[Active transport of organic acids in the proximal tubules of a surviving rat kidney normally and under certain actions. I. The effect of temperature, aeration conditions and Na ions].

The transport of a marker organic acid (fluorescein) in the intact proximal tubules was studied with the aid of contact microfluorimetry of the surface of surviving rat kidneys. The kinetics of transport at 20 and 37 degrees obeys to the Michaelis-Menten equation. The increase of oxygen content in the gas phase, from 21 to 100%, results in raising V max by 1.7 times, with an apparent Km being unchanged. With the 100% oxygen content taken as a gas phase, the fluorescein transport rate is maximal at 37--40 degrees, the temperature raising from 20 to 37 degrees results in decreasing Km by 4.5 times and in increasing V max by 45%. Both Na-free medium and the addition of strophantin K inhibit fluorescein uptake at temperature higher than 25 degrees only. At 37 degrees the omission of Na+ from the bath medium inhibits the fluorescein transport via Km augmentation with Vmax being unchanged. Thus, active transport of fluorescein is Na+-dependent in physiological range of temperature and the motive force of the transport is an electrochemical Na+-gradient created by means of Na+, K+-ATPase operation.