Glomerular hemodynamic and structural alterations in experimental diabetes mellitus

Elevated glomerular filtration rate (GFR) is a frequent finding in patients with early insulin-dependent diabetes mellitus (IDDM). The mechanisms responsible for this glomerular hyperfiltration in IDDM are unclear. Rats made diabetic with alloxan or streptozotocin, and treated daily with supplemental insulin, have moderate hyperglycemia and elevated GFR, and thus have been used to study mechanisms of glomerular hyperfiltration in diabetes. Renal micropuncture techniques have shown that single-nephron GFR (SNGFR) is elevated in moderately hyperglycemic diabetic rats. In some cases, this is because of elevated glomerular capillary pressure (Pgc), but in other cases, Pgc is normal despite elevated SNGFR. Several potential mediators of increased SNGFR have been examined, including hyperglycemia, increased glomerular prostaglandin production, and decreased sensitivity of the tubuloglomerular feedback mechanism. Renal failure is a common complication of human IDDM. Diabetic rats with long-term moderate hyperglycemia have been used to study the mechanism by which glomerular injury develops in diabetes mellitus. It has been postulated that glomerular hyperfiltration or some determinant of elevated GFR in early diabetes may ultimately cause glomerular damage, leading to a progressive loss of renal function (diabetic nephropathy). Diabetic rats with long-term moderate hyperglycemia, however, do not develop characteristic glomerular lesions of human diabetic nephropathy and, in fact, develop only minimal glomerular injury even after 1 year of diabetes. Thus, although the diabetic rat with moderate hyperglycemia may be useful to study the mechanisms of glomerular hyperfiltration in early diabetes, it may not be an appropriate model of renal failure in IDDM.     

Modulation of single-nephron GFR in the db/db mouse model of type 2 diabetes mellitus. II. Effects of renal mass reduction

This study examines for the first time the effects of uninephrectomy (Nx) on modulation of whole kidney glomerular filtration rate (GFR), single-nephron GFR (SNGFR), and progression of diabetic nephropathy in the db/db mouse model of type 2 diabetes mellitus. To characterize SNGFR and tubuloglomerular feedback (TGF) responses to Nx and chronic neuronal nitric oxide synthase inhibition in the db/db mouse, we studied the effects of Nx on whole kidney GFR, SNGFR, and TGF characteristics in db/db and wild-type (WT) mice after Nx or sham Nx. We also documented progression of glomerular changes over a 6-mo period. Whole kidney GFR and SNGFR were significantly higher in db/db Nx than db/db sham mice, without change in proximal tubule reabsorptive rates. The TGF responses, determined as proximal-distal SNGFR differences, were brisk: 12.1 +/- 1.0 vs. 8.4 +/- 0.6 nl/min in WT sham (P < 0.05), 15.7 +/- 1.0 vs. 12.0 +/- 1.0 nl/min in WT Nx (P < 0.05), and 17.8 +/- 1.3 vs. 14.3 +/- 1.0 nl/min in db/db Nx (P < 0.05) mice. Chronic ingestion of the neuronal nitric oxide synthase inhibitor S-methylthiocitrulline for 2-3 wk after Nx had no effect on SNGFR or the TGF response. These studies show further elevations in whole kidney GFR and SNGFR in these hyperglycemic morbidly obese db/db mice, with an intact TGF system after Nx. In addition, in the db/db Nx mice, 4-6 mo after Nx, there was an exacerbation of the lesions of diabetic nephropathy, as quantified by a significant increase in the ratio of mesangial surface area to total glomerular surface area.     

