Faculty version with model answers

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1 Faculty version with model answers Urinary Dilution & Concentration Bruce M. Koeppen, M.D., Ph.D. University of Connecticut Health Center 1. Increased urine output (polyuria) can result in a number of situations, including: (1) increased ingestion of water, (2) inadequate secretion of ADH by the posterior pituitary (central diabetes insipidus), (3) lack of response of the kidney to ADH (nephrogenic diabetes insipidus), (4) osmotic diuresis, and (5) administration of a diuretic that inhibits transport by the thick ascending limb of Henle s loop (loop diuretic). A. For each of these situations predict changes in solute excretion (i.e., increased, decreased, or unchanged), and values for urine osmolality (Uosm). Assume urine osmolality ranges from 50 to 1,200 mosm/kg H 2 O, and that the osmolality of the body fluids is 290 mosm/kg H 2 O. Condition Solute Excretion Uosm (mosm/kg H 2 O) Water Ingestion unchanged <100 Central DI unchanged <100 Nephrogenic DI unchanged <100 Osmotic Diuresis increased near 300 Loop Diuretic increased near 300 B. What are the mechanisms of water and solute handling by the kidneys in each of these situations? Use as your reference point a normal individual in antidiuresis, who is excreting a maximally concentrated urine (i.e., ADH levels are maximal, and urine osmolality is 1,200 mosm/kg H 2 O). The major emphasis of this question is to understand the handling of solute and water by the nephron, and especially how these can be regulated separately under normal conditions. In order to understand renal water handling it is useful to use the concept of solute-free water excretion and solute-free water reabsorption. Solute-free water is simply that, a volume of water containing no solute Note: this is an abstract concept, because all urine, even the most dilute, contains solute. However, the urine can be divided theoretically into two volumes. One that is solute free, and one that contains all the solute. At this point it is not necessary to know how to calculate these two virtual urine volumes (any textbook of renal physiology will provide details: See pp of Renal Physiology, 3 rd ed. By Koeppen and Stanton for example). Nevertheless, it is important to understand the requirements that allow the kidneys to excrete of solute-free water and the requirements that allow the kidneys to reabsorb of solute-free water. These are: Requirements for excretion of solute-free water (Urine Dilution) 1. Absence of ADH 2. Adequate delivery of solute (NaCl) and water to the diluting segments of the nephron, (thin ascending limb of Henle s loop, thick ascending limb of Henle s loop, and early distal tubule). Note that the most important segment is the thick ascending limb, because of its high transport rate. 3. Normal function of the diluting segments, again the thick ascending limb being most important. Bruce M. Koeppen, M.D., Ph.D., University of Connecticut Health Center -1-

2 Requirements for reabsorption of solute-free water (Urine Concentration) 1. Presence of ADH 2. Collecting duct responsive to ADH. 3. Adequate delivery of solute (NaCl) and water to the diluting segments of the nephron, (thin ascending limb of Henle s loop, Thick ascending limb of Henle s loop, and early diatal tubule). Note that the most important segment is the thick ascending limb, because of its high transport rate. 4. Normal function of the diluting segments, again the thick ascending limb being most important. Note: the NaCl reabsorbed by the thick ascending limb creates the hyperosmotic medullary interstitium that drives water reabsorption from the collecting duct. 5. Hyperosmotic medullary interstitium. This requires NaCl reabsorption by the thick ascending limb of Henle s loop, and normal blood flow in the vasa recta. For reference use the following data for the volume of glomerular filtrate reabsorbed by the various segments of the nephron (assume GFR = 180 L/day). Segment % of filtered load reabsorbed Volume reabsorbed (R) or excreted (E) Proximal tubule 67% 121 L (R) Loop of Henle (thin descending) 23% 41 L (R) Collecting Duct* <1% - 9.7% L (R) Final Urine 0.3% - 10% L (E) *depends on presence or absence of ADH Starting with the condition of maximally concentrated urine, the individual has: (1) maximal circulating levels of ADH, (2) maximal responsiveness of the collecting duct to ADH, and (3) a renal medulla where the interstital osmolality is maximal (approximately 1,200 mosm/kg H 2 O at the tip of the papilla). If GFR is 180 L/day, approximately 10% or 18L/day is being delivered to the beginning of the collecting duct (the osmolality of this fluid is approximately 100 mosm/kg H 2 O). Because the collecting duct is maximally