Serum iron can become elevated in a number of conditions (see e.g. Trinder et al. 2002), including acute iron poisoning, genetic disease (notably hereditary haemochromatosis, which affects around 1 in 300 people in Caucasian populations) and repeated blood transfusions. Iron overload is associated with both cardiac fibrosis and an increased risk of arrhythmias. However, the mechanisms by which iron causes these unwanted effects remain unclear. We have examined the effects of divalent (ferrous) iron (Fe²⁺) on Ca²⁺ handling and mechanical activity in rat ventricular myocytes. Rats were humanely killed and cells were isolated by collagenase-protease digestion and loaded with the Ca²⁺-sensitive fluorophores fura-2 or fluo-4, or the iron-sensitive fluorophore calcein. The cells were either field stimulated or voltage clamped in the amphotericin B-perforated patch configuration. In all experiments cells were superfused with Tyrode solution at room temperature and stimulated at a frequency of 0.5 Hz.

Iron-containing Tyrode solution contained 100 or 200 µM iron, together with 1-5 mM ascorbate to ensure that iron was in the reduced ferrous (+2) oxidation state. In field stimulation experiments, addition of Fe²⁺ reduced cell shortening and/or made cells refractory to stimulation. In voltage-clamp experiments, 100 or 200 µM Fe²⁺ caused a marked reduction in the amplitude of the Ca²⁺ current and a parallel reduction in the amount of cell shortening. Both effects were rapid and reversible. Interestingly, the reduction in Ca²⁺ current and shortening was not accompanied by any detectable changes in sarcoplasmic reticulum Ca²⁺ loading, suggesting that the decrease in trigger L-type Ca²⁺ current produced by Fe²⁺ was solely responsible for the decrease in shortening. Finally, we only rarely observed any irreversible effects of Fe²⁺ and ascorbate that might be ascribable to oxidant stress, although this might reflect the short duration of Fe²⁺ exposure and/or the relatively low concentration of Fe²⁺ used.

The effects of Fe²⁺ reported here are consistent with block of L-type Ca²⁺ channels by sub-millimolar concentrations of Fe²⁺. This contrasts with a previous report by Tsushima et al. (1999) who found that Fe²⁺ inhibited L-type Ca²⁺ currents in ventricular myocytes only at concentrations of 2 mM or greater. The reason for this discrepancy is not known. Overall, our results suggest that, like other di- and trivalent metal cations, Fe²⁺ is an effective blocker of voltage-gated Ca²⁺ entry in the heart.

This work was supported by the BHF and Royal Society