Electrochemical sensing of the secretion of indoxyl sulfate in a rat intestinal loop using a self-assembled monolayer-modified gold bead electrode

Indoxyl sulfate is a uremic toxin that accumulates in the plasma after a decline in renal function that might progress to chronic kidney disease (CKD). This accumulation is associated with the onset of dialysis and exacerbation of CKD and cardiovascular diseases. In this study, we aimed to demonstrate intestinal secretion as an excretion pathway of indoxyl sulfate in the severe stage of CKD using electrochemical sensing. A self-assembled monolayer (SAM) of 2-mercaptobenzimidazole on gold beads was successfully used for the separate sensing of indoxyl sulfate, uric acid (UA), and ascorbic acid. This electrode could detect indoxyl sulfate at concentrations as low as 50 nM in the presence of UA. No indoxyl sulfate secretion was observed in the intestinal loop of healthy rats or those intravenously administered saline. However, indoxyl sulfate secretion was detected in the 5/6 nephrectomized rats that showed high serum indoxyl sulfate levels, which also occurs in patients with CKD stage 4 or 5 and the healthy rats intravenously injected with indoxyl sulfate. These results suggest that indoxyl sulfate is excreted through the intestine when serum indoxyl sulfate level is high. The results of the present study showed that the SAM-modified gold bead electrode can be used as an easy and sensitive method for evaluating indoxyl sulfate secreted in the intestine over time.