Dysfunction of the endothelium dependent relaxation factor/nitric oxide (EDRF/NO) in blood vessels accompanies salt sensitive hypertension, diabetes mellitus (DM) and cardiovascular disease. Asymmetric dimethylarginine (ADMA) impairs EDRF/NO either by competing with the NOS substrate, L-arginine for membrane transport via system y+, or by competitive inhibition of NOS. Therefore, ADMA could underlie defective EDRF/NO responses that contribute to cardiovascular disease. However, the normal regulation of ADMA has not been extensively studied. ADMA is metabolized by dimethylarginine dimethylaminohydrolases (DDAH-1 and -2). We have found that DDAH-2 is heavily expressed in the kidney, especially in the macula densa cells and distal nephron. DDAH-1 is heavily expressed in the vascular endothelium and proximal tubules. Our studies show that Angiotensin II upregulate DDAH-2 (and thereby may reduce macula densa ADMA) but downregulates DDAH-1 (and thereby may increase endothelial ADMA). Dietary salt restriction and early insulinopenic DM both impairs EDRF/NO. This defect can be overcome by excess arginine, suggesting that NO may be inhibited by ADMA. Indeed, we have found that salt restriction decreases plasma arginine, yet increases plasma ADMA and impairs the EDRF/NO responses to acetylcholine of isolated mesenteric resistance vessels. In contrast, we have found that DM increases renal DDAH-2 expression, leading to reduced renal ADMA levels. A consequent increase in macula densa NO could block the vasoconstrictive tubuloglomrular feedback (TGF) response and thereby vasodilate the renal afferent arteriole selectively, leading to glomerular hyperfiltration. We propose to test the hypothesis that arginine delivery, transport and metabolism by NOS in the kidneys and microvascular resistance vessels is differentially regulated by DDAH thereby leading to site- and cell-specific generation of ADMA and NO during physiologic adaptations to changes in salt intake and pathophysiologic responses to insulinopenic DM. We have developed a gene silencing strategy targeting DDAH-1 and -2 in the rat to test the specific roles of these isoforms in physiologic studies of EDRF/NO in resistance vessels and TGF in the kidney. The first two aims test the hypothesis that dietary salt restriction limits NO generation in vascular endothelium because of a reduction in plasma arginine concentration and a decrease in DDAH-1 activity in endothelial cells. This could reduce the intracellular arginine: ADMA concentration, thereby inhibiting EDRF/NO. We propose to examine the specific roles of angiotensin and mineralocorticosteroid receptors.
The third aim tests the hypothesis that during early insulinopenic DM downregulation of DDAH-1 in endothelium underlies the defective EDRF/NO that precedes vasculopathy whereas upregulation of DDAH-2 in the macula densa enhances local NO that blocks the tubuloglomerular feedback (TGF) response causing hvperfiltration that precedes nephropathv.
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