic amino acid transport system in the basolateral membrane of the cat salivary epithelium.

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Abstract
The specificity and kinetics of L‐lysine influx across the basolateral surface of the cat salivary epithelium have been investigated in the perfused cat submandibular gland using a high‐resolution, paired‐tracer dilution technique. L‐lysine influx was measured at several different perfusate concentrations (0.05‐2.5 mM) and was found to be saturable. A Michaelis‐Menten analysis based on a single entry site gave a Km of 0.49 +/‐ 0.08 mM and a Vmax of 231 +/‐ 20 nmol/min X g. The uptake of L‐lysine was highly stereospecific and markedly inhibited by L‐arginine (0.25‐2.5 mM). The inhibitor constant (Ki) was 0.23 mM, suggesting that the carrier had a greater affinity for L‐arginine than L‐lysine. When the inhibitory effects of L‐histidine (0.5‐10 mM) were examined the Ki, estimated at 10 mM, was 4.6 mM. Nine other neutral amino acids (L‐alanine, L‐serine, L‐cysteine, glycine, L‐proline, L‐homoserine, L‐leucine, L‐phenylalanine and L‐glutamine), and an acidic amino acid (L‐aspartate) were also tested at 10 mM and, although several caused inhibition, the Ki was always at least 20 times higher than the measured Km for L‐lysine. It is concluded the carrier is highly specific for the L‐form of the basic amino acids. The sodium dependence of L‐lysine influx was investigated over a range of L‐lysine concentrations (0.05‐1 mM), and total removal of sodium from the perfusate had no effect on L‐lysine influx. In the presence of sodium, L‐homoserine, an amino acid not normally present in animal tissues, inhibited L‐lysine influx (Ki = 13 mM). This inhibition was not observed in the absence of sodium, and contrasts with the observation that the inhibitory action of L‐histidine was sodium independent. The present data suggest that a specific cationic amino acid transport system is operative in the basolateral membrane of the cat salivary epithelium. The properties of this system appear to be similar to the system y+ which has been described in several other cell types.