Structure and active transport in the plasma membrane of the tubules of frog kidney.
The active transport of organic anions through the plasma membrane of the proximal tubules of frog kidney was studied. For this purpose a marker anion, fluorescein, was used, its flow into the tubules registered by the increase of fluorescense. The kinetics of transport was measured as function of time, concentration of substrate, concentration of a competing acid (p-aminohippuric acid) and temperature. The process is inhibited by strophantin, a specific poison for (Na++K+)-dependent ATPase. These data show that fluorescein transport is effected with the participation of a charged carrier, probably by the downfield mechanism postulated by Mitchell. To confirm this mechanism, a passive flow of K+ was created inwards across the membrane of the proximal tubules by means of valinomycin. It led to the discharge of the membrane and to the inhibition of fluorescein transport. Anions are transported downfield across the membrane, probably in a state of complexes with two Na+ ions. A magnetic field of 10000-28000 oersted inhibits the fluorescein transport strongly. This can be regarded as a proof of the liquid-crystalline structure of biological membranes and demonstrates the importance of this structure for active transport.