Renal Pharmacology 1 and 2 Notes PDF

Renal Pharmacology 1 and 2 1 Renal Pharmacology 1 and 2 Lawrence H. Lash, Ph.D. Professor, Department of Pharmacology Office: 7312 Scott Hall T: 1-313-577-0475; E-mail: [email protected] Learning Objectives: OVERALL OBJECTIVE: Identify different classes of ph...

Renal Pharmacology 1 and 2 1 Renal Pharmacology 1 and 2 Lawrence H. Lash, Ph.D. Professor, Department of Pharmacology Office: 7312 Scott Hall T: 1-313-577-0475; E-mail: [email protected] Learning Objectives: OVERALL OBJECTIVE: Identify different classes of pharmacologic agents that can modify kidney function and apply knowledge of how these agents act to understand normal kidney function. SPECIFIC OBJECTIVES: 1. Identify the different classes of diuretic agents and apply this to understand the normal function of the specific nephron segments at which they act. 2. Differentiate amongst the different classes of diuretic agents as to cellular site of action, mechanism of action, and potential effects on sodium, volume, and acid-base homeostasis. 3. Distinguish the basis for use of potassium-sparing vs. potassium-wasting diuretics. 4. Describe the use of various pharmacologic agents to alter fluid balance. 5. Identify targets in the renin-angiotensin-aldosterone system that can modify renal function. 6. Evaluate the potential side effects in the use of NSAIDs with respect to kidney function. 7. Explain the use of different pharmacologic agents to stimulate mitochondrial biogenesis in the kidneys and understand their potential impact on kidney function.  Renal Pharmacology 1 and 2 2 Lecture Outline: I. Introduction. II. Diuretics. A. Overview: Formation of urine and sites of diuretic action. B. Proximal tubule: Carbonic anhydrase inhibitors, osmotic diuretics. C. Loop of Henle: Loop diuretics, NKCC2. D. Distal convoluted tubule: Thiazides, NCC, Ca2+ reabsorption. E. Collecting tubule system: Aldosterone, ENaC and ADH. III. Potassium-wasting and potassium-sparing diuretics. IV. Drugs that alter water balance: ADH agonists and antagonists. V. Drugs targeting the renin-angiotensin system (RAS). VI. Effects of NSAIDs on renal function. VII. Pharmacologic agents that modulate mitochondrial biogenesis. VIII. Summary.  Renal Pharmacology 1 and 2 3 I. Introduction. Question 1: Why talk about drugs if we are considering normal, kidney function and not treatment of renal or cardiovascular disease? Pharmacologic agents (e.g., drugs, herbal remedies) may not only serve as therapeutic agents, but also as investigative tools that have elucidated normal, physiological function. In some cases, we have learned that a specific protein is required only from studies where its activity has been specifically inhibited – e.g., thiazide and loop diuretics. Similarly, we have learned that a specific protein is required only from studies of individuals or experimental animals in whom the protein has a genetic mutation that markedly alters its activity. Example: Polycystins in Autosomal Dominant Polycystic Kidney Disease (ADPKD). Question 2: When will you study many of these drugs again and in what context? Repetition vs. reinforcement: Context matters! Example: The site and mechanism of action of many diuretic agents tells us a lot about the function of specific nephron segments in maintenance of electrolyte and fluid homeostasis. You will also learn about the use of many of these agents in the context of disease processes in the M2 curriculum: o Diuretics, RAAS drugs, vaptans and ENAC inhibitors for treatment of various forms of hypertension and volume disorders. o Use of drugs that modulate mitochondrial biogenesis to treat acute kidney injury (AKI) and chronic kidney disease (CKD).  