Membrane mechanisms for transepithelial amino acid absorption and secretion

Abstract

Although amino acids are normally almost completely absorbed by the proximal nephron, specific amino acids may also exhibit active secretion under the appropriate circumstances. This brief review examines details of the influx and efflux transport processes across both lumina and basolateral membranes of the proximal tubule and through the paracellular pathway as determinants of the direction and rate of transepithelial amino acid transport. Net absorption depends upon active uptake from the lumen into tubular cells by cotransport with Na+. At least some proximal tubule cells also actively accumulate amino acids from their peritubular environment by active transport across basolateral membranes. The net direction of transepithelial movement depends on the relative rates of passive amino acid exit from the cell across luminal and basolateral membranes. Since exit across the basolateral membrane appears to be facilitated whereas luminal exit is not, normally net absorption occurs. The minimum luminal amino acid concentration and excretion rate are also dependent on a relatively low paracellular amino acid permeability, especially in the latest segments of the proximal nephron where the transepithelial concentration gradient is the greatest.

Cystine Fluxes Across the Isolated Jejunal Epithelium in Cystinuria: Increased Efflux Permeability at the Luminal Membrane

“…In the context of the present study, the most relevant observation is that in cystinuric patients, renal cystine clearance may exceed GFR, whereas dibasic amino acid clearances always are less than GFR. Although the precise mechanism of cystine secretion in the tubule has not been established, several authors have suggested cell-to-lumen secretion of cystine or cysteine combined with reduced entry across the luminal membrane (1,8,24). Our results are consistent with secretion but not with reduced entry.…”

“…In the simplest absorption model amino acid is pumped into the cell against a concentration gradient and leaves it downhill. The balance between the two processes determines the apparent concentrative power of the cell (10,24,29). In the mucosal membrane of our control subjects, the uphill cystine transport was characterized by asymmetry of the apparent permeability coefficients (Pmc 5 times greater than Pcm).…”

Renal transport of taurine adapts to perturbed taurine homeostasis.

“…Net transepithelial flux is dependent on uptake at the luminal membrane and efflux at the antiluminal surface (17,18). We observed an adaptive response in the isolated brush-border membrane and in the Physiological Sciences: Rozen renal cortex slice; the latter preparation exposes the basolateral membrane primarily (19,20).…”

“…Because transport at the brush-border membrane controls net reabsorption of amino acids (17,18), it seems that adaptation of a membrane function represents a specific response to maintain whole-body taurine homeostasis.…”

Neutral Amino Acid Transport in Sheep External Intercostal Muscle

“…FoR.snenc, N. 8., KANEPS Guidotti et al 1978;Schafer and Barfuss 1980;Christensen 1982Christensen , 1984Christensen , 1985Shotwell et al 1983, for reviews). System A has broad specihcity for neutral amino acids and, in addition to its ability to transport aminoisobutyric acid (AIB), is able to transport N-methylated derivatives of amino acids such as N-methylaminoisobutyric acid (MeAIB) (Guidotti et al 1978;Shotwell et al 1983).…”