Glomerulotubular balance, dietary protein, and the renal response to glycine in diabetic rats

The glomerular filtration rate (GFR) normally increases during glycine infusion, which is a test of "renal reserve." Renal reserve is absent in diabetes mellitus. GFR increases after protein feeding because of increased tubular reabsorption, which reduces the signal for tubuloglomerular feedback (TGF). Dietary protein restriction normalizes some aspects of glomerular function in diabetes. Renal micropuncture was performed in rats 4-5 wk after diabetes was induced by streptozotocin to determine whether renal reserve is lost as a result of altered tubular function and activation of TGF, whether 10 days of dietary protein restriction could restore renal reserve, and whether this results from effects of glycine on the tubule. TGF activation was determined by locating single-nephron GFR (SNGFR) in the early distal tubule along the TGF curve. The TGF signal was determined from the ionic content of the early distal tubule. In nondiabetic rats, SNGFR in the early distal tubule increased during glycine infusion because of primary vasodilation augmented by increased tubular reabsorption, which stabilized the TGF signal. In diabetic rats, glycine reduced reabsorption, thereby activating TGF, which was largely responsible for the lack of renal reserve. In protein-restricted diabetic rats, the tubular response to glycine remained abnormal, but renal reserve was restored by a vascular mechanism. Glycine affects GFR directly and via the tubule. In diabetes, reduced tubular reabsorption dominates. In low-protein diabetes, the vascular effect is enhanced and overrides the effect of reduced tubular reabsorption.     

Long-Term Responses to Loss of Renal Mass: Tubular Changes: A Micropuncture Study of HCO3 Reabsorption by the Hypertrophied Proximal Tubule

In rats with renal failure produced by excision of one kidney and infarction of large portions of the other kidney, given a low calcium, high phosphorus diet for 2-3 weeks, GFR was reduced by 80 percent, the fractional excretion of sodium increased from 7 to 23 percent, that of bicarbonate from 16 to 23 percent and that of water from 4 to 13 percent. Single nephron GFR in the remaining nephrons was nearly doubled and end-proximal TF/P_In was depressed from 2.3 to 1.8, and proximal TF/P_HCO3 from 0.52 to 0.35, the latter figure corresponding to an increase of absolute proximal HCO₃ reabsorption from 1.7 to 3.5 nEq/min or from 2.8 to 3.2 Eq/L of single nephron glomerular filtrate. Acute parathyroidectomy had no influence on the fall of GFR or the rise of SNGFR in the remaining nephrons and failed to cause any significant changes in proximal tubular bicarbonate reabsorption. Parathyroidectomy, on the other hand, practically prevented the rise of the fractional excretion of sodium and of water and inverted the rise of the fractional excretion of bicarbonate to a fall. The data are interpreted to indicate that secondary hyperparathyroidism in renal failure impairs distal nephron bicarbonate and sodium reabsorption and, thus, contributes to the maintenance of sodium balance, but could possibly aggravate acidosis.     

A search for a defect of proximal transport in denervated kidneys of conscious dogs

The influence of renal nerves on proximal Na+ reabsorption was studied in clearance experiments with unilaterally renal-denervated conscious dogs prepared by surgical bladder division. Two types of experiments were made : A. maximal water diuresis, and B. Total blockade of distal NaCl reabsorption with ethacrynic acid and chlorothiazide. In maximal water diuresis CH2O + CNa was used as a measure of fluid delivery to the distal nephron. At similar GFR on both sides, the proximal reabsorption estimated as GFR--(CH2O + CNa) was 38.4 +/- 5.6 ml/min for the intact and 35.9 +/- 4.2 ml/min for the denervated kidney (n = 6, difference NS). After distal tubular blockade, proximal Na+ reabsorption calculated as filtered load minus urinary excretion was 3.84 +/- 0.43 mmol/min for the intact and 3.91 +/- 0.36 mmol/min for the denervated kidney (n = 6, difference NS). The fractional reabsorption of NA+ was 64.9 +/- 1.0% for the intact and 66.9 +/- 1.1% for the denervated kidney (difference NS). In contrast to data from renal denervation studies with anaesthetized animals, the present experiments did not show any difference in proximal reabsorption between the innervated- and denervated kidney. We conclude that in absence of anaesthesia renal efferent nerves have no major effect on NaCl transport in dog proximal tubule.     

