Immunohistochemical localization of haemoglobin binding sites in the distal tubule of the rat kidney.

Although it is well established that haemoglobin can be taken up by kidney tubular epithelium, the exact mechanism of the process has not been elucidated so far. We have undertaken a study to determine whether any specific binding sites for haemoglobin are present on the membranes of renal tubular cells. Paraffin sections of rat kidney cortex were incubated with haemoglobin, and the bound molecules were detected by means of a combined avidin-peroxidase and ImmunoMax method. Haemoglobin binding sites were observed in the apical membrane of distal tubules. Binding occurred for both rat haemoglobin and swine and human haemoglobins, and the proteins could compete with each other. Competition experiments with other proteins showed that the binding is specific for haemoglobin and that the net charge of the protein is not critical for the interaction. We failed to detect the binding sites in proximal tubules, where most of the filtered proteins are reabsorbed. The role of the binding sites in the distal nephron is unclear. Our findings may be essential for the further understanding of the pathomechanism of haemoglobin-induced acute renal failure.