In vivo microinjections of 55FeCl3 were made to assess renal iron (Fe2+/3+) transport in the anaesthetized rat. Following microinjection into proximal convoluted tubules (PCTs), 18·5 ± 2·9 % (mean ± s.e.m., n= 11) of the 55Fe was recovered in the urine. This recovery was not dependent on the injection site indicating that iron is not reabsorbed across the surface convolutions of the proximal tubule. Following microinjection into distal convoluted tubules (DCTs) 46·1 ± 6·1 % (n= 8) of the injected 55Fe was recovered. Taken together the recovery data from the PCT and DCT microinjection studies indicate that the transport of iron occurs in the loop of Henle (LH) and collecting duct system. In vivo luminal microperfusion was used to examine iron transport by the LH in more detail. In tubules perfused with 7 μmol l−155FeCl3, 52·7 ± 8·3 % (n= 8) of the perfused 55Fe was recovered in the collected fluid, indicating significant iron reabsorption in the LH. Addition of copper (Cu2+ as 7 μmol l−1 CuSO4), manganese (Mn2+ as 7 μmol l−1 MnSO4) or zinc (Zn2+ as 7 μmol l−1 ZnSO4) to the perfusate did not affect reabsorption of water, Na+ or K+, but increased recovery of 55Fe to 83·5 ± 6·8 % (n= 8, P < 0·04), 75·8 ± 5·9 (n= 6, not significant, n.s.) and 67·9 ± 3·8; (n= 9, n.s.), respectively. Thus, iron transport in the LH can be reduced by the addition of copper or manganese to the luminal perfusate suggesting that these ions may compete with iron for a common transport pathway. However, this pathway may not be shared by zinc.

Divalent metal transporter1 (DMT1; also known as DCT1 or NRAMP2) is an important component of the cellular machinery responsible for dietary iron absorption in the duodenum. DMT1 is also highly expressed in the kidney where it has been suggested to play a role in urinary iron handling. In this study, we determined the effect on renal DMT1 expression of feeding an iron-restricted diet (50 mg/kg) or an iron-enriched diet (5 g/kg) for 4 wk and measured urinary and fecal iron excretion rates. Feeding the low-iron diet caused a reduction in serum iron concentration and fecal iron output rate with an increase in renal DMT1 expression. Feeding an ironenriched diet had the converse effect. Therefore, DMT1 expression in the kidney is sensitive to dietary iron intake, and the level of expression is inversely related to the dietary iron content. Changes in DMT1 expression occurred intracellularly in the proximal tubule and in the apical membrane and subapical region of the distal convoluted tubule. Increased DMT1 expression was accompanied by a decrease in urinary iron excretion rate and vice versa when DMT1 expression was reduced. Together, these findings suggest that modulation of renal DMT1 expression may influence renal iron excretion rate. serum iron level; kidney; iron regulatory protein; SLC11A2; NRAMP2 IRON IS AN ESSENTIAL metal for life because it is a key constituent of a family of fundamental proteins, which includes hemoglobin, cytochromes, and NADH-coenzyme Q reductase. Maintaining the correct balance of iron is paramount to health because iron deficiency or excess results in morbidity and mortality. The molecular characterization of membrane-bound iron transporter proteins, in particular divalent metal transporter1 (DMT1; 9), also known as DCT1 (14) or NRAMP2 (13), has shed new light on some of the mechanisms of body iron homeostasis. DMT1 is the product of the

Our results indicate that the G185R mutation of DMT1 causes protein instability in the kidneys of b/b rats. Given that +/b and b/b rats excrete comparable amounts of iron, the lack of DMT1 protein is compensated by an alternative, yet to be identified, mechanism.

A luminal microperfusion technique was used to examine the K+ permeability of surface proximal convoluted tubules (PCT) in the kidney of anesthetized rats. Transtubular potassium concentration ([K+]) gradients were varied by altering the concentration of KCl in luminal perfusates, to which 32 mmol/l of the impermeant solute raffinose was also added to prevent net fluid reabsorption. The arithmetic mean transtubular [K+] gradient was highly predictive of net potassium flux, yielding an apparent K+ permeability of 31.9 ± 1.7 × 10−5 cm/s in the absence of fluid reabsorption. When compared using identical calculation techniques, we found this was not significantly different from the permeability derived in a previous study when fluid reabsorption was present [J. D. Kibble, M. Wareing, R. W. Wilson, and R. Green. Am. J. Physiol. 268 ( Renal Fluid Electrolyte Physiol. 27): F778–F783, 1995]. We conclude that fluid reabsorption does not affect the apparent permeability of the proximal tubule to potassium. The apparent permeability to86Rb, measured following its addition to luminal perfusates, was not significantly different from the value obtained for K+, suggesting that rubidium is a useful marker for net potassium movements in the PCT of the rat.

Abstract. In vitro studies have shown that glibenclamide sensitivity is conferred upon Kir 1.1 K+ channels when they are co-expressed with the cystic fibrosis transmembrane conductance regulator (CFTR). In rats, glibenclamide acts as a K+-sparing diuretic by a mechanism that involves blockade of Kir 1.1 channels in the distal nephron. To test whether interaction between Kir 1.1 and CFTR is required to mediate the renal effects of glibenclamide (15 mg/kg), clearance experiments were performed comparing wild type (WT) and Cftrtm2cam ▵F508 cystic fibrosis (CF) mice. Glibenclamide treatment was associated with an equivalent diuresis in both WT and CF mice. Glibenclamide was K+-sparing in both genotypes with no significant change in urinary K+ excretion observed. That glibenclamide was an effective K+-sparing diuretic in CF animals suggests that CFTR expression is not a requirement to mediate its renal actions in mice.