Modulation of single-nephron GFR in the db/db mouse model of type 2 diabetes mellitus

Hyperfiltration has been implicated in the progression toward diabetic nephropathy in type 2 diabetes mellitus (DM2). This study focuses for the first time on the in vivo modulation of single-nephron GFR (SNGFR) in the classic B6.Cg-m(+/+)Lepr(db)/J (db/db) mouse model of DM2. To obtain stable preparations, it was necessary to use a sustaining infusion of 3.3 ml.100 g body wt(-1) x h(-1), or higher. SNGFR (measured both proximally and distally) was greater in db/db vs. heterozygote (db/m) mice (P < 0.05) but not vs. the wild-type (WT) mice. The tubuloglomerular feedback (TGF) responses, determined as free-flow proximal vs. distal SNGFR differences, were significant in db/db mice (11.6 +/- 0.8 vs. 9.3 +/- 1.0 nl/min, P < 0.01), in db/m mice (8.0 +/- 0.8 vs. 7.2 +/- 0.6 nl/min, P < 0.02), and WT mice (9.9 +/- 0.6 vs. 8.9 +/- 0.7 nl/min, P < 0.05). After increasing the sustaining infusion in the db/db mice, to offset glycosuric urine losses, the SNGFR increased significantly, and the TGF response was abolished. In these volume-replete db/db mice, absolute fluid reabsorption measured both at the late proximal and distal tubular sites were significantly increased vs. db/m mice infused at 3.3 ml.100 g body wt(-1) x h(-1). After infusion of the neuronal nitric oxide synthase (nNOS) inhibitor S-methylthiocitrulline, SNGFR fell in both db/db and db/m mice. These studies show that SNGFR is elevated in this mouse model of DM2, is suppressed by nNOS inhibition, and is modulated by TGF influences that are altered by the diabetic state and responsive to changes in extracellular fluid volume.     

Acute and chronic effects of SGLT2 blockade on glomerular and tubular function in the early diabetic rat

Tubuloglomerular feedback (TGF) stabilizes nephron function from minute to minute and adapts to different steady-state inputs to maintain this capability. Such adaptation inherently renders TGF less efficient at buffering long-term disturbances, but the magnitude of loss is unknown. We undertook the present study to measure the compromise between TGF and TGF adaptation in transition from acute to chronic decline in proximal reabsorption (Jprox). As a tool, we blocked proximal tubule sodium-glucose cotransport with the SGLT2 blocker dapagliflozin in hyperglycemic rats with early streptozotocin diabetes, a condition in which a large fraction of proximal fluid reabsorption owes to SGLT2. Dapagliflozin acutely reduced proximal reabsorption leading to a 70% increase in early distal chloride, a saturated TGF response, and a major reduction in single nephron glomerular filtration rate (SNGFR). Acute and chronic effects on Jprox were indistinguishable. Adaptations to 10-12 days of dapagiflozin included increased reabsorption by Henle's loop, which caused a partial relaxation in the increased tone exerted by TGF that could be explained without desensitization of TGF. In summary, TGF contributes to long-term fluid and salt balance by mediating a persistent decline in SNGFR as the kidney adapts to a sustained decrease in Jprox.     

Hyperoncotic albumin infusion in experimental glomerulonephritis in rats: A micropuncture study.

The effect of infusion of hyperonciotic (25 g% salt-poor bovine plasma albumin) (0.06 g% of body weight) has been studied using clearance and micropuncture techniques in animals with two forms of immunologically induced experimental glomerulonephritis and in age-matched control rats. In control rats the significant natriuresis and diuresis after hyperoncotic albumin were associated with decreased fractional and absolute proximal reabsorption, decreased calculated efferent oncotic pressure, and a transitory rise in efferent arteriolar hydrostatic pressure. These findings, however, do not necessarily indicate a predominant role for peritubular "physical factors" in the control of proximal reabsorption. In glomerulonephritic rats a smaller diuresis and an insignificant natriuresis were found after hyperoncotic albumin, especially in anti-GBM nephritis, when no change in SNGFR or proximal absolute reabsorption occurred. In AICN there was a rise in SNGFR and a fall in absolute reabsorptive rate, especially in those nephrons with high filtration rates. There was no evidence that any alteration in efferent arteriolar hydrostatic pressure of calculated efferent oncotic pressure had occurred.     

Evidence against bicarbonate reabsorption in the ascending limb, particularly as disclosed by free-water clearance studies.

