Active sulfate reabsorption in the proximal convolution of the rat kidney: specificity, Na+ and HCO3- dependence

Using the standing droplet technique in the proximal convolution and simultaneous microperfusion of the peritubular capillaries, the decrease in luminal sulfate concentration with time and the zero net flux transtubular concentration difference of sulfate (delta CSO42-) at 45 s was determined - the latter being taken as a measure of the rate of active sulfate reabsorption. Starting with 0.5 mmol/l sulfate in both perfusates the delta CSO42- value of 0.35 mmol/l was approached exponentially with a half value time of 4.3 s. The delta CSO42- values in the early proximal and late proximal convolution did not deviate from each other. If the Na+ concentration in the perfusates was reduced, the delta CSO42- approached zero and extrapolated to a slightly negative value (Ci greater than Co). When 1 mmol/l ouabain was added to the perfusates delta CSO42- decreased by 66% (the latter experiments were performed in the golden hamster which is more sensitive to ouabain than the rat). 1 mmol/l thiosulfate diminished delta CSO42- by 68% and 1 mmol/l molybdate by 24%. Omitting or replacing bicarbonate by HEPES or glycodiazine reduced the sulfate reabsorption significantly, while acetazolamide (0.1 mmol/l) and increasing the CO2-pressure from 4.66 to 14.0 kPa (i.e. 5-15% CO2) had no effect. SITS 1 mmol/l had no effect on sulfate reabsorption. The data indicate that the sulfate reabsorption is driven by a Na+ gradient and inhibited by thiosulfate and molybdate, i.e. molecules which have a similar tetrahedral molecule structure. The sulfate reabsorption depends in an undefined manner on the presence of bicarbonate ions.