Vascular flow through tissues is regulated via a number of homeostatic mechanisms. Localized control of tissue blood flow, or autoregulation, is a key factor in regulating tissue perfusion and oxygenation. We show here that the net balance between two hypoxia-inducible factor (HIF) transcription factor isoforms, HIF-1α and HIF-2α, is an essential mechanism regulating both local and systemic blood flow in the skin of mice. We also show that balance of HIF isoforms in keratinocyte-specific mutant mice affects thermal adaptation, exercise capacity, and systemic arterial pressure. The two primary HIF isoforms achieve these effects in opposing ways that are associated with HIF isoform regulation of nitric oxide production. We also show that a correlation exists between altered levels of HIF isoforms in the skin and the degree of idiopathic hypertension in human subjects. Thus, the balance between HIF-1α and HIF-2α expression in keratinocytes is a control element of both tissue perfusion and systemic arterial pressure, with potential implications in human hypertension.HIF-alpha | arginase | vascular tone A utoregulation of vascular flow in peripheral tissues is essential both for controlling local tissue perfusion and for the regulation of systemic blood pressure. This dual role for peripheral blood flow is dependent in turn on a range of factors that act to adjust local vascular tone. A key element of this control is the balance between O 2 demand and O 2 supply (1). This balance causes increased need for oxygen to induce increased blood flow. This regulatory pathway in peripheral tissues has a direct impact on systemic arterial pressure, because peripheral vascular resistance in large part determines total vascular resistance in the arterial bed.The skin contains a very extensive series of vascular plexi. This vascular bed has a range of essential functions, which include regulating body temperature. Skin circulation also is altered in a number of disease states, including renal disease (2), hypercholesterolemia (3), peripheral vascular disease (4), heart failure, and hypertension (5). Identification of structural alterations to the subcutaneous microvasculature provides a powerful prognostic tool to predict cardiovascular events in hypertensive patients (6), and impaired microvascular vasodilation and capillary rarefaction is associated with familial predisposition to hypertension (7).The heterodimeric transcription factors hypoxia-inducible factoralpha (HIF-1α) and HIF-2α are essential for the maintenance of cellular oxygen homeostasis (8). In response to hypoxia, stabilized HIF-1α and HIF-2α proteins initiate the expression of genes that alleviate hypoxic stress, including genes promoting cell growth, adhesion, and migration, new vessel formation, and the development of vascular networks (9,10).Recent data from a number of groups have indicated that HIF-1α and HIF-2α can act in a dualistic manner to regulate a range of responses in vivo; these interactions include functionally opposing interactions with the Myc t...