Bicarbonate reabsorption in the thick ascending limb of Henle's loop was examined by studies of free-water clearance (CH2O) and free-water reabsorption (TcH2O). During maximal water diuresis in the dog, CH2O/GFR was taken as an indes of sodium reabsorption in, and urine flow (V/GFR) as an index of delivery of filtrate to, this scarbonate, infusion of a nonreabsorbable solute (hypotonic mannitol) and administration of an inhibitor of bicarbonate reabsorption (acetaent, but less than that achieved with hypotonic saline infusion. This suggests that sodium that sodium bicarbonate is not reabsorbed in the ascending limb. Rather, it is the sodium chloride, swept out of the proximal tubule by osmotic diuresis due to nonreabsorbed mannitol or sodium bicarbonate, that is reabsorbed in the ascending limb thereby increasing CH2O, whereas the nonreabsorption of mannitol and sodium bicarbonate results in a depressed CH20 per unit V when compared with hypotonic saline. V/GFR is not a satisfactory index of delivery to the ascending limb during osmotic diuresis, since it includes water obligated by nonreabsorbable solutes. When a better index of delivery, the sum of the clearances of chloride (CC1) and free-water (CH2O) is used, hypotonic bicarbonate infusion, hypotonic mannitol infusion and acetazolamide administration increase CH2O/GFR per unit delivery to the same extent as odes hypotonic saline infusion. Studies in dogs and rats on TcH2O also indicate that sodium bicarbonate is an impermeant solute in the ascending limb. Osmotic diuresis due to sodium bicarbonate diuresis, produced either by inhibition of sodium bicarbonate reabsorption (acetazolamide, L-lysine mono-hydrochloride) or infusion of sodium bicarbonate, or mannitol diuresis both produced marked chloruresis and increased TcH2O to the same extent as did hypertonic saline infusion. If chloride excretion was almost eliminated by hemodialysis against a chloride-free dialysate (dogs) or prolonged feeding of a salt-free diet (rats), TcH2O formation was unimpaired if hypertonic saline was infused but virtually obliterated during mannitol or sodium bicarbonate diuresis. Sodium reabsorption in the ascending limb, therefore, appears to be dependent upon chloride as the accompanying anion. At any given rate of bicarbonate excretion, more cloride is delivered out of the proximal tubule (as estimated from CC1 + CH2O) with hypotonic sodium bicarbonate infusion than with acetazolamide administration. This suggests that magnitude of the chlorutesis accompanying bicarbonate diuresis depends, not only on osmotic diuresis due to nonreabsorbed sodium bicarbonate, but also on the extent to which concomitant changes in effective extracellular volume influence overall sodium chloride reabsorption.     

Renal hemodynamic response to L-arginine in uncomplicated, type 1 diabetes mellitus: the role of buffering anions and tubuloglomerular feedback

According to the "tubulocentric" hypothesis of the glomerular hyperfiltration of diabetes mellitus (DM), tubuloglomerular feedback (TGF) is the critical determinant of the related renal hemodynamic dysfunction. To examine the role of TGF in human type 1 DM, 12 salt-replete healthy (C) and 11 uncomplicated DM individuals underwent measurements of glomerular filtration rate (GFR), renal blood flow (RBF), and lithium-derived absolute "distal" sodium delivery (DDNa). Measurements were made during two 3-h infusions of 0.012 mmol·kg(-1)·min(-1) l-arginine (ARG) buffered with either equimolar HCl (ARG.HCl) or citric acid (ARG.CITR). Our hypothesis was that changes in TGF signaling would be directionally opposite ARG.HCl vs. ARG.CITR according to the effects of the ARG-buffering anion on DDNa. Similar changes in C and DM followed ARG.CITR, with declines in DDNa (-0.26 ± 0.07 mmol/min C vs. -0.31 ± 0.07 mmol/min DM) and increases in RBF (+299 ± 25 vs. +319 ± 29 ml·min(-1)·1.73 m(-2)) and GFR (+6.6 ± 0.8 vs. +11.6 ± 1.2 ml·min(-1)·1.73 m(-2)). In contrast, with ARG.HCl, DDNa rose in both groups (P = 0.001), but the response was 73% greater in DM (+1.50 ± 0.15 mmol/min C vs. +2.59 ± 0.22 mmol/min DM, P = 0.001). RBF also increased (P = 0.001, +219 ± 20 ml·min(-1)·1.73 m(-2) C, +105 ± 14 DM), but ΔRBF after ARG.HCl was lower vs. ARG.CITR in both groups (P = 0.001). After ARG.HCl, ΔRBF also was 50% lower in DM vs. C (P = 0.001) and GFR, unchanged in C, declined in DM (-7.4 ± 0.9 ml·min(-1)·1.73 m(-2), P = 0.02 vs. C). After ARG.HCl, unlike ARG.CITR, DDNa increased in C and DM, associated with less ΔRBF and ΔGFR vs. ARG.CITR. This suggests that the renal hemodynamic response to ARG is influenced substantially by the opposite actions of HCl vs. CITR on DDNa and TGF. In DM, the association of ARG.HCl-induced exaggerated ΔDDNa, blunted ΔRBF, and the decline in GFR vs. C shows an enhanced TGF dependence of renal vasodilatation to ARG, in agreement with a critical role of TGF in DM-related renal hemodynamic dysfunction.     

