acidosis (EA) has profound effects on vascular homeostasis, including vascular bed-specific alterations in vascular tone. Regulation of gene expression by EA has been observed in a variety of cells including vascular endothelial cells. Whether EA regulates gene expression in vascular smooth muscle cells (VSMCs) is not known. Heme oxygenase (HO)-1 is expressed in vascular cells, and its expression is regulated by cellular stressors such as heat, radiation, and hypoxia. Increased HO-1 expression in VSMCs leads to increased production of CO and its second messenger cGMP, which are important regulators of vascular tone and paracrine interactions in the vasculature. We examined whether EA regulates the expression of HO-1 in VSMCs. Exposure of VSMCs to acidic medium (pH 6.8) significantly increased HO-1 mRNA and protein compared with exposure to medium of physiological pH (pH 7.4). The acidic induction of HO-1 expression was time dependent and involved both transcriptional activation of the HO-1 gene and enhanced stability of HO-1 mRNA. Nitric oxide did not appear to mediate this response. We conclude that HO-1 is transcriptionally and posttranscriptionally upregulated by EA in VSMCs. This induction is time dependent and reversible. We speculate that EA, as an important tissue and cellular stressor for VSMCs, may elicit changes in gene expression patterns that contribute to the maintenance or disruption of vascular homeostasis. transcriptional activation; ribonucleic acid stabilization EXTRACELLULAR ACIDOSIS (EA) is a common clinical state that has profound cardiovascular effects. Although extracellular pH is normally maintained within narrow limits around 7.4, a number of acute and chronic disorders disrupt the acid-base homeostatic mechanisms and lead to systemic acidemia. In addition, local tissue acidosis is found in areas of inflammation, ischemia, and/or hypoxia as well as within solid tumors (36). Systemic acidemia leads to vascular bed-specific alterations in vascular tone and modulation of cardiac contractility. Local tissue acidosis is an important modulator of tumor invasiveness (20) and susceptibility of tumors to radiation, chemotherapy (20), and antiangiogenic therapy (34). The cellular and molecular events mediating the effects of acidosis on vascular homeostasis are incompletely understood. Because acidosis often coexists with hypoxia, the role of acidosis as an independent modulator of vascular cell behavior has, until recently, been overlooked. However, recent studies in vitro and in vivo support the idea that EA has cellular effects independent of hypoxia (2,9,21,27,32,35).Heme oxygenase (HO) and its enzymatic products biliverdin and CO have received increasing recognition as biologically important modulators of cellular interactions in the vasculature. The inducible form of this enzyme, HO-1, is ubiquitously distributed in mammalian tissues, and its expression is regulated by a number of nonheme inducers such as cytokines, heavy metals, hormones and endotoxin (1). In addition, a number of cellular ...