Is a reduction in albuminuria associated with renal and cardiovascular protection? A post hoc analysis of the ALTITUDE trial.

AIMS: To investigate whether the degree of albuminuria reduction observed in the ALTITUDE trial is associated with renal and cardiovascular protection, and secondly, whether the reduction in albuminuria was too small to afford clinical benefit. METHODS: In a post hoc analysis of the ALTITUDE trial in 8561 patients with type 2 diabetes and chronic kidney disease or cardiovascular disease we examined the effect of albuminuria changes at 6 months on renal and cardiovascular outcomes using Cox proportional hazard regression. RESULTS: The median change in albuminuria in the first 6 months in the aliskiren arm of the trial was -12% (25th to 75th percentile: -48.7_to_ +41.9%) and 0.0% (25th to 75th percentile: -40.2_to_55%) in the placebo arm. Changes in albuminuria in the first 6 months were linearly associated with renal and cardiovascular endpoints: a >30% reduction in albuminuria in the first 6 months was associated with a 62% reduction in renal risk and a 25% reduction in cardiovascular risk compared with an increase in albuminuria. The association between changes at 6 months in albuminuria and renal or cardiovascular endpoints was similar in the two treatment groups (p for interaction >0.1 for both endpoints). CONCLUSIONS: The addition of aliskiren to angiotensin-converting enzyme inhibitor/angiotensin receptor blocker therapy resulted in albuminuria changes that were associated with renal and cardiovascular risk changes. This did not translate into renal or cardiovascular protection because the overall reduction in albuminuria in the aliskiren arm was too small and nearly similar to that in the placebo arm.

Increased serum potassium affects renal outcomes: a post hoc analysis of the Reduction of Endpoints in NIDDM with the Angiotensin II Antagonist Losartan (RENAAL) trial.

AIMS/HYPOTHESIS: To assess the effect of an angiotensin receptor blocker (ARB) on serum potassium and the effect of a serum potassium change on renal outcomes in patients with type 2 diabetes and nephropathy. METHODS: We performed a post hoc analysis in patients with type 2 diabetes participating in the Reduction of Endpoints in NIDDM with the Angiotensin II Antagonist Losartan (RENAAL) study. Renal outcomes were defined as a composite of doubling of serum creatinine or end-stage renal disease. RESULTS: At month 6, 259 (38.4%) and 73 (10.8%) patients in the losartan group and 151 (22.8%) and 34 (5.1%) patients in the placebo group had serum potassium ≥5.0 mmol/l and ≥5.5 mmol/l, (p < 0.001), respectively. Losartan was an independent predictor for serum potassium ≥5.0 mmol/l at month 6 (OR 2.8; 95% CI 2.0-3.9). Serum potassium at month 6 ≥ 5.0 mmol/l was in turn associated with increased risk for renal events (HR 1.22; 95% CI 1.00-1.50), independent of other risk factors. Adjustment of the overall treatment effects for serum potassium augmented losartan's renoprotective effect from 21% (6-34%) to 35% (20-48%), suggesting that the renoprotective effects of losartan are offset by its effect on serum potassium. CONCLUSIONS/INTERPRETATION: In this study, we found that treatment with the ARB losartan is associated with a high risk of increased serum potassium levels, which is in turn associated with an increased risk of renal outcomes in patients with diabetes and nephropathy. Whether additional management of high serum potassium would further increase the renal protective properties of losartan is an important clinical question.

Renal risk scores: progress and prospects.

Worldwide adoption of the Kidney Disease Outcomes Quality Initiative classification for chronic kidney disease (CKD) and widespread use of the estimated glomerular filtration rate to assess renal function have identified large numbers of patients with previously undiagnosed CKD. It is clear, however, that this is a heterogeneous group and that only a small minority of such patients ever progress to end-stage renal disease. There is thus an urgent need for a simple method of risk assessment that can be applied to all patients with CKD to identify those few at greatest risk. The magnitude of baseline proteinuria has long been recognized as an important predictor of renal prognosis. Furthermore, several studies have found that change in proteinuria after initiation of antihypertensive treatment as well as achieved level of proteinuria correlate with prognosis. Thus, proteinuria has emerged as the single most important marker of renal risk. Many other factors have been identified as risk factors for CKD progression. Several attempts have been made to combine a relatively small number of risk factors into a risk score to predict renal outcomes in specific groups of patients. Validation of these risk scores as well as further studies are now required to develop a renal risk score applicable to a more general population of patients with CKD. Similar methodology could be applied to assess the important issue of the cardiovascular risk associated with CKD.