Renal Pharmacology 1 and 2 4 II. Diuretics. A. Overview: Formation of urine and sites of diuretic action. Introduction and rationale: Abnormalities in fluid volume and electrolyte composition are common and important clinical disorders. Drugs that block specific transport functions of the renal tubules are valuable clinical tools in the treatment of these disorders. Many of these drugs have also been applied to understand the normal regulatory mechanisms involved in ion and fluid homeostasis. Although various agents that increase urine volume (diuretics) have been described since antiquity, it was not until 1937 that carbonic anhydrase inhibitors were first described and not until 1957 that a much more useful and powerful diuretic agent (chlorothiazide) became available. The pharmacology of diuretic agents: Many diuretics exert their effects on specific membrane transport proteins in renal tubular epithelial cells. Other diuretics exert osmotic effects that prevent water reabsorption (e.g., mannitol), inhibit enzymes (e.g., acetazolamide), or interfere with hormone receptors in renal epithelial cells (e.g., vaptans, or vasopressin antagonists). The physiology of each nephron segment is closely linked to the basic pharmacology of the drugs acting there. Thus, knowledge of how these drugs act has helped to define the functions of various nephron segments in electrolyte and fluid homeostasis. Definitions: Technically, a “diuretic” is an agent that increases urine volume. A “natriuretic” causes an increase in renal sodium excretion. An “aquaretic” increases excretion of solute-free water. Because natriuretics almost always also increase water excretion, they are usually just called diuretics. Osmotic diuretics and vasopressin (antidiuretic hormone; ADH) antagonists are aquaretics and are not directly natriuretic. Most recently, an entirely new class of agents has been developed that block urea transport. These agents result in increased urine output and increased urea excretion but not increased excretion of electrolytes. Even though they are technically aquaretics,  Renal Pharmacology 1 and 2 5 they have also been referred to as urearetics. These agents are not yet available for therapy but are in early investigational stages. Figure 1. Overview of sites of action of major diuretics and principal transport activities affected in each nephron segment. Sites of action of major diuretic agents: 1 – Carbonic anhydrase inhibitors (PCT). 2 – Osmotic agents (PCT, Thin descending limb, CD). 3 – Loop agents (TAL). 4 – Thiazides (DCT). 5 – Aldosterone antagonists (CT). 6 – ADH antagonists (CD). 7 – Adenosine (Glomerulus, PCT, TAL, CD). The major functions of different nephron segments with respect to water, electrolyte, and organic ion transport are summarized in Table 1:  Renal Pharmacology 1 and 2 6 Table 1. Major segments of the nephron and their water and ion transport functions. Primary Transporters and Segment Functions Water Diuretic with Drug Targets at Permeability Major Action Apical Membrane Glomerulus Formation of glomerular Extremely None None filtrate high Reabsorption of 65% of Na+/H+ (NHE3)1, Carbonic Proximal Very high filtered Na+/K+/Ca2+, and carbonic anhydrase convoluted Mg2+; 85% of NaHCO3, and anhydrase; inhibitors, tubule nearly 100% of glucose and Na+/glucose Adenosine (PCT) amino acids. cotransporter 2 antagonists (SGLT2) (under Isosmotic reabsorption of investigation) water. Secretion and reabsorption Proximal Very high Acid and base None of organic acids and bases, straight transporters including uric acid and most tubules diuretics Thin Passive reabsorption of High Aquaporins None descending water limb of Henle’s loop Active reabsorption of 15– Thick Very low Na/K/2Cl Loop diuretics 25% of filtered ascending (NKCC2) Na+/K+/Cl−; secondary limb of reabsorption of Ca2+ and Henle’s loop Mg2+ Active reabsorption of 4–8% Distal Very low Na/Cl (NCC) Thiazides of filtered Na+ and Cl−; Ca2+ convoluted reabsorption under tubule parathyroid hormone control Na+ reabsorption (2–5%) Na channels K+-sparing Cortical Variable2 coupled to K+ and H+ (ENaC), K diuretics; collecting secretion channels,1 H+ adenosine tubule transporter,1 antagonists aquaporins (under investigation) Water reabsorption under Medullary Variable2 Aquaporins Vasopressin vasopressin (ADH) control collecting antagonists duct 1 – Not currently a target for pharmacologic agents. 2 – Controlled by vasopressin (antidiuretic hormone [ADH]) activity.  