Additional studies have been made of the accumulation of S35 by renal cortical tissue incubated in media containing radiosulfate. This process was found to occur in several mammalian species in addition to the rat, but was not observed as a significant occurrence in three species of lower vertebrates. In the case of rat renal tissue, S35 uptake was found to be sensitive to the pH and osmolar concentration of the medium. The character of the anions present in conjunction with K+ affected it as well. Various factors known to be related to in vitro accumulative processes, as well as to renal sulfate reabsorption by the intact dog, were tested on rat kidney cortex to assess the effect on radiosulfate uptake. In general, all substances tested (amino acids, metabolic intermediates, ATP, metabolic inhibitors, competitive inhibitors for PAH accumulation in vitro) were found to lessen S35 uptake, or to be without effect upon it. The one striking exception was phlorhizin, which enhanced markedly S35 uptake in vitro, as it does sulfate reabsorption in vivo. Some implications of these findings have been discussed.

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