Chronic effect of insulin-like growth factor I on renin synthesis, secretion, and renal function in fetal sheep

In the adult, insulin-like growth factor I (IGF-I) increases glomerular filtration rate (GFR) and renal blood flow (RBF) during both acute and chronic treatment. To study its effects on the developing kidney, chronically catheterized fetal sheep (120 +/- 1 days gestation) were infused intravenously for up to 10 days with 80 microgram/h IGF-I (n = 5) or vehicle (0.1% BSA in saline, n = 6). In contrast to previous acute studies in adult rats and humans, after 4 h of IGF-I fetal GFR and RBF were unchanged. Fractional sodium reabsorption increased (P < 0.05). However, by 4 days, GFR per kilogram had risen by 35 +/- 13% (P < 0.05), whereas RBF remained unchanged. Tubular growth and maturation may have occurred, as proximal tubular sodium reabsorption increased by ~35% (P < 0.005). Therefore, despite a marked increase in filtered sodium (~30%, P < 0.05), fractional sodium reabsorption did not change. Although the effects of IGF-I on renal function were delayed, plasma renin activity and concentration were both elevated after 4 h and remained high at 4 days (P < 0.05). Despite this, arterial pressure and heart rate did not change. Kidneys of IGF-I-infused fetuses weighed ~30% more (P = 0.05) and contained ~75% more renin than control fetuses (P < 0.005). Thus, in the fetus, the renal effects of long-term IGF-I infusion are very different from the adult, possibly because IGF-I stimulated kidney growth.     

Insulin induces the correlation between renal blood flow and glomerular filtration rate in diabetes: implications for mechanisms causing hyperfiltration

Glomerular filtration rate (GFR) and renal blood flow (RBF) are normally kept constant via renal autoregulation. However, early diabetes results in increased GFR and the potential mechanisms are debated. Tubuloglomerular feedback (TGF) inactivation, with concomitantly increased RBF, is proposed but challenged by the finding of glomerular hyperfiltration in diabetic adenosine A(1) receptor-deficient mice, which lack TGF. Furthermore, we consistently find elevated GFR in diabetes with only minor changes in RBF. This may relate to the use of a lower streptozotocin dose, which produces a degree of hyperglycemia, which is manageable without supplemental suboptimal insulin administration, as has been used by other investigators. Therefore, we examined the relationship between RBF and GFR in diabetic rats with (diabetes + insulin) and without suboptimal insulin administration (untreated diabetes). As insulin can affect nitric oxide (NO) release, the role of NO was also investigated. GFR, RBF, and glomerular filtration pressures were measured. Dynamic RBF autoregulation was examined by transfer function analysis between arterial pressure and RBF. Both diabetic groups had increased GFR (+60-67%) and RBF (+20-23%) compared with controls. However, only the diabetes + insulin group displayed a correlation between GFR and RBF (R(2) = 0.81, P < 0.0001). Net filtration pressure was increased in untreated diabetes compared with both other groups. The difference between untreated and insulin-treated diabetic rats disappeared after administering N(ω)-nitro-l-arginine methyl ester to inhibit NO synthase and subsequent NO release. In conclusion, mechanisms causing diabetes-induced glomerular hyperfiltration are animal model-dependent. Supplemental insulin administration results in a RBF-dependent mechanism, whereas elevated GFR in untreated diabetes is mediated primarily by a tubular event. Insulin-induced NO release partially contributes to these differences.     