Renal Pharmacology 1 and 2 7 Diuretic drugs can be divided into 6 major physiological classes: Class 1: Osmotic diuretics (e.g., mannitol) Class 2: Proximal tubule diuretics – Carbonic anhydrase inhibitors (e.g., acetazolamide) Class 3: Loop diuretics – Na-K-2Cl inhibitors (e.g., furosemide, bumetanide, ethacrynic acid) Class 4: Distal convoluted tubule diuretics – Na-Cl inhibitors (e.g., chlorothiazide, hydrochlorothiazide, metolazone, chlorthalidone) Class 5: Collecting duct diuretics – Na channel blockers (e.g., amiloride, triamterene) Class 6: Aldosterone antagonists (e.g., spironolactone) Table 2. Changes in urinary electrolyte patterns and body pH in response to diuretic drugs. Urinary electrolytes Diuretic class NaCl NaHCO3 K+ Body pH Carbonic anhydrase inhibitors + +++ + Decrease Loop agents ++++ 0 + Increase Thiazides ++ + + Increase Loop agents plus thiazides ++++ + ++ Increase K+-sparing agents + (+) – Decrease +, increase; –, decrease; 0, no change; Decrease = acidosis; Increase = alkalosis.  Renal Pharmacology 1 and 2 8 B. Proximal tubule: Carbonic anhydrase inhibitors, osmotic diuretics. Figure 2. Transport processes in the PCT that are drug targets. PCT is primary site for transport of NaHCO3, NaCl, glucose, amino acids, and other organic solutes. (Na++K+)-ATPase on basolateral plasma membrane: Primary active transport system that establishes Na+ ion gradient for energization of secondary active transport systems – e.g., SGLT2 (a Na+-D-glucose cotransporter on the apical (luminal) plasma membrane, NHE3 on apical plasma membrane). You should be able to readily appreciate the impact of inhibition of Carbonic Anhydrase on Na+ and fluid balance from Figure 2. SGLT2 inhibitors: Approved to treat diabetes; because this carrier is responsible for reabsorbing much of the glucose filtered by the glomeruli, inhibition of SGLT2 has diuretic properties and results in increased Na+ and glucose excretion.  Renal Pharmacology 1 and 2 9 C. Loop of Henle: Loop diuretics, NKCC2. Figure 3. Transport processes in the TAL that are drug targets. The thick ascending limb of Henle’s loop (TAL) actively reabsorbs NaCl from the tubular lumen; Unlike the proximal tubules and thin descending limb, the TAL is nearly impermeable to water (called Diluting Segment ). The NaCl transport system on the apical membrane is a Na+/K+/2Cl– cotransporter (called NKCC2); selectively blocked by diuretics called loop agents. Excess K+ accumulates in the TAL cell due to action of NKCC2; get back-diffusion of K+ into the tubular urine through the ROMK channel; ROMK channel creates a lumen- positive driving force that drives the paracellular reabsorption of divalent cations (i.e., Mg2+ and Ca2+).  Renal Pharmacology 1 and 2 10 Figure 4. Structures of two major loop diuretics. Note that the shaded methylene group on ethacrynic acid is reactive and can form an adduct with free sulfhydryl (–SH) groups of cysteinyl residues on target proteins. Table 3. Relative potency of loop diuretics. Drug Equivalent Dose1 Furosemide 20 mg Torsemide 10 mg Bumetanide 0.5 mg Ethacrynic acid ~ 50 mg 1. Doses are approximate due to variability of furosemide bioavailability. From: Basic & Clinical Pharmacology, 14th Edition by Katzung, Trevor & Masters, McGraw Hill, New York, 2018.  Renal Pharmacology 1 and 2 11 D. Distal convoluted tubule: Thiazides, NCC, Ca2+ reabsorption. Figure 5. Ion transport pathways across the luminal and basolateral plasma membranes of the distal convoluted tubule (DCT) cell. As in virtually all renal epithelial tubular cells, the (Na++K+)-ATPase on the basolateral membrane is a primary active transporter that provides the primary driving force for most secondary active transporters by establishing the transmembrane Na+ ion gradient. The Na+/Cl– cotransporter (NCC) on the apical membrane is the primary transporter for those ions in the DCT. The NCC is the specific target for the thiazide diuretics (see Figure 6). Unlike in the TAL, K+ ions do not recycle across the apical membrane, so there is no electrochemical potential to drive divalent cations. Instead, Ca2+ ions are actively reabsorbed via an apical Ca2+ channel and a basolateral Ca2+/Na+ exchanger and Ca2+- ATPase. Parathyroid hormone (PTH) regulates this process.  