responsive to ADH, and ADH levels are elevated the permeability of the collecting duct to H 2 O is high, and water is reabsorbed, driven by the osmotic gradient between the tubular fluid and the surrounding interstitial fluid (290 mosm/kg H 2 O in the cortex, and 1,200 mosm/kg H 2 O) at the tip of the papilla). The net result is that a small volume (<1 L) of concentrated urine will be excreted (osmolality equal to interstital fluid osmolality of 1,200 mosm/kg H 2 O at the tip of the papilla). Water Ingestion: Following ingestion of a water load, body fluid osmolality will fall, and thereby inhibit the secretion of ADH. Because ADH has a short plasma half-life, circulating levels will quickly decrease. With decreased ADH levels the collecting ducts will become increasingly impermeable to water (endocytosis of water channels from apical membranes), and as a result less water will be reabsorbed. In the limit, where ADH levels are zero, virtually all of the filtrate delivered to the collecting duct will continue down the collecting duct and be excreted (because the collecting duct is not completely impermeable water, even in the total absence of ADH, some of the 18L of tubular fluid [approximately 1-2 L] will be reabsorbed). The net result is the excretion of a large volume of dilute urine. With prolonged ingestion of water, blood flow through the vasa recta will increase, and this will result in the washout of the medullary interstitium. Also, interstital urea will diffuse into the medullary collecting duct, which is permeable to urea even in the absence of ADH, and now contains dilute urine (i.e., there is a favorable gradient for urea to diffuse from the medullary interstitum, where its concentration is high, into the lumen of the collecting duct). In the extreme, medullary interstitial osmolality at the tip of the papilla can fall to 600 mosm/kg H 2 O (Note: the major solute washed out of the interstitium is urea). In Bruce M. Koeppen, M.D., Ph.D., University of Connecticut Health Center -2-

3 terms of the requirements for excretion and reabsorption of solute-free water (see above), it should be recognized that the only thing that has changed is the ADH levels (from high to low). In this situation, solute excretion is unaffected by water excretion. This could be determined if the solute excretion rate was determined (it would be unchanged whether the urine was concentrated or dilute). The best way to understand how solute excretion is maintained in the face of large changes in urine volume, is to recognize that the vast majority of solute (e.g., NaCl) reabsorption occurs prior to the collecting duct. In addition, solute excretion is under the control of other systems (e.g., sympathetic nerves, renin-angiotensin-aldosterone, ANP, etc.), and not ADH. Central DI: Central diabetes insipidus can occur as a result of trauma to the pituitary (e.g., basilar skull fracture) or more rarely as an autosomal dominant inherited disorder. In either case there is inadequate or no secretion of ADH. If central DI was induced acutely in an individual producing a concentrated urine, the scenario for water and solute handling by the nephron would be similar to that described above for the water load. Thus, with decreased ADH levels the collecting ducts will become increasingly impermeable to water (endocytosis of water channels from apical membranes), and as a result less water will be reabsorbed. In the limit, where ADH levels are zero, virtually all of the filtrate delivered to the collecting duct will continue down the collecting duct and be excreted (because the collecting duct is not completely impermeable water, even in the total absence of ADH, some of the 18L of tubular fluid (approximately 1-2 L) will be reabsorbed). This will result in an increase in body fluid osmolality, and the stimulation of thirst. Solute excretion will be unaffected. In terms of the requirements for excretion and reabsorption of solute-free water (see above), the only thing that has changed is ADH levels (from high to low). Extra background information: Recent genetic studies of the autosomal dominant form of inherited diabetes insipidus have shown that there are multiple mutations that can cause reduced or no secretion of ADH. Recall that ADH is synthesized as a prepro-hormone in the preoptic and paraventricular nuclei of the hypothalamus. A pro-hormone is produced by cleavage of a signal peptide and addition of carbohydrate side chains. The pro-hormone is further modified within the neurosecretory granules of the posterior pituitary, yielding the ADH molecule, neurophysin, and a glycoprotein (all are released when ADH secretion is stimulated). The ADH gene is located on chromosome 20, and consists of 3 exons. Exon 1 encodes a signal peptide, the ADH