14.1 Philip A Marsden, MD Nitric oxide and the kidney Dep. Of Nephrology R1 In Ah Choi

Major role as a messenger molecule 1) In the blood vessel - basal and calcium-agonist stimulated release of NO  bioactivity of endothelium-derived relaxing factor(EDRF) 2) In the kidney, other solid organs - tonic vasodilator, working essentially instantaneously ** higher concentrations - can be toxic, including damaging cellular constituents (such as DNA) - inducing hypotension in those with sepsis Nitric oxide (NO) INTRODUCTION

NO, a molecular gas, is formed by three isoform enzyme, nitric oxide synthase (NOS) 1) nNOS (NOS1) - neuronal NOS - constitutively active, producing low level of NO 2) iNOS (NOS2) - inducible or macrophage NOS - inducible by inflammatory stimuli - extremely large amount 3) eNOS (NOS3) - endothelial NOS - constitutively active, producing low level of NO Nitric oxide synthase (NOS) BASIC PHYSIOLOGY

Functional Mechanism of Nitric oxide (NO) - paracrine mediator (different from AGII, ADH; endocrine) - produced and released by individual cells - readily penetrates the biological membranes of neighboring cells - modulating a number of signaling cascades - extremely short half-life, its effects locally and transiently Nitric oxide BASIC PHYSIOLOGY

The most recognized cellular target of NO BASIC PHYSIOLOGY NO guanylate cyclase cGMP in cytosol GTP + +

Other cellular target of NO BASIC PHYSIOLOGY - interacts with thiol groups on proteins and small molecules  formation of S-nitrosothiols - target Fe/S groups at the catalytic centers of proteins, including hemoglobin - formation of peroxynitrite from NO and superoxide radical  cellular toxicity ; posttranslational changes in the tyrosine residues of proteins  effects of NO depend upon - concentration of NO - local environment, the presence of thiols and superoxide

Expression of nitric oxide in the kidney ** all three NOS isoforms can be expressed in the kidney 1) nNOS - macula densa, the inner medullary collecting duct 2) iNOS - localized to several tubule segments (thick medullary ascending limb, distal convoluted tubule, proximal tubule) - glomerulus, interlobular and arcuate arteries 3) eNOS - expressed in the endothelium of glomerular capillaries, afferent and efferent arterioles, and intrarenal arteries ** greatest enzymatic activity for NO production in the kidney in the inner medullary collecting duct (IMCD): 3~6 fold higher than in the glomeruli

NITRIC OXIDE AND THE KIDNEY 1) regulation of renal hemodynamics 2) modulation of fluid and electrolyte transport 3) regulation of damage in response to injury Important of renal actions of NO in the kidney

1) Renal hemodynamics - eNOS is expressed to a variable extent in endothelial cells of afferent arteriole, glomerulus, and efferent arteriole ** endothelial-derived NO - plays a major role in maintaining arteriolar dilation by paracrine control of renal glomerular vascular resistance and mesangial cell tone Effects on renal flow

1) Renal hemodynamics - acute systemic NOS inhibition  increases in afferent and efferent arteriolar resistances  fall in glomerular capillary ultrafiltration coefficient - chronic systemic blockade of NO bioactivity  significant glomerular capillary hypertension  efferent arteriolar resistance, increased AGII, ET-1

1) Renal hemodynamics In animal model - deletion of the genes for eNOS  hypertension - chronic inhibition of NO synthesis  hypertension - overexpress eNOS transgenic mice  hypotension Inhibition of NO synthesis or deletion of eNOS genes  significant reductions in renal plasma flow and glomerular filtration rate parallel the development of hypertensoin Effects on systemic blood pressure

1) Renal hemodynamics - enhanced glomerular filtration, renal plama  blocked by nonselective NOS blocker - increased eNOS, not iNOS in glomerulus  the hemodynamic effects were due to enhanced eNOS activity in diabetic nephropathy In clinical setting

2) Solute and water transport Increased salt intake  intrarenal NO synthesis ↑  facilitating renal sodium excretion acute or chronic blockade of NO synthesis  impairs urinary sodium excretion, even at concentrations that do not affect renal glomerular or systemic hemodynamics ex) in human, chronic administration of a Nos inhibitor  40% reduction in the fractional excretion of sodium Salt balance

2) Solute and water transport - NO inhibits sodium entry in the cortical collecting duct Na-H exchange in the proximal tubule Na-K-2Cl cotransporter in the outer medulla Na-K-ATPase activity in varied nephronal setments - NO impairs the responsiveness of the collecting tubule to ADH  facilitating the excretion of water Mechanism

3) Tubuloglomerular feedback - NO syntesized by nNOS in the macula densa blunts the tubuloglomerular feedback (TGF) response ** TGF response : NaCl ↑ in macula densa  GFR ↓  maintain distal flow Mechanism

3) Tubuloglomerular feedback high NaCl in macula densa  increased release of NO from macula densa  countering the afferent arteriole constriction elicited in the TGF response ** the response is appropriately blunted with a high salt diet  maintenance of glomerular filtration promotes excretion of the excess salt Mechanism

4) Role in renal injury - tonic vasodilator, inhibit platelet activation and adhesion - minimize injury in glomerulonephritis - inactivation of the eNOS gene  increased the sensitivity to glomerular injury - varies based upon the cell-type and NO isoform 1) eNOS

4) Role in renal injury - exacerbate damage - by mesangial, tubular epithelial cell - in response to inflammatory stimuli - at basal condition, do not express anty of the NOS isoforms in mesangial cell - documented in human GN, animal models of glomerular injury, in vitro experiments using inflammatory cytokines ex) TNF exposure to mesangial cells  30 fold increase in cGMP first 8hrs maximal by 24hrs 2) iNOS

4) Role in renal injury Inflammatory stimuli NO↑ Suppressing eNOS vasoconstriction Cell damage PEROXYNITRATE SUPEROXIDE