Importance of efferent arteriolar vascular tone in regulation of proximal tubule fluid reabsorption and glomerulotubular balance in the rat.

Micropuncture study was performed in 21 mildly volume-expanded Munich-Wistar rats before and during partial aortic constriction to examine the effects of endogenous prostaglandins (PG) and angiotensin II (AII) on single nephron glomerular filtration rate (SNGFR) and absolute proximal reabsorption rate (APR). Animals received either vehicle (group 1), indomethacin (group 2), or indomethacin plus saralasin (group 3). Before aortic constriction, these inhibitors were without effect on values of SNGFR and APR. In group 1 rats, reduction in mean renal arterial perfusion pressure (RAP) to approximately 65 mm Hg resulted in marked and proportional declines in SNGFR and APR. With equivalent reduction in RAP in group 2 rats, however, SNGFR fell to a lesser extent and APR tended to increase slightly above preconstriction values. Indomethacin administration was therefore associated with disruption of glomerulotubular balance. In view of the roughly equivalent declines in afferent arteriolar resistance measured in groups 1 and 2, the magnitude of increase in efferent arteriolar resistance (R(E)) appeared to be of major importance in determining the observed presence or absence of glomerulotubular balance. Thus, the lesser fall in SNGFR in group 2 than in group 1 was a result of the higher value for glomerular capillary hydraulic pressure in group 2, a consequence of the higher value of R(E). The higher average value for APR during reduced RAP in group 2 than in group 1 is also attributable to this pronounced rise in R(E), the effect of which was to augment the net reabsorptive pressure both by favoring higher postglomerular oncotic pressure and lower downstream (peritubular capillary) hydraulic pressure. Since intrarenal release of AII is enhanced when RAP declines, and because AII is known to raise R(E) selectively, it is likely that endogenous AII brought about the marked increase in R(E) in group 2, which was readily demonstrable only in indomethacin-treated rats, presumably because endogenous PG synthesis was suppressed. In keeping with this conclusion, when the action of endogenous AII was inhibited by saralasin in group 3 rats, reduction in RAP failed to induce a rise in R(E), so that net filtration and reabsorption pressures again declined proportionally, as did SNGFR and APR. The present evidence therefore suggests that glomerulotubular balance is influenced to an important extent by the prevailing vasomotor tone of the efferent arteriole.

Mechanism of inhibition of proximal tubule fluid reabsorption after exposure of the rat kidney to the physical effects of expansion of extracellular fluid volume.

The natriuresis and concomitant decline in absolute proximal reabsorption (APR) that occur in rats in response to saline loading are blunted markedly when renal perfusion pressure is reduced immediately before, but not after, the volume load. To ascertain the mechanism responsible for these differences between early clamp (EC) vs. late clamp (LC), intracapillary and interstitial determinants of peritubular capillary uptake of APR were measured in seven LC and seven EC Munich-Wistar rats before and after isotonic saline loading (80% body wt). With volume expansion in LC animals, we observed a marked decline in APR (averaging 11+/-1 nl/min), associated with large increases in urinary sodium excretion rate, which averaged 8+/-2 mueq/min. In EC, the changes in urinary sodium excretion rate (+1+/-0 mueq/min) and APR (-3+/-1 nl/min) with volume expansion were smaller in magnitude. Since peritubular capillary reabsorption coefficient and mean peritubular transcapillary hydraulic pressure difference did not change with saline loading in LC, the marked fall in APR was attributed primarily to a measured large decline in mean peritubular transcapillary oncotic pressure difference (deltapi). Despite an equivalent mean fall in deltapi with volume expansion in EC, near-constancy of APR was found to be associated with a simultaneous and equivalent decline in mean peritubular transcapillary hydraulic pressure difference (a consequence of decreased mean peritubular capillary hydraulic pressure), which effectively offset the fall in deltapi. These results demonstrate the importance of hydraulic pressure patterns of the peritubular capillaries in modulating APR and are consistent with the view that Starling forces across the postglomerular microcirculation play a fundamental role in determining APR.