molecule and the amino terminal portion of neurophysin. Exon 2 encodes the central region of neurophysin, and exon 3 the carboxy terminal of neurophysin. Mutations have been identified in all regions of the ADH gene (signal peptide, ADH molecule and neurophysin). In each of these situations, there is defective trafficking of the peptide, with abnormal accumulation in the endoplasmic reticulum. It is believed that this abnormal accumulation in the ER results in cell death. Nephrogenic DI: Nephrogenic diabetes insipidus results when the collecting duct is unresponsive to ADH. This can arise from a number of systemic disorders, and more rarely occurs as a result of inherited disorders. Regardless of the cause, the effect on water and solute handling by the nephron is identical to that seen in central DI. Thus, the collecting ducts are impermeable to water (water channels are not expressed on the apical membranes), and as a result less water will be reabsorbed. In the limit, virtually all of the filtrate delivered to the collecting duct will continue down the collecting duct and be excreted (because the collecting duct is not completely impermeable water, even in the total absence of ADH, some of the 18L of tubular fluid (approximately 1-2 L) will be reabsorbed). This will result in an increase in body fluid osmolality, and the stimulation of the thirst sensation. Solute excretion will be unaffected. In terms of the requirements for excretion and reabsorption of solute-free water (see above), the students should recognize that the only thing that has changed is ADH (in this case circulating levels of ADH are elevated, but the collecting duct does not respond to ADH). Bruce M. Koeppen, M.D., Ph.D., University of Connecticut Health Center -3-

4 Extra background information: Recent molecular studies have shown that many of the acquired forms of nephrogenic DI are the result of decreased expression of the water channels (aquaporin 2) in the collecting duct. To date, decreased expression of aquaporin 2 has been documented in nephrogenic DI associated with: hypokalemia, lithium ingestion, nephrotic syndrome, ureteral obstruction, low protein diet, and hypercalcemia (Interestingly, over expression of aquaporin has also been documented in states where water retention occurs. These include, SIADH, chronic renal failure, cirrhosis, and pregnancy). The inherited forms of nephrogenic DI essentially reflect mutations in the ADH receptor (V2), or the aquaporin 2 molecule. The V2 receptor is located on the X chromosome. Thus, these inherited forms are x-linked. Most of the mutations in the V2 receptor result in trapping of the receptor inside the cell, only a few cases result in the surface expression of a receptor that will not bind ADH. The gene for aquaporin 2 is located on chromosome 12, and is inherited as a autosomal recessive defect. This is much less common form of inherited nephrogenic DI, and usually results in the production of a protein that cannot be trafficked to the apical membrane. Osmotic diuresis: Osmotic diuresis can occur under a number of situations; the most common being in diabetes mellitus. In this situation, the filtered load of glucose increases as a result of the hyperglycemia. This increased filtered load exceeds the capacity of the proximal tubule to reabsorb glucose, and glucose remains in the tubule lumen where it is an osmotically active particle. As a result, water and solute (NaCl) reabsorption is reduced, and a larger than normal volume of tubular fluid is delivered into the loop of henle. Some, but not all of this extra fluid will be reabsorbed by the more distal nephron segments, and as a result there is an increase in urine water and solute (glucose plus NaCl) excretion. To understand the effect of glucose on solute and water excretion, it is necessary to review proximal tubule transport. Under normal conditions, the reabsorption of solute by the proximal tubule results in a small decrease in the osmolality of the tubular fluid (tubular fluid osmolality < interstital osmolality by a few millosmoles). This results in water reabsorption, because of the high water permeability of this portion of the nephron. As water is reabsorbed, additional solute (NaCl) is reabsorbed via the paracellular pathway by the process of solvent drag. Thus, if glucose cannot be completely reabsorbed, it will remain in the tubular lumen and impair osmotic water reabsorption, and as well as the solvent drag component of NaCl reabsorption. This will deliver more solute and water to the loop of Henle. With an osmotic diuresis, urine osmolality would decrease in this situation from a maximal value of 1,200 mosm/kg H 2 O to a value near that of plasma (300 mosm/kg H 2 O). This reflects the fact that fluid leaving the