Lopez, Vasthi, Patricia Siques, Julio BritCarmen Vallejos, Nelson Naveas, Claudia Carvallo, Fabiola León-Velarde, and Nelson Carvajal. Upregulation of arginase expression and activity in hypertensive rats exposed to chronic intermittent hypobaric hypoxia. High Alt. Med. Biol. 10:373-381, 2009.- The aim of this study was to analyze the activity and expression levels of arginase I and II and to monitor the cardiovascular and hematological responses in tolerant and intolerant rats exposed to chronic intermittent hypobaric hypoxia (CIHH). Male Wistar rats (age: 3.0 ± 0.4 months, weight: 250 ± 25 g; n Combining double low line 30) were randomly divided into two groups: CIHH2 - 2 (2 days hypoxia, 2 days normoxia, n Combining double low line 20) and NX (normoxia, n Combining double low line 10). The hypoxia was simulated in a hypobaric chamber at 428 torr. Tolerance was determined according to a previous protocol. Arginase activity was measured in lung and heart tissues, and the expression levels were determined by a (RT-PCR) assay in lung tissue. Results showed that the intolerants rats had lower body weight, higher hematocrit (Hct) (74 ± 4% vs. 61 ± 2%, p < 0.05), higher values of systolic blood pressure (SBP) (183 ± 3.7 mmHg vs. 147 ± 5.4 mmHg, p < 0.05), and higher arginase activity. In addition, RT-PCR analysis from lung tissue showed an overexpression of arginase II in the intolerant group (p < 0.01). However, tolerants had similar values as the NX group (p Combining double low line ns). Further, a correlation was found between arginase activity and SBP in the heart (r2 Combining double low line 0.596, p < 0.001). An upregulation of arginase type II could be pivotal in understanding the pathogenesis of systolic hypertension and probably other phenomena associated with intermittent hypobaric hypoxia. A schematic explanation of these relations is proposed. © 2009, Mary Ann Liebert, Inc.