Postglomerular vascular protein concentration: evidence for a causal role in governing fluid reabsorption and glomerulotublar balance by the renal proximal tubule.

We tested the relationship between postglomerular microvascular protein concentration and rates of sodium and water transfer by rat proximal tubules. Using recently described microperfusion techniques, efferent arterioles and branch peritubular capillaries of normal hydropenic rats were perfused with colloid-free Ringer's solution, and isoncotic (9.0-10.0 g/100 ml) and hyperoncotic (15 g/100 ml) albumin-Ringer's solutions. Reabsorption in adjacent proximal tubules was studied using free-flow techniques, with initial collections obtained during normal blood perfusion, recollections during experimental microperfusion, and in some tubules, repeat recollections after microperfusion and spontaneous resumption of blood perfusion. Colloid-free perfusion resulted in a uniform inhibition of proximal reabsorption (absolute and fractional). Despite identical techniques, substitution of isoncotic and hyperoncotic perfusates resulted, on average, in unchanged and increased rates of reabsorption, respectively. These findings of direct linear changes in reabsorption in response to changes in postglomerular protein concentrations usually occurred in the absence of significant changes in filtered load, and were nearly always found to be reversible within minutes of cessation of experimental perfusion. Given this evidence of a causal relationship between postglomerular oncotic pressure and proximal reabsorption, we undertook to determine whether this relationship is responsible for the parallel adjustments in proximal reabsorption that follow changes in GFR (glomerulotubular balance). Using a separate group of hydropenic rats, proximal reabsorption was studied, initially during partial aortic constriction (during which renal perfusion pressure, single nephron GFR, absolute proximal reabsorption, and calculated filtration fraction all were reduced below levels prior to constriction), and again while adjacent efferent arteriolar and peritubular capillary protein concentrations, but not GFR, were restored to normal (preconstriction) levels by microperfusion with 9-10 g/100 ml albumin-Ringer's solution. During this dissociation of GFR and postglomerular protein concentration, absolute and fractional proximal reabsorption nearly always increased in parallel with the changes in the latter, thereby demonstrating that glomerulotubular balance is mediated, at least in part, by changes in postglomerular oncotic pressure brought about by changes in filtration fraction.

Sodium-hydrogen exchange and glucose transport in renal microvillus membrane vesicles from rats with diabetes mellitus.

Diabetes mellitus is associated with important changes in renal hemodynamics and transport function. Disturbances in solute transport have also been characterized in nonrenal tissues during hyperglycemia and insulinopenia. The purpose of this study was to determine if diabetes is associated with adaptive changes in function of the brush-border membrane of the proximal tubule. We studied Na+ and glucose transport in rat microvillus membrane vesicles isolated from the renal cortex of streptozotocin-induced and BB/W autoimmune diabetic rats. Untreated diabetes was associated with an increase in pH-stimulated total and amiloride-sensitive 22Na+ uptake into vesicles. Insulin treatment returned vesicle 22Na+ uptake to control levels. The increased Na+/H+ exchange was shown to be a result of increased net renal acid production rather than a specific response to insulinopenia because treatment with NaHCO3 also returned 22Na+ uptake to control levels. On the other hand, Na+-glucose cotransport, which was depressed in vesicles from untreated diabetics, returned to control levels with insulin but not NaHCO3 administration. This decreased Na+-glucose cotransport was not secondary to reduction in transport sites in untreated diabetics. These results show that in diabetes mellitus, increased Na+/H+ exchange activity is not the direct result of insulinopenia. However, the diabetic state appears to alter the functioning of the luminal Na+-glucose cotransporter.

Therapeutic advantage of converting enzyme inhibitors in arresting progressive renal disease associated with systemic hypertension in the rat.