proximal tubule and entering the loop of Henle always has an osmolality similar to that of plasma, reflecting the iso-osmotic nature of proximal tubule reabsorption (this is true regardless of the final urine osmolality). As delivery is increased by the osmotic diuresis, it becomes increasingly difficult for the more distal nephron segments to modify their transport and compensate (transport becomes saturated). In the limit, fluid exiting the loop of Henle (and more distal nephron segments) begins to look more and more like the fluid which entered (i.e., has an osmolality similar to that of plasma). Loop diuretic: A loop diuretic will impair the function of the thick ascending limb of Henle s loop (inhibits the Na + -2Cl - -K + symporter in the apical membrane). The net effect will be an increase in the excretion of both solute (NaCl) and water. Loop diuretics impair the ability of the kidney to both dilute and concentrate the urine, and as a result the urine osmolality approaches that of plasma. Understanding how the loop diuretics impair the ability of the kidneys to dilute urine is straightforward. The thick ascending limb is the major nephron site where dilution of the tubular fluid occurs. Inhibiting the Na + -2Cl - -K + symporter decreases the reabsorption of NaCl and thus dilution of the tubular fluid does not occur. Because fluid entering the loop of Henle is isoosmotic to plasma, it will then leave the loop of Henle, and ultimately the kidneys with a similar osmolality. Urine osmolality also approaches that of plasma if the loop diuretic is administered Bruce M. Koeppen, M.D., Ph.D., University of Connecticut Health Center -4-

5 to an individual excreting a concentrated urine. This occurs because inhibition of thick ascending limb transport will result in a decrease in the osmolality of the medullary interstitium. Without a hyperosmotic medullary interstitium water cannot be reabsorbed from the collecting duct, despite its high water permeability (ADH levels are elevated), and thus the urine cannot be concentrated. The increase in urine flow that occurs with administration of loop diuretics results from the fact that the medullary interstitial osmotic gradient, which as noted, is the result of thick ascending limb transport, also drives water reabsorption from the thin descending limb of Henle s loop. Thus, urine volume goes up because of decreased reabsorption at the level of both the thin descending limb of Henle s loop and the collecting duct. In terms of the requirements for excretion or reabsorption of solute-free water, the parameter altered in this situation is transport by the thick ascending limb of Henle s loop. 2. Two women ingest different diets, and as a result have different urinary solute excretion rates. In order to maintain solute balance on her diet, woman A excretes 400 mosmoles/day of solute, while woman B excretes 1,000 mosmoles/day of solute. Both woman have normal renal function, and can excrete urine with an osmolality ranging between 50 mosm/kgh 2 O and 1,200 mosm/kg H 2 O. A. What is the range of urine volume these women can excrete? Minimum Urine Volume Maximum Urine Volume Woman A 0.33 L 8.0 L Woman B 0.83 L 20.0 L The general formula used in these equations is: Urine volume = mosmoles excreted/urine osmolality Woman A: Minimum volume = 400 mosmoles/1,200 mosm/kg H 2 O = 0.33 L Maximum volume = 400 mosmoles/50 mosmoles/kg H 2 O = 8.0L Woman B: Minimum volume = 1,000 mosmoles/1,200 mosm/kg H 2 O = 0.83 L Maximum volume = 1,000 mosmoles/50 mosmoles/kg H 2 O = 20.0L B. What would be the consequence if both women were given 10 L of water to drink in a 24 hour period? Woman A can only excrete 8 L of water (her excretion is constrained by the low solute excretion rate and the minimum osmolality of the urine). As a result of ingesting 10 L of water she will excrete 8 L of dilute urine (50 mosm/kg H 2 O, and retain 2 L. The retained water will dilute her body fluids resulting in a decrease in plasma osmolality and serum [Na + ]. In contrast, woman B will easily excrete the water load, since her capacity is 20 L/day. She will be able to maintain water balance, and there will be no change in her plasma osmolality or serum [Na + ]. 3. Two men are treated chronically (several months) with a diuretic. Man A is treated with a loop diuretic, and Man B is treated with a thiazide diuretic. Both men eat the same diet, which contains approximately 100 meq of Na + on a daily basis. Bruce M. Koeppen, M.D., Ph.D., University of Connecticut Health Center -5-

