Effects of Fe(2+) on ion channels: Na(+) channel, delayed rectified and transient outward K(+) channels

Iron accumulation in bronchial epithelial cells is dependent on concurrent sodium transport

Airway epithelial cells prevent damaging effects of extracellular iron by taking up the metal and sequestering it within intracellular ferritin. Epithelial iron transport is associated with transcellular movement of other cations including changes in the expression or activity of Na, K-ATPase and epithelial Na(+) channel (ENaC). Given this relationship between iron and Na(+), we hypothesized that iron uptake by airway epithelial cells requires concurrent Na(+) transport. In preliminary studies, we found that Na(+)-free buffer blocked iron uptake by human airway epithelial cell. Na(+) channels inhibitors, including furosemide, bumetanide, and ethylisopropyl amiloride (EIPA) significantly decreased epithelial cell concentrations of non-heme iron suggesting that Na(+)-dependent iron accumulation involves generalized Na(+) flux into the cells rather than participation of one or more specific Na(+) channels. In addition, efflux of K(+) was detected during iron uptake, as was the influx of phosphate to balance the inward movement of cations. Together, these data demonstrate that intracellular iron accumulation by airway epithelium requires concurrent Na(+)/K(+)exchange.

The effects of ferric iron on voltage-gated potassium currents in molluscan neurons

In isolated neurons of Helix pomatia, a two-microelectrode voltage clamp technique was used to study the effect of Fe3+ on voltage-gated potassium currents: a low-threshold fast-inactivating current (I(A)) and a high threshold slow-inactivating current with calcium-dependent (I(C)) and calcium-independent (I(DR)) components. Extracellular application of FeCl3 rapidly, reversibly and dose-dependently reduced the amplitude of I(A), I(C) and I(DR) with IC50 values of 49, 45 and 70 microM, respectively. Complete inhibition of K+ currents was reached at 100-500 microM Fe3+. The threshold for the total slow-inactivating potassium current shifted in a positive direction by 10-30 mV in the presence of Fe3+ (50-300 microM). Our work is the first demonstration of the strong blocking effect of Fe3+ on potassium currents of neuronal membrane.

Left ventricular systolic function during stress echocardiography exercise in subjects with asymptomatic hereditary hemochromatosis

There is no information available on left ventricular (LV) systolic function and the response to stress echocardiography in asymptomatic subjects with hereditary hemochromatosis (HH). To evaluate this topic, 43 asymptomatic subjects with HH homozygous for the C282Y HFE gene mutation (22 untreated subjects [group A] and 21 long-term treated subjects [group B]) were compared with 21 age- and gender-matched normal volunteers negative for HFE mutations. Contractile reserve, as a measure of LV systolic function, was assessed using continuous echocardiographic imaging and electrocardiography during supine bicycle exercise. Nineteen subjects in group A had repeat tests after 6 months of induction phlebotomy therapy to assess the effect of iron removal. Exercise performance and hemodynamic variables of supine bicycle exercise were comparable between subjects with HH and controls. LV contractile reserve of asymptomatic subjects with HH was not impaired at either a 75-W submaximal exercise level (mean +/- SD difference in ejection fraction from baseline 13.8 +/- 6.2%, 11.5 +/- 6.8%, and 13.4 +/- 7.8% in groups A, B, and C, respectively; p = NS for all by analysis of variance) or at peak exercise (difference in ejection fraction from baseline 18.9 +/- 6.9%, 18.4 +/- 7.8%, and 20.3 +/- 8.1% in groups A, B, and C, respectively; p = NS for all by analysis of variance). However, the incidence of abnormal ischemic stress electrocardiographic responses was more frequent in subjects with HH as a whole (33%) compared with normal subjects (10%). Stress imaging revealed no regional wall motion abnormalities, suggesting that these were false-positive results. Iron removal by induction phlebotomy did not affect stress echocardiographic performance. In conclusion, LV systolic function during exercise in asymptomatic subjects with HH is preserved, and 6-month induction phlebotomy does not affect stress echocardiographic performance.

Fe2+ decreases the taurine-induced Cl- current in acutely dissociated rat hippocampal neurons

The effects of ferrous ions (Fe(2+)) on taurine-induced Cl(-) current (I(tau)) recorded from single neurons, which was freshly isolated from the rat hippocampal CA1 area, were studied with conventional whole-cell recording under voltage-clamp conditions. Using standard pharmacological approaches, we found that the currents gated by concentrations of taurine (<or=10 mM), which existed in about 90% of the hippocampal neurons tested, were predominantly mediated by strychnine-sensitive glycine receptors. When co-applied with taurine, Fe(2+) effectively depressed I(tau) in a concentration-dependent manner, with an IC(50) of 3.76 mM and Hill coefficient of 1.01, while preincubation with 1 mM Fe(2+) alone did not affect the following membrane currents elicited by taurine. The result suggests that resting taurine-gated channels are insensitive to Fe(2+). Since internal cell dialysis with 3 mM Fe(2+) failed to modify I(tau), it was deduced that the site of action of Fe(2+) is extracellular. Furthermore, the Lineweaver-Burke double reciprocal plot of normalized response to taurine against the concentration of taurine illustrated that the depression of I(tau) was noncompetitive, therefore Fe(2+) may act on the glycine receptor-chloride ionophore complex at a site distinct from where taurine binds. Various concentrations of Fe(2+) ranging from 0.1 to 20 mM depressed I(tau) and this extracellular depression was independent of membrane voltage. These results indicate that Fe(2+) decreases I(tau) in acutely dissociated rat hippocampal neurons and the inhibition of glycine receptors by Fe(2+) might be one possible approach through which Fe(2+) induces seizures.