Micropuncture and morphologic studies were performed in six groups of male Munich-Wistar rats after removal of the right kidney and segmental infarction of two-thirds of the left kidney. Groups 1 and 4 received no specific therapy. Groups 2 and 5 were treated with the angiotensin I-converting enzyme inhibitor, enalapril, 50 mg/liter, in the drinking water. Groups 3 and 6 were treated with reserpine (5 mg/liter), hydralazine (80 mg/liter), and hydrochlorothiazide (25 mg/liter). All rats were fed standard chow. Groups 1-3 underwent micropuncture study 4 wk after renal ablation. Untreated group 1 rats exhibited systemic hypertension and elevation of the single nephron glomerular filtration rate (SNGFR) due to high average values for the mean glomerular transcapillary hydraulic pressure gradient (delta P) and glomerular plasma flow rate (QA). In group 2 rats, treatment with enalapril prevented systemic hypertension and maintained delta P at near-normal levels without significant reduction in SNGFR and QA. In contrast, triple drug therapy normalized systemic hypertension, but failed to lower delta P in group 3 rats. Groups 4-6 were followed for 12 wk after renal ablation. Untreated group 4 rats demonstrated continuous systemic hypertension, progressive proteinuria, and glomerular structural lesions, including mesangial expansion and frequent areas of segmental sclerosis. In group 5 rats, treatment with enalapril maintained systemic blood pressure at normal levels over the 12-wk period and dramatically limited the development of proteinuria and glomerular lesions. Despite equivalent systemic blood pressure control in group 6 rats, failure of triple drug therapy to control glomerular hypertension was associated with progressive proteinuria and glomerular lesions comparable to those seen in untreated group 4 rats. Thus, unless glomerular capillary hypertension is corrected, control of systemic blood pressure is insufficient to prevent progressive renal injury in rats with reduced renal mass.

Early enhancement of fluid transport in rabbit proximal straight tubules after loss of contralateral renal excretory function.

To assess the renal functional adaptation to reduced excretory capacity, we studied whole kidney and single nephron function in anesthetized volume-replete rabbits after unilateral (left kidney) nephrectomy (UNX), ureteral obstruction (UO), or ureteroperitoneostomy (UP). At 24 h, despite the absence of measurable hypertrophy of the contralateral (right) kidney, these procedures significantly increased p-aminohippurate clearance (45-54%) and inulin clearance (CIN) (64-110%) compared with sham-operated control animals. In each group, whole kidney sodium reabsorption increased in proportion to the rise in CIN. To determine whether the intrinsic transport capacity of proximal tubule segments is altered by these maneuvers, we measured fluid volume reabsorption rate (Jv) in isolated superficial proximal straight tubule (PST) segments perfused in vitro, comparing each control tubule (obtained by biopsy of the left kidney immediately before an experimental maneuver) with a corresponding tubule segment obtained 24 h or 7 d later from the contralateral kidney. Control tubule Jv in sham-24 h animals averaged 0.48 +/- 0.04 nl/(min X mm). Jv did not change significantly at 24 h or 7 d after sham maneuvers but increased significantly at 24 h after UNX [delta Jv = 0.13 +/- 0.03 nl/(min X mm)], UO [delta Jv = 0.10 +/- 0.04 nl/(min X mm)], and UP [delta Jv = 0.13 +/- 0.04 nl/(min X mm)]. Jv remained increased by similar amounts at 7 d after UNX and UO. To evaluate whether an increase in glomerular filtration rate (GFR) might be the stimulus to this augmentation in Jv values, methylprednisolone (MP) (15 mg/kg per d) was administered daily to sham-operated animals, a maneuver which induced a 73% rise in CIN by day 5. This procedure also produced a significant increase in Jv in PST at 5 d [delta Jv = 0.16 +/- 0.05 nl/(min X mm)]. The increase in Jv evident in each group at 5 or 7 d was paralleled by an equivalent change in tubule cell volume and apparent tubule luminal surface area in UNX-7d and MP-5d; no such increments in these indices, or in apparent tubule serosal surface area were evident at 24 h in any group. Thus, a 50% reduction in renal excretory function in the rabbit provokes adjustments in renal plasma flow rate and GFR in the contralateral kidney, which are evident by 24 h. The concurrent change in Jv in PST is closely related to CIN or some associated hemodynamic process, but does not appear to require an increase in tubule cell volume or apparent surface area. The ability to detect these small in vivo changes in Jv may derive from the enhanced sensitivity of paired-kidney experiments using tubule segments obtained by renal biopsy.