6 A. How much Na + does each of these men excrete in a day, and why (assume Na + loss via sweat and feces is negligible)? To maintain Na + balance, both men will excrete approximately 100 meq/day of Na +. Normally the kidneys excrete 90%+ of the daily ingested Na + load. The remainder is lost in the feces and in sweat (loss of Na + via these non-renal routes can increase significantly with diarrhea and with increased sweating). It should be emphasized that when individuals are treated chronically, with a diuretic they do not exhibit a sustained natriuresis, as they do during the initial phase of treatment. After a short period of negative Na + balance (i.e., intake < excretion), they again come into steady-state balance (i.e., intake = excretion). The ability to re-establish steady-state balance results from the diuretic-induced decrease in effective circulating volume. During the period of negative Na + balance (initial action of the diuretic), the effective circulating volume is decreased. As a result, there is a decrease in GFR and an increase in proximal tubule reabsorption. This in turn delivers less Na + to those portions of the nephron where the diuretic is acting, and Na + excretion returns to pre-diuretic levels. This is illustrated below for a loop diuretic. Pre-diuretic Initial 65% 9% 65% 9% 25% 1% 15% 11 % Chronic 75% 9% 1% 15% It should also be noted that while these figures do not show changes in distal Na + reabsorption, there will in fact be some additional Na + reabsorbed at these sites. This is especially so for the early distal tubule, which enhances its reabsorption of Na + when delivery is increased as a result of loop-acting diuretics. Recent studies have shown that this enhanced distal tubule reabsorption is the result of increased expression of the NaCl symporter. B. Both men are deprived of water. Man A is able to concentrate his urine to 400 mosm/kg H 2 O, and man B is able to concentrate his urine to 1,100 mosm/kg H 2 O. How do you explain the difference in response of these men to water deprivation? Man A was treated with a loop diuretic. With inhibition of NaCl reabsorption by the thick ascending limb, there is less NaCl deposited in the medullary interstitium. As a result the osmolality of the interstitium declines, and water reabsorption from the collecting duct in the presence of ADH is reduced (the urine osmolality mirrors the decreased osmolality of the medullary interstitium). Thus, the loop diuretic inhibits both the excretion of solute-free water, as well as the reabsorption of solute-free water. Man B was treated with a thiazide diuretic, which in contrast, inhibits NaCl reabsorption in the early distal tubule. Since this segment is Bruce M. Koeppen, M.D., Ph.D., University of Connecticut Health Center -6-

7 located in the cortex of the kidney, medullary interstitial osmolality is not effected. Thus, when ADH is present, the urine can be concentrated. Thus, thiazide diuretics inhibit the excretion of solute-free water, but not solute-free water reabsorption. See also the requirements for reabsorption of solute-free water in question #1. C. Both men are water loaded. Man A is only able to dilute his urine to 300 mosm/kg H 2 O, while man B can only dilute his urine to 250 mosm/kg H 2 O. How do you explain the inability of these men to maximally dilute their urine? Man A is treated with a loop diuretic. Since the thick ascending limb has the highest rate of solute reabsorption of all the water impermeable segments of the nephron (i.e., thin ascending limb, thick ascending limb and early distal tubule), inhibition of solute transport at this site will greatly impair the kidneys ability to dilute the urine. As a result the urine osmolality will approach that of plasma (300 mosm/kg H 2 O), which is the osmolality of the glomerular filtrate as well as that of the tubular fluid exiting the proximal tubule. Man B is treated with a thiazide diuretic, which acts only on cortical nephron segments (early distal tubule). This will impair urine dilution, but not as significantly, since the thick ascending limb is still intact. D. Prior to starting diuretic therapy, both men had a serum [Na + ] of 142 meq/l. Now man B s serum [Na + ] is 132 meq/l, and man A s serum [Na + ] is unchanged. How do you explain the difference in serum [Na + ] in these men? The first thing to recognize is that the hyponatremia in individual B is the result of positive water balance (intake > excretion). In this situation the problem is related to impaired excretion of solute-free water. Both diuretics impair the ability of the kidney to excrete solute-free water. However, the loop diuretic also impairs the ability of the kidney to reabsorb solute-free water. Thiazide diuretics do not appreciably alter the medullary interstitial osmotic gradient, whereas this gradient is dissipated with loop diuretics (see above). Therefore, with ADH levels elevated, due to the diuretic-induced volume depletion (i.e., non-osmotic release of ADH), water can be reabsorbed from the medullary collecting duct in the man taking the thiazide diuretic. Because the loop diuretic dissipates the medullary interstitial osmotic gradient, this will not occur in man A. It should be noted that hyponatremia (a reflection of positive water balance) can occur with both diuretics, when large volumes of water are ingested or administered intravenously. However, on typical diets (and volumes of liquids ingested) hyponatremia is more commonly seen in patients treated with thiazide diuretics for the reason noted above. 