Renal Pharmacology 1 and 2 12 Figure 6. Hydrochlorothiazide and related agents. The thiazides were discovered in 1957 in an effort to synthesize more potent carbonic anhydrase inhibitors. It subsequently became clear that the thiazides primarily inhibit transport of NaCl rather than NaHCO3 and that their site of action is predominantly in the DCT rather than the PCT. Some thiazides do retain significant carbonic anhydrase inhibitory activity. Like carbonic anhydrase inhibitors and loop diuretics, all the thiazides have an unsubstituted sulfonamide group. E. Collecting tubule system: Aldosterone, ENaC and ADH. These cells connect the DCT to the renal pelvis and ureter.  Renal Pharmacology 1 and 2 13 Consists of several sequential tubular segments: connecting tubule, collecting tubule, and collecting duct Composed of principal cells and intercalated cells: The Principal cells are the major sites of Na+, K+, and water transport. The Intercalated cells (a and b subtypes) are the primary sites of H+ (a cells) or HCO3– (b cells) secretion. The a and b intercalated cells are very similar except that the membrane localization of H+-ATPase and Cl–/HCO3– exchanger are reversed. Figure 7. Ion transport pathways across the luminal and basolateral plasma membranes of collecting tubule and collecting duct cells – I (Na+, K+, H+, and Cl–). Although these tubule segments may be anatomically distinct, the gradations in physiological function are more gradual; in terms of diuretic action, Katzung suggests thinking of these cells as a single segment of the nephron containing several distinct cell types.  Renal Pharmacology 1 and 2 14 Distinct pathways for handling of Na+, K+, and Cl– ions in Principal cells of collecting tubule: No apical cotransport system for Na+ and other ions Separate ion channels for Na+ and K+, which exclude anions, leading to net movement of charge across the membrane Na+ entry into Principal cell predominates over K+ entry, leading to a 10-50 mV lumen-negative electrical potential, which drives Cl– ions into blood via a paracellular route and draws K+ out of cell via apical K+ channel. Na+ that enters cells transported back into blood by the (Na++K+)-ATPase on the basolateral membrane Thus, upstream diuretics that increase Na+ delivery to the collecting tubules also increase K+ secretion. This mechanism, combined with aldosterone secretion due to volume depletion, is the primary basis for most diuretic-induced K+ wasting. [See Section III for more on K+-wasting and K+-sparing diuretics] Reabsorption of Na+ ions by the epithelial Na channel (ENac) and its coupled secretion of K+ are regulated by aldosterone. Aldosterone: Steroid hormone that modulates gene transcription to increase the activities of both apical membrane channels and the basolateral (Na++K+)-ATPase. Ultimate result of aldosterone action is to dramatically increase both Na+ reabsorption and K+ secretion.  Renal Pharmacology 1 and 2 15 Figure 8. Effects of aldosterone on late distal tubule and collecting duct and diuretic mechanism of aldosterone antagonists. A. Overview of aldosterone’s influence on Na+ retention: Occurs via interaction with the mineralocorticoid receptor (MR), aldosterone affects myriad renal pathways that handle Na+. Key to numbered items influenced by ALDO: 1. Activation of membrane-bound Na+ channels 2. Na+ channel (ENaC) removal from the membrane inhibited 3. De novo synthesis of Na+ channels 4. Activation of membrane-bound (Na++K+)-ATPase 5. Redistribution of (Na++K+)-ATPase from cytosol to membrane 6. De novo synthesis of (Na++K+)-ATPase 7. Changes in permeability of tight junctions 8. Increased mitochondrial production of ATP Note: Cortisol also has affinity for the mineralocorticoid receptor but is inactivated in the cell by 11-β-hydroxysteroid dehydrogenase (HSD) type II. B. Details of aldosterone’s influence on membrane ENaC: ERK signaling phosphorylates components of ENaC, making them susceptible to interaction with Nedd4-2, a ubiquitin-protein ligase that ubiquitinates ENaC, leading to its degradation. The Nedd4-2 interaction with ENaC occurs via several proline-tyrosine- proline (PY) motifs of ENaC. ALDO enhances expression of the serum and glucocorticoid-regulated kinase-1 (SGK1) and the glucocorticoid-induced leucine zipper protein (GILZ; TSC22D3). SGK-1 phosphorylates and inactivates Nedd4-2; 14-3-3 dimers bind to the phosphorylated sites in Nedd4-2 and stabilize them. Phosphorylated Nedd4-2 no longer interacts well with the PY motifs of ENaC. As a result, ENaC is not ubiquitinated and remains in the membrane, leading to increased Na+ entry into the cell. GILZ stabilizes SGK1, enhancing its effects, and decreases ERK signaling and ENaC phosphorylation, events that prime ENaC for degradation; these effects all lead to less ubiquitination and more active ENaC in the cell membranes of the distal tubule and collecting duct. Abbreviations: AIP, aldosterone-induced proteins; ALDO, aldosterone; CH, ion channel; BL, basolateral membrane; LM, luminal membrane; MR, mineralocorticoid receptor.  