Micropuncture study of nephron function in the rhesus monkey.

The function of the proximal and distal tubule was studied in the rhesus monkey during antidiuresis and during the diuresis after furosemide administration (during which extracellular fluid volume was maintained).In the proximal tubule, fluid to plasma ratios for sodium, potassium, and osmolality approximated unity. During antidiuresis, about 30% of the filtered water remained at the end of the accessible portion of this segment (92% of length). Fluid was hypotonic to plasma throughout the distal tubule. 25% of the filtered water was present in the early distal tubule. Small but significant water reabsorption (about 8% of filtered) occurred in remainder of this segment. Tubule fluid to plasma potassium concentration ratios tended to increase along the distal tubule, and the amount of potassium, relative to the amount filtered, increased from 13% in the early portion of this segment to 26% in the late portion.After furosemide was administered animals excreted about one-third of the filtered sodium and water. Despite this diuresis, electrolyte and water reabsorption along the proximal tubule did not differ from values obtained in control animals. Osmolality and sodium concentration of fluid from the distal tubule approached those of plasma. 22% of the filtered sodium (twice the control values) reached the distal tubule, whereas the fraction of filtered water remaining was only slightly increased. These findings indicate that, after the administration of this drug, inhibition of sodium reabsorption occurred in the water-impermeable segment of the nephron, rather than in the proximal tubule. After furosemide administration, all tubule fluid to plasma potassium concentration ratios in the distal tubule were equal to or greater than one, suggesting inhibition of active potassium reabsorption at or prior to this site.Fluid to plasma bicarbonate concentration ratios from the midportion of the proximal tubule were consistently less than one in normal monkeys. After acetazolamide was administered, the bicarbonate concentration of samples of tubule fluid recollected from these same sites was the same as, or higher than in plasma. This fact demonstrates the inhibition of bicarbonate reabsorption in this portion of the tubule.

The relationship between glomerular filtration rate and sodium reabsorption by the proximal tubule of the rat nephron.

We have tested two of the hypotheses proposed to explain the adjustment in sodium reabsorption in the proximal tubule that follows a change in the rate of glomerular filtration (glomerulotubular balance). Using the recollection micropuncture technique, we were able to measure the immediate and late changes in reabsorptive rate after an acute alteration in filtration rate produced by aortic constriction and release of constriction. It was found that fractional reabsorption, as measured by the inulin tubule fluid to plasma (TF/P) ratio, increased after aortic constriction and decreased after release, but that in most instances, absolute reabsorptive rate changed in parallel to glomerular filtration rate. The change was similar whether the collections were made less than 1 or more than 5 min after the change in blood pressure. The rapid time course of this adjustment in reabsorptive rate is viewed as evidence against an intrarenal humoral feedback mechanism. In the same experiments we measured the (TF/P)(In), transit time, and flow rate of fluid in single nephrons before and during aortic constriction or release of aortic constriction. The change in reabsorptive rate and the simultaneous change in calculated cross-sectional area of the tubule lumen were rarely proportional, i.e., C/pir(2) was not constant. In other experiments, these same measurements were made before and during periods of increased ureteral pressure. Despite large increments in calculated cross-sectional area, the absolute rate of reabsorption either remained relatively unchanged or fell in proportion to the change in filtration rate. It is concluded that under these conditions, reabsorptive rate is governed by some factor other than tubule geometry.

Adaptation of Na+-H+ exchange in renal microvillus membrane vesicles. Role of dietary protein and uninephrectomy.