Elucidating mechanisms underlying altered renal autoregulation in diabetes

Previous studies have reported that high-salt intake paradoxically activates tubuloglomerular feedback (TGF) in type 1 diabetes. Using Zucker lean (ZL) and diabetic fatty (ZDF) rats on normal and high-salt diets, renal hemodynamics and the renin-angiotensin system (RAS) were characterized. On normal salt diet, glomerular filtration rate (GFR) was higher in ZDF than ZL rats. Autoregulation of GFR was less efficient and lithium clearance was lower in ZDF rats than ZL rats. Salt load reduced GFR in ZDF rats with restoration of lithium clearance and partial improvement in autoregulatory index (AI). The administration of 8-cyclopentyl-1,3-dipropylxanthine, a selective adenosine-1 receptor antagonist to ZDF rats on a high-salt diet abolished the improvement of AI in GFR. However, this effect was seen by neither (Cx40)GAP27 nor (Cx37,43)GAP27, which inhibits connexin (Cx) 40 or Cx37. Renal ANG II was higher in ZDF than ZL rats on normal salt diet, but the difference was eliminated by a salt load. The present data provide the first demonstration for a salt paradox in type 2 diabetes and implicate that in addition to Cx alterations, an enhanced proximal reabsorption attenuates TGF, underlying glomerular hyperfiltration and RAS activation. These data suggest that a high-salt diet standardizes distal delivery in diabetes, suppressing the RAS, and improving GFR autoregulation and hyperfiltration through adenosine.     

Effect of Acetazolamide on Obesity-Induced Glomerular Hyperfiltration: A Randomized Controlled Trial

Aims

Obesity is an important risk factor for the development of chronic kidney disease. One of the major factors involved in the pathogenesis of obesity-associated kidney disease is glomerular hyperfiltration. Increasing salt-delivery to the macula densa is expected to decrease glomerular filtration rate (GFR) by activating tubuloglomerular feedback. Acetazolamide, a carbonic anhydrase inhibitor which inhibits salt reabsorption in the proximal tubule, increases distal salt delivery. Its effects on obesity-related glomerular hyperfiltration have not previously been studied. The aim of this investigation was to evaluate whether administration of acetazolamide to obese non diabetic subjects reduces glomerular hyperfiltration.

Materials and Methods

The study was performed using a randomized double-blind crossover design. Obese non-diabetic men with glomerular hyperfiltration were randomized to receive intravenously either acetazolamide or furosemide at equipotent doses. Twelve subjects received the allocated medications. Two weeks later, the same subjects received the drug which they had not received during the first study. Inulin clearance, p-aminohippuric acid clearance and fractional lithium excretion were measured before and after medications administration. The primary end point was a decrease in GFR, measured as inulin clearance.

Results

GFR decreased by 21% following acetazolamide and did not decrease following furosemide. Renal vascular resistance increased by 12% following acetazolamide, while it remained unchanged following furosemide administration. Natriuresis increased similarly following acetazolamide and furosemide administration. Sodium balance was similar in both groups.

Conclusions

Intravenous acetazolamide decreased GFR in obese non-diabetic men with glomerular hyperfiltration. Furosemide, administered at equipotent dose, did not affect GFR, suggesting that acetazolamide reduced glomerular hyperfiltration by activating tubuloglomerular feedback.