Renal Pharmacology 1 and 2 16 Figure 9. Ion transport pathways across the luminal and basolateral plasma membranes of collecting tubule and collecting duct cells – II (water). Highlights water transport in the collecting tubule and collecting duct, which is the site in nephron where final urine concentration is determined. Principal cells contain a regulated system of aquaporin-2 (AQP2) water channels; permeability is controlled by antidiuretic hormone (ADH; also called arginine vasopressin or AVP). ADH acts by regulating insertion of preformed AQP2 channels into the apical membrane. ADH binds to vasopressin receptors on basolateral plasma membrane: Vasopressin receptors in kidney are V2 receptors. Vasopressin receptors in vasculature and CNS are V1-type receptors. ADH binding to receptor leads to a Gs protein-coupled cAMP-mediated process that stimulates AQP2 insertion into the apical (luminal) plasma membrane. ADH increases water permeability, leading to water reabsorption and a more concentrated urine.  Renal Pharmacology 1 and 2 17 In the absence of ADH, the collecting tubule and duct are impermeable to water, which results in formation of a dilute urine. III. Potassium-wasting and potassium-sparing diuretics. Loop diuretics and thiazide diuretics are K+-wasting and can lead to hypokalemia. Two classes of diuretics that are K+-sparing: ENaC inhibitors: Amiloride and triamterene. ALDO antagonists: Spironolactone Figure 10. Structures of two prototype potassium-sparing diuretics. Amiloride and triamterene: Direct inhibitors of Na+ influx in the cortical collecting tubule.  Renal Pharmacology 1 and 2 18 Triamterene is extensively metabolized in the liver; both the active form and metabolites undergo renal excretion; thus, drug has short half-life. Amiloride: No metabolism; relatively longer half-life. Spironolactone: Synthetic steroid; competitive antagonist to aldosterone. Active metabolites produced in liver. Binds to and inhibits androgen receptor; side effects in males (e.g., gynecomastia and reduced libido). K+-sparing diuretics are most useful in states of mineralocorticoid excess or hyperaldosteronism. Contraindications to use of K+-sparing diuretics: Patients with chronic renal insufficiency are particularly susceptible to hyperkalemia. Concomitant use of RAAS inhibitors (e.g., beta-blockers, ACE inhibitors, ARBIs; see Section V) increases the likelihood of hyperkalemia. Patients with liver disease may have impaired metabolism of triamterene and spironolactone; doses must be carefully adjusted. Strong CYP3A4 inhibitors can markedly increase blood levels of eplerenone (spironolactone analogue) but not spironolactone.  Renal Pharmacology 1 and 2 19 IV. Drugs that alter water balance: ADH agonists and antagonists. In those cases where enhanced ADH function is desired (e.g., treatment of diabetes insipidus), ADH agonists can be used, as illustrated in Table 4. Table 4. Vasopressin (ADH) agonists. More often, however, antagonism of ADH is desired, such as in congestive heart failure or syndromes of inappropriate ADH secretion. As noted above, there are 2 main types of vasopressin receptors (V1, 2 subtypes – V1a and V1b; and V2); the latter is selectively localized in the kidneys. Until recently, there were 2 nonselective agents, lithium and demeclocycline (a tetracycline antimicrobial drug) that were used as ADH inhibitors. Demeclocycline is used more often than lithium due to the many adverse effects of the latter drug. Demeclocycline is now being rapidly replaced by a new group of specific ADH receptor antagonists, the vaptans (Table 5).  