Bidirectional active transport of thiosulfate in the proximal convolution of the rat kidney

Using the standing droplet method in the late proximal convolution and simultaneous microperfusion of the peritubular capillaries, the zero net flux transtubular concentration difference of thiosulfate at 45 s was determined, the latter being taken as a measure of active thiosulfate transport. Under control conditions, in the presence of Na+, near zero delta c values were observed. When 1 mmol/l carinamide or paraaminohippurate (PAH) were added to the perfusates significant reabsorptive delta c arose. However, when 7.5 mmol/l sulfate was added to the Na+ containing perfusates or when the perfusates were Na+-free secretory delta c values were observed. Tested under Na+-free conditions, the secretory delta c was not influenced by simultaneously present 5 mmol/l of SO2-4 but was diminished by 50 mmol/l SO2-4. PAH (1 mmol/l), carinamide (0.2 mmol/l) and probenecid (1 mmol/l) decreased the secretory delta c by 48, 65 and 48%, respectively. The PAH secretion was not influenced, when thiosulfate or sulfate up to 50 mmol/l was added to both perfusates. Under Na+-free conditions the delta c of thiosulfate in early loops of the proximal convolution is higher than in late loops, while for PAH this pattern is reversed. Taken together with the previously published inhibition of sulfate reabsorption by thiosulfate the data indicate 1. thiosulfate is reabsorved by the Na+-dependent sulfate transport system and 2. thiosulfate is simultaneously secreted by a carinamide-, probenecid- and PAH-sensitive secretory system. The secretory system might also be shared by sulfate. The thiosulfate net flux is the result of the difference in the activity of the counteracting transporters, located at the luminal and contraluminal cell side. Is it possible that the higher activity of the transporter at one cell side leads to a reversal of the flux through the transporter at the other cell side.