Trial Registration

ClinicalTrials.gov NCT01146288     

p22phox in the macula densa regulates single nephron GFR during angiotensin II infusion in rats

Angiotensin II (ANG II) infusion increases renal superoxide (O(2)(-)) and enhances renal vasoconstriction via macula densa (MD) regulation of tubuloglomerular feedback, but the mechanism is unclear. We targeted the p22(phox) subunit of nicotinamide adenine dinucleotide phosphate (NADPH) oxidase (NOX) with small-interfering RNA (siRNA) to reduce NADPH oxidase activity and blood pressure response to ANG II in rats. We compared single nephron glomerular filtration rate (SNGFR) in samples collected from the proximal tubule (PT), which interrupts delivery to the MD, and from the distal tubule (DT), which maintains delivery to the MD, to assess MD regulation of GFR. SNGFR was measured in control and ANG II-infused rats (200 ng.kg(-1).min(-1) for 7 days) 2 days after intravenous injection of vehicle or siRNA directed to p22(phox) to test the hypothesis that p22(phox) mediates MD regulation of SNGFR during ANG II. The regulation of SNGFR by MD, determined by PT SNGFR-DT SNGFR, was not altered by siRNA in control rats (control + vehicle, 13 +/- 1, n = 8; control + siRNA, 12 +/- 2 nl/min, n = 8; not significant) but was reduced by siRNA in ANG II-treated rats (ANG II + vehicle, 13 +/- 2, n = 7; ANG II + siRNA, 7 +/- 1 nl/min, n = 8; P < 0.05). We conclude that p22(phox) and NADPH oxidase regulate the SNGFR during ANG II infusion via MD-dependent mechanisms.     

Effect of glucagon and glomerulopressin on the renal function of the dog.

Glucagon was infused through the porta or through the left renal artery in dogs. Another group of dogs were infused with glomerulopressin through the left renal artery. It was observed that glucagon when infused through the portal vein enhanced the glomerulopressin production and the glomerular filtration rate (GFR). When glucagon was infused intrarenally it did not alter GRF but it had a direct tubular action decreasing sodium reabsorption in the proximal tubule. Glomerulopressin infused intrarenally increased GRF and potassium excretion. The results suggest that the increase in GFR was due to increase in glomerulopressin activity. There are three reasons for this statement: a) GRF increased when glomerulopressin activity was high, but not when there was a low activity, 5) intrarenally infused glomerulopressin produced a very significant change in the GFR of the infused kidney, while the GRF of the contralateral kidney remained unchanged and c) intrarenally administered glucagon had no effect on GFR.     

Volume expansion during NOS substrate donation with L-arginine: regulatory offsetting of renal response?

The responses to infusion of nitric oxide synthase substrate (L-arginine 3 mg.kg(-1).min(-1)) and to slow volume expansion (saline 35 ml/kg for 90 min) alone and in combination were investigated in separate experiments. L-Arginine left blood pressure and plasma ANG II unaffected but decreased heart rate (6 +/- 2 beats/min) and urine osmolality, increased glomerular filtration rate (GFR) transiently, and caused sustained increases in sodium excretion (fourfold) and urine flow (0.2 +/- 0.0 to 0.7 +/- 0.1 ml/min). Volume expansion increased arterial blood pressure (102 +/- 3 to 114 +/- 3 mmHg), elevated GFR persistently by 24%, and enhanced sodium excretion to a peak of 251 +/- 31 micromol/min, together with marked increases in urine flow, osmolar and free water clearances, whereas plasma ANG II decreased (8.1 +/- 1.7 to 1.6 +/- 0.3 pg/ml). Combined volume expansion and L-arginine infusion tended to increase arterial blood pressure and increased GFR by 31%, whereas peak sodium excretion was enhanced to 335 +/- 23 micromol/min at plasma ANG II levels of 3.0 +/- 1.1 pg/ml; urine flow and osmolar clearance were increased at constant free water clearance. In conclusion, L-arginine 1) increases sodium excretion, 2) decreases basal urine osmolality, 3) exaggerates the natriuretic response to volume expansion by an average of 50% without persistent changes in GFR, and 4) abolishes the increase in free water clearance normally occurring during volume expansion. Thus L-arginine is a natriuretic substance compatible with a role of nitric oxide in sodium homeostasis, possibly by offsetting/shifting the renal response to sodium excess.     