Renal Pharmacology 1 and 2 20 Table 5. Vasopressin (ADH) antagonists. There are several orally active agents, tolvaptan, satavaptan, lixivaptan, and mozavaptan, that are V2-receptor selective. Tolvaptan is approved by the U.S. FDA for treatment of hyponatremia and as an adjunct to standard diuretic therapy in patients with congestive heart failure. Figure 11. Chemical structure of V2-selective vaptans. Redrawn from: J. Clin. Endocrinol. Metab. 2013;98(4):1321-1332.  Renal Pharmacology 1 and 2 21 Figure 12. Vasopressin- induced change in water permeability in the collecting duct and the action of vaptans. After arginine vasopressin binds to the vasopressin 2 receptor (VR2), a G-protein mediated increase in intracellular cyclic 3’,5’-cAMP leads to activation of protein kinase A (PKA). In turn, the latter phosphorylates (P) aquaporin 2 (AQP-2) water channels contained in preformed vesicles, effecting their movement along cytoskeletal elements from the intracellular compartment to the apical membrane. Exocytic insertion of AQP-2 into the apical membrane increases apical permeability to water. Following the osmotic gradient from the tubular lumen to medullary interstitium, tubular fluid water enters the cell through apical AQP-2 and exits through constitutively active basolateral AQP-3 and AQP-4. Vaptans compete with arginine vasopressin for the V2 receptor binding site. Binding of a vaptan prevents initiation of the sequence of events described and leaves the collecting duct impermeable to water. Source: Am. J. Kidney Dis. 2013;62(2):364- 376. V. Drugs targeting the renin-angiotensin system. The Renin-Angiotensin-Aldosterone System (RAAS) participates in the pathophysiology of hypertension, congestive heart failure, myocardial infarction, and diabetic nephropathy. This realization has led to a thorough exploration of the RAAS and the development of new approaches for inhibiting its actions. As you have been told about the basic functions of the RAAS by Dr. Rossi, my focus here is to emphasize components of the system that are pharmacologic targets.  Renal Pharmacology 1 and 2 22 Figure 13. Components of the RAS system. The heavy arrows show the classical pathway, and the light arrows indicate alternative pathways. Receptors involved: AT1, AT2, AT4, Mas, MrgD, and PRR. AP, aminopeptidase; E, endopeptidases; PCP, prolylcarboxylpeptidase. Numerous, potential pharmacologic targets should be evident by examination of this scheme. Targets include the key enzymes that act at several steps in the pathway as well as receptors to which several products bind.  Renal Pharmacology 1 and 2 23 Figure 14. Regulation of the RAS pathway. Control of renin secretion: Renin is secreted by the granular cells within the JG apparatus and is regulated by the following pathways (Figure 14): 1. The macula densa pathway 2. The intrarenal baroreceptor pathway 3. The β1 adrenergic receptor pathway Pharmacologic agents that alter the RAS pathway: 1. Renin inhibitors 2. ACE inhibitors 3. AT1 receptor blockers 4. Agonists and antagonists of b1-adeneric receptors in JGA cells 5. Indirect modulators: a. Diuretics b. Vasodilators c. NSAIDs  Renal Pharmacology 1 and 2 24 Figure 15. Regulation of renin release from the JGA cells by processes in the macula densa. Key pharmacologic targets in JGA and macula densa cells: 1) AT1-R 2) P2Y-R 3) b1-adrenergic receptor 4) Adenosine (ADO) receptor 5) Prostaglandin receptor 6) nNOS 7) COX-2 8) NKCC2 (loop diuretics)  Renal Pharmacology 1 and 2 25 Figure 16. Inhibitors of the RAS. Three major types of inhibitors: Direct renin inhibitors (DRIs), angiotensin converting enzyme inhibitors (ACEIs), and angiotensin receptor blockers (ARBs). Figure 17. Structures of representative RAS inhibitors.  Renal Pharmacology 1 and 2 26 One last word about RAS… Figure 18. Opposing effects of modulating RAS components. VI. Effects of NSAIDs on renal function. Nonsteroidal anti-inflammatory drugs (NSAIDs): Common drugs used for analgesic and anti-inflammatory effects. Adverse renal events occur in approximately 1-5% of all patients using NSAIDs. Considering that estimated annual use of NSAIDs in U.S. is > 70 million prescriptions and 30 billion OTC doses, upwards of 2.5 million patients annually expected to experience a nephrotoxic event from NSAID use. NSAIDs and prostaglandins: NSAIDs inhibit cyclooxygenase-1 and/or -2 (COX-1, COX-2).  