The ablation of renal mass and institution of a high protein diet both lead to renal cortical hypertrophy and increased glomerular filtration rate (GFR). We studied Na+ transport in rat microvillus membrane vesicles isolated from uninephrectomized or sham operated rats fed 6% (low), 24% (standard), or 40% (high) protein diets. The feeding of high protein, as compared with low protein, was associated with a 50% increase in rates of pH-stimulated 22Na+ transport in isolated vesicles from sham and uninephrectomized animals. Values for the standard protein diet were intermediate to values for high and low protein. At each level of dietary protein intake, vesicular Na+ transport was greater in the uninephrectomized than in sham rats. The high protein diet was also associated with increased vesicular 22Na+ flux inhibitable by 1 mM amiloride. Increases in total and amiloride sensitive flux were also noted in the absence of a pH gradient. Conductive Na+ and H+ transport were not altered, nor were sodium-glucose and sodium-alanine cotransport. Kinetic studies revealed evidence for an increased Vmax of Na+-H+ exchange in uninephrectomized animals fed a 40 vs. a 6% protein diet whereas Km was unchanged. Supplements of NaHCO3 in the 40% protein diet, to adjust for an increased rate of net acid excretion, did not prevent the increased rates of Na+-H+ exchange. However, treatment with actinomycin D (0.12 mg/kg) prevented the increased Na+-H+ activity as well as the increased renal mass and GFR noted 24 h after unilateral nephrectomy. Na+-H+ exchange rate was closely correlated with GFR (r = 0.961; P less than 0.005) and renal mass (r = .986; P less than 0.001). These observations provide evidence for modification of the luminal membrane Na+-H+ exchanger in response to changes in dietary protein content and nephron number.

The dynamics of glomerular ultrafiltration in the rat.

Using a unique strain of Wistar rats endowed with glomeruli situated directly on the renal cortical surface, we measured glomerular capillary pressures using servo-nulling micropipette transducer techniques. Pressures in 12 glomerular capillaries from 7 rats averaged 60 cm H(2)O, or approximately 50% of mean systemic arterial values. Wave form characteristics for these glomerular capillaries were found to be remarkably similar to those of the central aorta. From similarly direct estimates of hydrostatic pressures in proximal tubules, and colloid osmotic pressures in systemic and efferent arteriolar plasmas, the net driving force for ultrafiltration was calculated. The average value of 14 cm H(2)O is lower by some two-thirds than the majority of estimates reported previously based on indirect techniques. Single nephron GFR (glomerular filtration rate) was also measured in these rats, thereby permitting calculation of the glomerular capillary ultrafiltration coefficient. The average value of 0.044 nl sec(-1) cm H(2)O(-1) glomerulus(-1) is at least fourfold greater than previous estimates derived from indirect observations.

On the mechanism of inhibition in fluid reabsorption by the renal proximal tubule of the volume-expanded rat.

We undertook to determine the extent to which the inhibition in absolute proximal fluid reabsorption in response to expansion of extracellular volume with noncolloid-containing solutions is the result of concomitant reductions in postglomerular (efferent arteriolar) protein concentration. Selective elevation of efferent arteriolar oncotic pressure in volume-expanded rats (Ringer's 10% body weight) to levels slightly in excess of normal by microperfusion with 9-10% albumin-Ringer's solution nearly completely reversed the inhibition in absolute and fractional reabsorption in adjacent proximal tubules. In contrast, during similar microperfusion with a 6-7% albumin solution, no increase in proximal reabsorption was measured. We interpret these findings to indicate that the bulk of the inhibition in absolute proximal reabsorption in response to volume expansion with colloid-free solutions is causally mediated by the accompanying parallel decline in postglomerular vascular protein concentration.

Interleukin 1 induces prolonged L-arginine-dependent cyclic guanosine monophosphate and nitrite production in rat vascular smooth muscle cells.