Effects of hypothermic kidney preservation on the isolated perfused kidney: a comparison of reperfusion methods

Two isolated-perfused kidney methods were used to study the effects of hypothermic preservation on renal function in dog kidneys. The isolated-machine-perfused kidney (IMPK) used an in vitro perfusion technique--the perfusate was a Krebs-bicarbonate type delivered to the kidney at 37 degrees C by a mechanical pump at a constant pressure (100 mm Hg). The isolated-blood-perfused kidney (IBPK) utilized transplantation of the preserved kidney to the femoral vasculature. Renal function (urine analysis) was determined over a 1-hr reperfusion interval and included GFR (creatinine clearance), urine formation, and Na+ reabsorption. Kidneys preserved for only 24 hr by cold storage in either Collins'--C3 solution or in hypotonic citrate and kidneys hypothermically perfused for 24 hr demonstrated greater retention of renal function when reperfused by blood (IBPK) than with the in vitro perfusate (IMPK). The GFR was reduced by 38-58% when tested with the IBPK, but by 80-90% when tested with the IMPK. Na+ reabsorption was normal (97%) with blood reperfusion but was reduced to 36-50% in cold-stored kidneys and 82% in hypothermically perfused kidneys determined by machine reperfusion (IMPK). However, kidneys perfused for 72 hr demonstrated more similar renal functions when tested by either IMPK or IBPK. GFR was reduced to 20% (IBPK) and 11% (IMPK) and Na+ reabsorption averaged 76-85% (IBPK or IMPK). These results suggest that either reperfusion method is suitable for determining the effects of renal preservation on kidney function in kidneys preserved for 72 hr but, for short-term preserved kidneys (24 hr), the IBPK model may be preferred.     

The effect of renal arterial infusion of albumin and dextran on tubular fluid reabsorption in the dog.

Albumin or Dextran solutions of varying concentration were infused into the renal artery of hydropenic dogs. Their effect on urine flow, sodium excretion, creatinine and PAH clearance, single nephron GFR, fractional and absolute fluid reabsorption in the proximal convolution, reabsorptive t1/2, and hydrostatic pressures in the proximal tubules and adjacent capillaries was compared with a similar infusion of isotonic saline solution. Six, 9, 12, 18 and 25% albumin and 6% Dextran solution did not significantly change the measured parameters. Infusion of 9 and 12% Dextran solution elicited a decrease in water and sodium excretion as well as absolute and fractional proximal tubular fluid reabsorption to a 5% level of significance. Infusion of 18% Dextran was accompanied by a marked decrease in total and proximal reabsorption combined with a decline of GFR, PAH clearance, and hydrostatic pressures in tubules and peritubular capillaries. The results do not support the hypothesis of a direct action of oncotic pressure on tubular fluid reabsorption; the above described effects of Dextran seem to be accounted for by its other "pharmacological" effect.     

Increased glomerular filtration rate in early metabolic syndrome is associated with renal adiposity and microvascular proliferation

Metabolic syndrome (MetS) is associated with glomerular hyperfiltration and is a risk factor for chronic kidney disease, but the underlying mechanisms are poorly defined. This study tested the hypothesis that increased glomerular filtration rate (GFR) in early MetS is associated with renal adiposity and microvascular proliferation. Twelve MetS-prone Ossabaw pigs were randomized to 10 wk of a standard (lean, n = 6) or atherogenic (MetS, n = 6) diet. Kidney hemodynamics and function, perirenal fat volume, and tubular dynamics were assessed in vivo by multidetector computed tomography (CT) and blood oxygen level-dependent (BOLD)-MRI. Microvascular architecture was assessed ex vivo with micro-CT. Candidate injury mechanisms were evaluated in kidney tissue by Western blotting and histology. Basal GFR, renal blood flow, and renal cortical perfusion and volume were elevated in the MetS group. Perirenal and kidney tissue fat, proximal-nephron intratubular fluid concentration, and endothelial nitric oxide synthase expression were increased in MetS. GFR levels correlated with tissue triglyceride levels. Elevated spatial density of 20- to 40-μm cortical microvessels was accompanied by mild oxidative stress, inflammation, and with proximal tubular vacuolization. Medullary size and perfusion were relatively preserved, and BOLD-MRI showed intact medullary tubular response to furosemide. Increased GFR in early MetS is associated with renal adiposity and microvascular proliferation, which involve mainly the renal cortex and precede significant activation of oxidative stress and inflammation. Renal adiposity and proliferative microvessels may represent novel therapeutic targets for preserving renal function in early MetS.