Renal Pharmacology 1 and 2 27 COX enzymes act on arachidonic acid to generate various prostaglandins and thromboxanes, which primarily cause vasodilation in the kidneys. The reduction in PG synthesis can lead to reversible renal ischemia, a decline in glomerular hydraulic pressure (the major driving force for glomerular filtration), and AKI. This occurs via an NSAID-induced attenuation of renal vasodilation. In healthy patients, PGs play little role in renal hemodynamics. However, PG synthesis is increased in the setting of prolonged renal vasoconstriction, which serves to protect the glomerular filtration rate (GFR). PG synthesis is increased in the following conditions: o Chronic kidney disease (CKD), especially stage 3 or worse (i.e., estimated GFR [eGFR] < 60 mL/min/1.73 m2) o Volume depletion from aggressive diuresis, vomiting, or diarrhea o Effective arterial volume depletion due to heart failure, nephrotic syndrome, or cirrhosis o Older age o Severe hypercalcemia with associated renal arteriolar vasoconstriction NSAID-induced inhibition of PG-mediated afferent vasodilation and reduction in peritubular blood flow may also increase the risk of ischemic acute tubular necrosis (ATN) or other nephrotoxin-induced tubular injury from drugs such as aminoglycosides, amphotericin B, hydroxyethyl starch, and radiocontrast material. Function of aldosterone antagonists, such as spironolactone, depends on renal PG synthesis. Accordingly, NSAIDs blunt the efficacy of aldosterone antagonists. Risk factors for NSAID-induced AKI: Patients with CKD Use of diuretics, ACE inhibitors, or ARBs increase risk. Generally recommended to limit or avoid NSAID use among patients with reduced eGFR (especially those with eGFR < 60 mL/min/1.73 m2). Even recommend limiting use on patients with moderately reduced eGFR (i.e., eGFR of 60 to 89 mL/min/1.73 m2). This would include many older patients who are otherwise considered healthy.  Renal Pharmacology 1 and 2 28 VII. Pharmacologic agents that modulate mitochondrial biogenesis. In the lecture on “Metabolic Energetics and Drug Metabolism in the Kidney,” we talked about proteins and transcription factors that can regulate mitochondrial biogenesis and how alteration of mitochondrial biogenesis can greatly impact renal function. In particular, the sirtuins have become important pharmacologic targets to improve kidney function. Table 6 presents a summary of several agents that have been discovered. Therapeutic approaches to enhance sirtuin function in the kidneys involve 4 types of agents: Sirtuin-activating compounds (STACs): Natural compounds Sirtuin-activating compounds (STACs): Synthetic compounds NAD+-boosting therapies Alternative strategies Table 6. Therapeutic approaches to enhance expression and activity of sirtuins in kidney disease. [Adapted from: Morigi et al. (2018) Sirtuins in renal health and disease. J. Am. Soc. Nephrol. 29, 1799- 1809.] Compound Target Biologic Effects Experimental Models Natural STACs Resveratrol SIRT1 Protects renal mitochondrial function by reducing Sepsis-associated AKI SIRT3 SOD2 and oxidative stress; prolongs animal survival Curcumin SIRT3 Improves renal function and reduces tubular Cisplatin-induced AKI damage, preserving mitochondrial bioenergetics, redox balance and dynamics Silybin SIRT3 Improves renal function and reduces tubular Cisplatin-induced AKI necrosis and cell apoptosis, preserving mitochondrial functions Stanniocalcin SIRT3 Reduces renal damage by activating AMPK and Ischemia-reperfusion UCPs injury (IRI) Honokiol SIRT3 Decreases tubular damage and improves animal Sepsis-associated AKI survival, reducing oxidative stress, NF-kB signaling, and inflammatory cytokines SIRT3 Attenuates angiotensin II-induced renal function Hypertensive impairment and fibrosis by decreasing KLF15- nephropathy dependent extracellular matrix expression Synthetic STACs SRT1720 SIRT1 Inhibits tubular endoplasmic reticulum stress via Unilateral ureteral SIRT1-dependent increase of HO-1 and obstruction (UUO) thioredoxin, slowing renal fibrosis SRT2183 SIRT1 Reduces fibrosis and apoptosis in renal medulla UUO  Renal Pharmacology 1 and 2 29 SRT3025 SIRT1 Attenuates proteinuria and GFR decline, Remnant kidney reducing glomerulosclerosis and tubulointerstitial disease NAD+-boosting