The cytokine interleukin 1 (IL-1) inhibits contractile responses in rat aorta by causing endothelium-independent and prolonged activation of soluble guanylate cyclase. The present study tested whether IL-1 activates guanylate cyclase by inducing prolonged production of nitric oxide in cultured rat aortic vascular smooth muscle cells (VSMC). IL-1 induced a marked time-dependent increase in cyclic guanosine monophosphate (cGMP) in VSMC which was significant at 6 h, and increased progressively for up to 36 h. This effect of IL-1 was abolished when protein synthesis was inhibited with cycloheximide or actinomycin D, suggesting that the effect of IL-1 involves new protein synthesis. IL-1-induced cGMP accumulation was inhibited by the soluble guanylate cyclase inhibitors, methylene blue, LY83583, and hemoglobin and by the L-arginine analogue NGmonomethyl-L-arginine (L-NMMA). The inhibitory effect of L-NMMA was reversed by a 10-fold excess of L-arginine, but not by D-arginine. Nitrite, an oxidation product of nitric oxide, accumulated in the media of VSMC incubated with IL-1 for 24 h in the presence of L-arginine, whereas both IL-1-induced cGMP accumulation and nitrite production were attenuated in VSMC incubated in L-arginine-deficient medium. In L-arginine-depleted VSMC, IL-1-induced cGMP accumulation was restored to control levels by a 15-min incubation with L-arginine. These results demonstrate that IL-1 activates guanylate cyclase in rat VSMC by inducing production of nitric oxide via a pathway dependent on extracellular L-arginine.

Mechanisms underlying transition from acute glomerular injury to late glomerular sclerosis in a rat model of nephrotic syndrome.

Functional and morphologic measurements were performed in Munich-Wistar rats after a single central venous injection of puromycin aminonucleoside (PA) or saline vehicle (sham). During phase I, PA rats exhibited overt nephrotic syndrome and impaired glomerular filtration, primarily due to a reduction in the glomerular capillary ultrafiltration coefficient. The morphologic counterpart of the latter consisted of effacement of glomerular epithelial cell foot processes and decrease in the number of filtration slit diaphragms. Administration of the angiotensin I converting enzyme inhibitor (CEI) enalapril to PA rats did not ameliorate glomerular dysfunction. During phase II, PA rats exhibited spontaneous resolution of proteinuria, impaired function, and morphologic abnormalities. However, PA rats now demonstrated marked glomerular capillary hypertension and continued, albeit lesser, reductions in the ultrafiltration coefficient. Concurrent CEI administration modestly lowered systemic arterial pressure, and normalized the glomerular capillary hydraulic pressure and ultrafiltration coefficient. Additional rats were studied during phase III, 70 wk after injection. In PA rats, prior glomerular hypertension was associated with development of recurrent proteinuria and extensive glomerular sclerosis, whereas concurrent CEI administration limited these parameters to values comparable to those in sham rats. Glomerular hypertension thus may explain the development of glomerular sclerosis and renal failure long after an episode of acute glomerular injury.

Role of atrial natriuretic peptide in adaptation of sodium excretion with reduced renal mass.

The kidney maintains constancy of body fluid volume by regulating urinary sodium (Na) excretion. In chronic renal failure, the reduction in glomerular filtration rate (GFR) is accompanied by an increase in Na excretion per nephron if dietary Na intake is not changed. Reduction in Na intake in proportion to reduced GFR obviates this adaptive increase in tubule Na excretion. To examine the potential role of endogenous atrial natriuretic peptide (ANP) in modulating the enhanced Na excretion per nephron in chronic renal failure, we studied rats subjected to 5/6 nephrectomy or sham operation on low, normal, and high Na intakes. Urinary Na excretion increased with increasing dietary Na in all groups, and Na excretion per nephron was increased in 5/6 nephrectomized rats as compared with sham-operated rats on the higher Na intakes. Plasma ANP levels were unaffected by dietary Na manipulations in sham-operated rats, but rose progressively in 5/6 nephrectomized rats with increasing Na intake. Despite extensive nephron reduction, however, plasma ANP levels failed to rise in uremic rats on low Na diets and in this group Na excretion per nephron also failed to rise. We conclude that enhanced ANP secretion may play an important role in promoting the adaptive increase in Na excretion per nephron in chronic renal failure. Restriction of dietary Na in the setting of reduced GFR obviates the stimulation of ANP secretion as well as the adaptive increase in Na excretion rate per nephron.

Atrial natriuretic peptide transcription, secretion, and glomerular receptor activity during mineralocorticoid escape in the rat.

