Iron is essential to the cell. Both iron deficiency and overload
impinge negatively on cardiac health. Thus, effective iron homeostasis
is important for cardiac function. Ferroportin (FPN), the only
known mammalian iron-exporting protein, plays an essential role
in iron homeostasis at the systemic level. It increases systemic iron
availability by releasing iron from the cells of the duodenum,
spleen, and liver, the sites of iron absorption, recycling, and storage
respectively. However, FPN is also found in tissues with no known
role in systemic iron handling, such as the heart, where its function
remains unknown. To explore this function, we generated mice
with a cardiomyocyte-specific deletion of Fpn. We show that these
animals have severely impaired cardiac function, with a median
survival of 22 wk, despite otherwise unaltered systemic iron status.
We then compared their phenotype with that of ubiquitous hepcidin
knockouts, a recognized model of the iron-loading disease hemochromatosis.
The phenotype of the hepcidin knockouts was far
milder, with normal survival up to 12 mo, despite far greater iron
loading in the hearts. Histological examination demonstrated that,
although cardiac iron accumulates within the cardiomyocytes of
Fpn knockouts, it accumulates predominantly in other cell types
in the hepcidin knockouts. We conclude, first, that cardiomyocyte
FPN is essential for intracellular iron homeostasis and, second, that
the site of deposition of iron within the heart determines the severity
with which it affects cardiac function. Both findings have
significant implications for the assessment and treatment of cardiac
complications of iron dysregulation.