therapies NMN SIRT1 Protects young and old mice from AKI; preserves Cisplatin-induced AKI renal function and reduces tubular damage and and IRI apoptosis by enhancing mitochondrial biogenesis AICAR SIRT3 Improves renal function and reduces tubular Cisplatin-induced AKI damage, preserving mitochondrial dynamics through activation of AMPK signaling and PKC- 1a Alternative strategies CR SIRT1 Improves renal function and reduces tubular Cisplatin-induced AKI damage by reducing apoptosis and preserving mitochondrial function MSCs SIRT3 Stimulates renal tubular repair by preserving Cisplatin-induced AKI mitochondrial functional integrity and their exchange among tubular cells Abbreviations: AICAR, 5-aminoimidazole-4-carboxamide ribonucleoide; AMPK, 5’ AMP-activated protein kinase; CR, caloric restriction; HO-1, heme oxygenase 1; KLF15, Krüppel-like factor 15; MSCs, mesenchymal stromal cells; NMN, nicotinamide mononucleotide; PGC-1a, peroxisome proliferator- activated receptor g coactivator; STAC, sirtuin-activating compound; UCP2, uncoupling protein 2. Another view of sirtuin function in the kidneys:  Renal Pharmacology 1 and 2 30 VIII. Summary. Properties of key diuretic agents: 1. Carbonic anhydrase inhibitors – e.g., acetazolamide Mechanism of action: inhibition of enzyme prevents dehydration of H2CO3 and hydration of CO2 in the PCT. Effects: reduces HCO3– reabsorption, causing a self-limited diuresis; reduces body pH; can cause hyperchloremic metabolic acidosis. 2. SGLT2 inhibitors – e.g., canagliflozin Mechanism of action: Inhibition of sodium/glucose cotransporter 2 (SGLT2) in PCT results in decreased Na+ and glucose reabsorption. Effects: Reduction in serum glucose concentration and reduced Na+ reabsorption causes a mild diuresis.  Renal Pharmacology 1 and 2 31 3. Loop diuretics – e.g., furosemide Mechanism of action: Inhibition of Na/K/2Cl transporter in TAL. Effects: Marked increase in NaCl excretion, some K+ wasting, hypokalemic metabolic alkalosis can occur, increased urinary Ca and Mg. 4. Thiazides – e.g., hydrochlorothiazide Mechanism of action: Inhibition of the Na/Cl transporter in the DCT. Effects: Modest increase in NaCl excretion + some K+ wasting, hypokalemic metabolic alkalosis can occur, decreased urinary Ca. 5A. Potassium-sparing diuretics – e.g., spironolactone Mechanism of action: Pharmacologic antagonist of aldosterone in collecting tubules, weak antagonism of androgen receptors. Effects: Reduces Na retention and K wasting in kidney; potential toxicity – hyperkalemia and gynecomastia. 5B. Potassium-sparing diuretics – e.g., amiloride Mechanism of action: Blocks eNaC in collecting tubules. Effects: Reduces Na retention and K wasting, increases Li clearance. 6. Osmotic diuretics – e.g., mannitol Mechanism of action: Physical osmotic effect on tissue water distribution. Effects: Marked increase in urine flow, initially hyponatremia but then hypernatremia. 7. Vasopressin (ADH) antagonists – e.g., tolvaptan Mechanism of action: Selective antagonist at V2 ADH receptors. Effects: Reduces water reabsorption, increases plasma Na concentration. Targeting of renal enzymes and pathways to modulate blood pressure: Diuretics: lower blood pressure by depleting the body of Na+ and reducing blood volume and perhaps by other mechanisms. Sympathoplegic agents: lower blood pressure by reducing peripheral vascular resistance, inhibiting cardiac function, and increasing venous pooling in capacitance vessels. Direct vasodilators: reduce pressure by relaxing vascular smooth muscle, thus dilating resistance vessels and—to varying degrees—increasing capacitance as well. Agents that block production or action of angiotensin and thereby reduce peripheral vascular resistance and (potentially) blood volume.  Renal Pharmacology 1 and 2 32 The fact that these drug groups act by different mechanisms permits the combination of drugs from two or more groups with increased efficacy and, in some cases, decreased toxicity. Targeting mitochondrial biogenesis to improve kidney function: Focus primarily on activation of sirtuin 1 or 3 in renal mitochondria. Both natural compounds (e.g., resveratrol) and synthetic derivatives used. Based on adaptability of renal mitochondria to energy needs of renal cells.