Iron Is Filtered by the Kidney and Is Reabsorbed by the Proximal Tubule

Area of Science:

• Nephrology
• Renal Physiology
• Mineral Metabolism

Background:

• The kidney plays a crucial role in maintaining mineral homeostasis.
• Iron (Fe) reabsorption in the renal proximal convoluted tubule (PCT) has been speculated but not definitively demonstrated in vivo.
• Understanding Fe handling by the PCT is vital for comprehending overall Fe balance.

Purpose of the Study:

• To determine the iron concentration profile within the S2 renal proximal convoluted tubule (PCT) lumen.
• To investigate whether the S2 PCT actively transports iron.
• To elucidate the characteristics of the Fe reabsorption pathway in the PCT.

Main Methods:

• In vivo renal micropuncture was performed on Wistar rats.
• Tubular fluid was collected from superficial PCTs for Fe concentration measurement.
• Microperfusion experiments using solutions containing Fe and other minerals (Zn, Cu, Mn, Cd) were conducted.

Main Results:

• The Fe concentration gradient along the S2 PCT indicated significant Fe reabsorption.
• PCTs reabsorbed 105.2±12.7 fmol.mm-1.min-1 Fe, 435±52 pmol.mm-1.min-1 Na, and 2.7±0.2 nl.mm-1.min-1 water.
• Fe reabsorption was independent of ascorbate and unaffected by the presence of Cu, Mn, Cd, or Zn, but was inhibited by excess unlabeled Fe, suggesting a specific transport mechanism.

Conclusions:

• The S2 PCT in rats actively reabsorbs iron in vivo.
• The PCT utilizes a specific pathway for Fe reabsorption that is insensitive to common divalent cations like Cu, Mn, Cd, and Zn.
• These findings provide the first direct evidence of renal iron filtration and subsequent tubular reabsorption in a living mammal.