The mechanisms that mediate renal "escape" from the sodium-retaining effects of mineralocorticoids are incompletely understood. This study was undertaken to determine whether atrial natriuretic peptide (ANP) may play a role in the escape phenomenon. Immunoreactive ANP in rat plasma increased 2.5-fold above baseline values at 12 and 24 h after a single depot injection of desoxycorticosterone acetate in oil and returned to baseline thereafter. In addition, specific pre-pro-ANP messenger RNA content in rat atria was significantly elevated as early as 12 h after mineralocorticoid administration and remained elevated at 24, 48, and 72 h, indicating a prompt and sustained increase in ANP biosynthesis. Renal glomerular ANP receptor density was down-regulated appropriately with rising plasma ANP levels, and receptor affinity was unchanged. Thus, mineralocorticoid administration in the rat is a powerful stimulus for ANP release and for atrial myocyte ANP synthesis, which suggests a potential role for this hormone in overriding mineralocorticoid-induced renal sodium retention.

Effects of interferon-gamma on nitric oxide synthase activity and endothelin-1 production by vascular endothelial cells.

Given the pivotal role suggested for IFN-gamma in immune diseases of the vascular wall, we investigated the effects of IFN-gamma on nitric oxide (NO) and endothelin-1 (ET-1) expression in bovine aortic endothelial cells (BAEC). We have previously reported that TNF-alpha enhanced NO synthase activity in BAEC as assessed by quantifying release of bioactive NO with reporter monolayers and measuring conversion of L-[14C]arginine to L-[14C] citrulline. In murine macrophages IFN-gamma synergizes with TNF-alpha or lipopolysaccharide to induce robust increases in calcium-independent NO synthase activity. In this study we have found that IFN-gamma alone failed to have a significant effect on NO synthase activity in BAEC. In contrast to murine macrophages, IFN-gamma inhibited TNF-alpha-stimulated induction of endothelial NO synthase activity in a concentration-dependent manner. This observation suggests that there is major difference in the response of BAEC and murine macrophages to IFN-gamma. A second major aim of this study was to determine the effect of IFN-gamma on preproET-1 mRNA expression and ET-1 secretion rates in BAEC. IFN-gamma alone had little or no effect on ET-1 mRNA levels and basal ET release when measured for 8 h. However, cotreatment with IFN-gamma potentiated the stimulatory effect of TNF-alpha on BAEC ET-1 mRNA transcript levels and ET release. In contrast, pretreatment of cells with IFN-gamma for 16-24 h blunted the stimulatory effect of TNF-alpha. These findings suggest that endothelial cell expression of vasoactive mediators is modified by the temporal interplay of at least two immune mediators, IFN-gamma and TNF-alpha.

Atrial natriuretic peptides inhibit conductive sodium uptake by rabbit inner medullary collecting duct cells.

The inner medullary collecting duct (IMCD) effects net sodium reabsorption under the control of volume regulatory hormones, including atrial natriuretic peptides (ANP). These studies examined the mechanisms of sodium transport and its regulation by ANP in fresh suspensions of IMCD cells. Sodium uptake was inhibited by amiloride but insensitive to furosemide, bu-metanide, and hydrochlorthiazide. These results are consistent with uptake mediated by a sodium channel or Na+/H+ exchange. To determine the role of sodium channels, cells were hyperpolarized by preincubation in high potassium medium followed by dilution into potassium-free medium. Membrane potential measurements using the cyanine dye, Di(S)-C3-5 verified a striking hyperpolarization of IMCD cells using this protocol. Hyperpolarization increased the apparent initial rate of sodium uptake fourfold. Amiloride and ANP inhibited potential-stimulated sodium uptake 73% and 65%, respectively; the two agents together were not additive. Addition of 5 mM sodium to hyperpolarized cells resulted in a significant amiloride-sensitive depolarization. Half-maximal inhibition of potential-driven sodium uptake occurred at 3 X 10(-7) M amiloride, and 5 X 10(-11) M ANP. We conclude that sodium enters IMCD cells via a conductive, amiloride-sensitive sodium channel, which is regulated by ANP. ANP inhibition of luminal sodium entry in the IMCD appears to contribute to the marked natriuretic effect of this hormone in vivo.