Renal tubular mechanisms of organic solute transport

Abstract

Using micropuncture techniques in isolated renal tubules it is possible to measure the active transport, passive backflux, electrical potential difference, co transport with ions such as sodium (NA+) linked with substances such as amino acids and glucose. Studies using a variety of sugars and substituted sugars show that some configurations confer preferential selective transport which presuppose specific binding sites. The affinity of the binding site increases with Na+ concentration which also influences the effect of phlorrhizin on D glucose uptake. While the D glucose influx into brush border membrane vesicles is Na+ dependent and could be inhibited by phlorrhizin but only to a small extent by its aglucone phoretin the D glucose uptake by basolateral membrane vesicles is almost completely Na+ independent and could be inhibited by phloretin and not by phlorrhizin. Studies with double microperfused rat kidney tubules reveal separate transport systems for basic aminoacids, acidic aminoacids, and for the iminoglycine group. Again amino acid transport is linked to Na+ transport and evidence that hydrogen ions might also be involved is being sought. Conclusions about the transport of organic acids and bases are less certain and suggest multiple mechanisms involving A.T.P. pumps linked on occasion with Na+ or H+ (they may be inhibited by ouabain) and by non ionic diffusion. Finally, renal pinocytosis of larger molecular weight substances is reviewed and carrier phospholipids and proteins are mentioned. (Parsons - London)