The juxtaglomerular apparatus (JGA) contains a well defined L-arginine- NO signalling pathway between sites of generation in the endothelium or the macular densa (MD) and the afferent arteriole. Neuronal NOS in the MD is activated by solute reabsorption and blunts tubuloglomerular feedback (TGF). We propose to investigate the interaction of reactive oxygen species (ROS), e.g., oxygen radicals (O2.-) and H2O2, with these signalling pathways. We report that the SHR is a robust model of oxidative stress in the JGA, where enhanced ROS generation leads to defective NO signalling despite apparent ongoing NO generation. Our hypothesis is that excessive action of angiotensin II (Ang II) acting on AT1 receptors in the JGA of the SHR mobilizes intracellular Ca++ that activates endothelial and nNOS. However, a coincident increase in ROS generation by oxidases such as NADPH oxidase or NOS itself reduces bioactive [NO] and enhances TGF. The reduction in [NO] potentiates TGF- induced vasoconstriction of the afferent arteriole, reduces RBF, and thereby decreases O2 delivery. Moreover, a reduction in [NO] enhances mitochondrial respiration and diminishes Na+ reabsorped per O2 used (Q/Na). A consequent fail in pO2 limits the ongoing ROS generation and restrain damage by prolonged oxidant stress. We propose an escalating set of protocols.
Specific Aim 1 will combine video-microscopic measurements of vessel diameter with quantitative fluorescence techniques for O2.- and H2O2 in rabbit isolated, perfused afferent arterioles. We will use paraquat for dose-dependent generation of O2.-, and the membrane permeable superoxide dismustase (SOD) mimetic tempol for dismutation to H2O2. We will investigate the effects of Ang II on Q.- (which can be degraded by tempol administration) and H2O2 (which can be degraded by catalase), and the L-arginine-NO system. We can vary [O2] to study directly its effects on oxidases and NOS.
Specific Aim 2 will combine microperfusion studies of TGF with novel measures of nephron pO2 in SHR. We will assess MD signalling via NO from responses to inhibition of nNOS and via O2.-from response to dismutation with tempol. Variation in nephron pO2 will permit study of its regulation of oxidase activity.
Specific Aim 3 will investigate the regulation of oxidase activity in the JGA by prolonged effects of Ang II. We will combine a molecular biology study of NADPH oxidase, SOD, and NOS isoforms with micropuncture studies of TGF and pO2 during prolonged inhibition of AT1 and AT2 receptors or ROS to address directly the hypothesis that Ang II via A1T activates ROS-generating oxidases in proportion to pO2 that underlie the functional defect in NO signaling in this model of oxidative stress.
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