Several tissues of northern elephant seals (Mirounga angustirostris) routinely experience ischemia/reperfusion events during dives without evidence of damage often observed in terrestrial mammals. Notably, this species has the highest known mass-specific blood volume of mammals, while also possessing carbon monoxide (CO) concentrations in blood and skeletal muscle that can be 10x higher than values measured in healthy humans. Given the emerging interest of CO as a cellular protectant against oxidative stress, this study investigated the potential cytoprotection associated with endogenous CO production in elephant seal and human cell cultures, specifically targeting the heme degradation pathway between species. Arterial endothelial cells from elephant seal placenta and human umbilicus were exposed to free heme (0-25µM) to stimulate heme oxygenase (HO) activity, the enzyme directly responsible for endogenous CO production during heme degradation. Viability of seal cells was 11-28% higher in the presence of heme compared to human cells, where baseline HO-1 protein content was 16x higher in seal cells. Seal cells were more responsive to low doses of heme, where peak Hmox1 gene expression was upregulated 654-fold compared to human HMOX1 at 53-60-fold. Moreover, confocal analysis showed heme treatments correlated with increased HO-1 nuclear content for both species, where this increase was 2.66x higher in seal cells. Despite these dramatic differences in HO-1 protein content and gene expression between the two species, HO activity was similar between species at low concentrations of heme (heme limited treatment), while maximum HO activity (surplus heme treatment) was 4.2x lower for seal cell lysates. This could be explained by (1) sequestration of heme by other proteins (ex. heme regulatory motifs on HO-2 can bind heme without degrading it, where the HO-2 content between the two species is still unknown), (2) higher rates of HO-1 nuclear translocation in seal cells, where nuclear HO-1 is not known to be catalytically active, or (3) lower HO activity serving as an adaptation in seal cells to prevent excessive release of free ferrous iron (Fe 2+ ) from heme molecules. To investigate #3, we found baseline total iron content (Fe 2+ and Fe 3+ ) and accumulation following heme exposure were higher in seal cells, where previous studies report lower ferritin levels (which sequesters Fe 2+ ) in elephant seal blood relative to human values. Despite increases in total iron content following heme treatments, there was no difference in hydrogen peroxide (H 2 O 2 ) levels in seal cells, while low doses of heme decreased H 2 O 2 levels in human cells to values comparable to seals. Meanwhile, heme treatments significantly reduced superoxide anion (O 2 - ) levels in both species, though O 2 - was still significantly higher in human cells. This reduction in reactive oxygen species (ROS) suggests (1) byproducts of HO activity (i.e., CO, biliverdin) help mitigate oxidative stress (e.g., direct degradation of H 2 O 2 , inhibition of ROS-producing enzymes, stimulation of signaling pathways) or (2) nuclear translocation of HO-1 following heme treatments enables transcription of antioxidants in both species. This study highlights a natural mechanism through which ischemia/reperfusion-tolerant animals may avoid injury associated with oxidative stress and supports further exploration of the HO/CO pathway for therapies in human medicine. Funded by NSF Award #1927616 This abstract was presented at the American Physiology Summit 2026 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.
Cotoia et al. (Fri,) studied this question.