In the present study we investigated cardiac hypertrophy and cardiac complications in mice subjected to hyperoxia. Results demonstrate that there is a significant increase in average heart weight to tibia length (22%) in mice subjected to hyperoxia treatment vs. normoxia. Functional assessment was performed in mice subjected to hyperoxic treatment, and results demonstrate impaired cardiac function with decreased cardiac output and heart rate. Staining of transverse cardiac sections clearly demonstrates an increase in the cross-sectional area from hyperoxic hearts compared with control hearts. Quantitative real-time RT-PCR and Western blot analysis indicated differential mRNA and protein expression levels between hyperoxia-treated and control left ventricles for ion channels including Kv4.2 (Ϫ2 Ϯ 0.08), Kv2.1 (2.54 Ϯ 0.48), and Scn5a (1.4 Ϯ 0.07); chaperone KChIP2 (Ϫ1.7 Ϯ 0.06); transcriptional factors such as GATA4 (Ϫ1.5 Ϯ 0.05), Irx5 (5.6 Ϯ 1.74), NFB1 (4.17 Ϯ 0.43); hypertrophy markers including MHC-6 (2.17 Ϯ 0.36) and MHC-7 (4.62 Ϯ 0.76); gap junction protein Gja1 (4.4 Ϯ 0.8); and microRNA processing enzyme Drosha (4.6 Ϯ 0.58). Taken together, the data presented here clearly indicate that hyperoxia induces left ventricular remodeling and hypertrophy and alters the expression of Kv4.2 and MHC6/7 in the heart. ion channel regulation; hyperoxia; heart; hypertrophy; potassium channel; redox PATIENTS in critical or intensive care units (ICU) with acute lung injury or cardiac disease are often administered 100% O 2 for treatment. Recent studies indicate that hyperoxia induces cardiac injury due to dysfunctional lung and compromised pulmonary functioning (37), even though the exact nature of this problem remains unknown. Here, we evaluated changes in expression of the ion channel and key transcriptional factors in the heart that occur with hyperoxia and likely play a role in cardiovascular remodeling.Potassium channels and their auxiliary subunits such as potassium channel interacting protein-2 (KChIP2) are abundantly expressed in the heart (5, 7, 35). It is established that the potassium channels Kv4.2 and Kv1.5 are responsive to oxygen changes (29, 39). In the present study, we investigate whether hyperoxia alters expression of the transcription factors Irx5 and Mef2c, which are implicated to play a direct role in regulating Kv4.2 expression (7,15,22). Cardiac-specific markers used to identify hypertrophy and transcriptional changes (9, 22) were also evaluated by assessing myosin heavy chain-6, and -7 (MHC6, MHC7), zinc finger transcription factor (GATA4), histone-lysine N-methyltransferase (Ezh2), and Six-1 expression levels with hyperoxia.Key inflammatory mediators such as TNF␣ and NFB are central regulators or master switches for many pathological processes (10,20,30). Recent evidence indicates that NFB regulates KChIP2, which in turn regulates Kv4.2 expression (26). Therefore we assessed the levels of Kv4.2, KChIP2, and NFB in the mouse heart subjected to hyperoxia. We hypothesized that hyperoxia induces cardi...