4. Three different patients are in positive water balance. As a result they all have hyponatremia, with a plasma [Na + ] = 116 meq/l (nl = meq/l). The urine osmolality of these patients is measured. Patient #1: Uosm = 300 mosm/kg H 2 O Patient #2: Uosm = 500 mosm/kg H 2 O Patient #3 Uosm = 50 mosm/kg H 2 O A. Match each patient with one of the following diagnoses. Diagnosis Patient # Compulsive Water Drinking Diuretic Administration S.I.A.D.H Bruce M. Koeppen, M.D., Ph.D., University of Connecticut Health Center -7-

8 B. What are the mechanisms by which hyponatremia is generated in each of these patients? It is important to emphasize that all three of these patients are in a state of positive water balance (i.e., the intake of solute-free water exceeds the capacity of the kidneys to excrete solute free water). Hyponatremia is not a problem of Na + balance, rather is a problem of water balance. Compulsive water drinking: Hyponatremia occurs because the ingestion of solute-free water exceeds the capacity of the kidneys to excrete solute-free water. Compulsive water drinking occurs in approximately 5-15% of patients with significant psychiatric disease. Diuretic administration: If the diuretic is either a loop diuretic or a thiazide diuretic they will inhibit the separation of solute and water in the thick ascending limb of Henle s loop or early distal tubule. This will decrease the ability to excrete solute-free water and dilute the urine. In addition, with chronic use, volume depletion will occur (decreased ECF volume). This will have several effects that also impair the kidneys ability to dilute the urine. The decreased volume, via low-pressure baroreceptors in the atria and pulmonary vessels, will stimulate ADH secretion (i.e., non-osmotic release of ADH). In addition, the decreased volume will decrease GFR, which in turn decreases the delivery of solute and water to the loop of Henle. This decreased delivery also limits the separation of solute and water and thus the ability to dilute the urine. Thus urinary dilution is impaired because: 1. The diuretic inhibits separation of solute and water. 2. Delivery of solute and water to the loop of Henle is reduced. 3. ADH levels are elevated. If water ingestion exceeds the capacity of the kidneys to dilute the urine and thereby excrete solute-free water, hyponatremia will result. As noted in problem #3, the development of hyponatremia is more commonly seen in patients treated with a thiazide diuretic. SIADH: In SIADH (syndrome of inappropriate secretion of antidiuretic hormone) ADH secretion is persistently elevated, and does not respond to the physiological regulatory factors which normally control secretion (i.e., body fluid osmolality and ECF volume). This persistent secretion of ADH will lead to water reabsorption by the collecting duct causing hyponatremia if water intake is excessive. 5. A patient has the following laboratory data: Plasma [Na + ] = 135 meq/l Plasma [glucose] = 90 mg/dl Plasma BUN = 80 mg/dl A. What is the estimated plasma osmolality of this patient (hint: the molecular weight of glucose = 180 g/mole and for blood urea nitrogen (BUN) = 28 g/mole)? Posm = 2 x [Na + ] + [glucose] / 18 + [BUN] / 2.8 = 304 mosm/kg H 2 O Note: the molecular weights for glucose and BUN reflect the fact that the traditional units for these parameters are expressed in mg/dl. Also the molecular weight of BUN is not that of urea, rather it is the molecular weight of the nitrogen atoms in the urea molecule. Bruce M. Koeppen, M.D., Ph.D., University of Connecticut Health Center -8-

9 B. Would you expect the plasma [ADH] to be increased or decreased in this patient? Why? Based on the calculated plasma osmolality of 304 mosm/kg H 2 O, it would be predicted that ADH levels would be increased. However, they would actually be suppressed in this situation, because the plasma osmolality is elevated by the increased levels of urea, and. urea is an ineffective osmole for ADH secretion. The osmoreceptors respond primarily to the osmotic pressure generated by the plasma [Na + ]. In this situation the effective osmolality of the plasma is only 270 mosm/kg H 2 O (i.e., 2 x [Na + ]). Bruce M. Koeppen, M.D., Ph.D., University of Connecticut Health Center -9-