Alterations in local blood flow control mechanisms may play a major role in the transition from an elevated cardiac output to a maintained elevation in vascular resistance in volume-expanded forms of hypertension. An enhanced constriction of arterioles, in response to increased O2, availability has been demonstrated in many forms of hypertension, but little is known regarding the mechanisms that mediate O2-induced constriction of microvessels. Cytochrome P450 (CYP450) 4A omega-hydroxylase, which catalyzes the formation of 20-HETE (a vasoconstrictor metabolite of arachidonic acid), may act as an 02 sensor in the microcirculation. This study investigates the role of CYP450 4A omega-hydroxylase and 20-HETE in mediating O2-induced constriction of arterioles in the microcirculation of the Dahl S rat, a genetic model of salt sensitive hypertension, and in SS.BN13 consomic rats, in which chromosome 13 of the normotensive Brown Norway rat is substituted into the Dahl S genetic background. The SS.BN13 consomic rats are 98% identical to the Dahl S rat genetically, but do not exhibit elevated blood pressure in response to high salt diet. The overall hypothesis to be tested is that the enhanced response of arterioles to elevated PO2 in Dahl S hypertensive rats is due to one or a combination of 3 factors: increased 20-HETE production, increased sensitivity of arterioles to the vasoconstrictor effects of 20-HETE, and/or altered expression of CYP450 4A omega-hydroxylase. The effect of 20-HETE inhibition, on arteriolar responses to elevated PO2 will be determined in the in situ cremaster muscle of Dahl S and SS.BN13 rats on high salt (HS) and low salt (LS) diets. Cytochrome P450-4A omega-hydroxylase isoforms in arterioles and parenchymal cells of Dahl S rats and SS.BN13 rats on high and low salt diets will be assessed by RT-PCR and Western blotting, and changes in 20-HETE production in response to elevated PO2 will be measured in arterioles and parenchymal cells. Arteriolar constriction in response to exogenous 20-HETE will also be compared in the in situ microcirculation of Dahl S rats and SS.BN13 rats on high salt and low salt diets for various periods of time. These studies should provide an increased understanding of how the mechanisms that regulate vascular tone during changes in 02 availability, are altered during hypertension and during increases in dietary salt intake