Increased prostaglandin production by glomeruli isolated from rats with streptozotocin-induced diabetes mellitus

Effects of hyperglycemia and mannitol infusions on renal hemodynamics in normal subjects

Early diabetes mellitus is characterized by an increase in glomerular filtration rate and effective renal plasma flow which may initiate and potentiate glomerular injury which ultimately results in diabetic nephropathy. To investigate the role of hyperglycemia per se in mediating these hemodynamic changes, eight healthy adults had inulin clearance, creatinine clearance, and para-aminohippurate (PAH) clearance after steady state conditions of hyperglycemia were achieved (549 +/- 86). Clearances were repeated during equivalent osmotic diuresis with Mannitol. Euvolemia was maintained by quantitative fluid and electrolyte replacement of urinary losses. Mean inulin clearances were 87 +/- 6, 87 +/- 5, and 81 +/- 4 ml/min during control, glucose, and mannitol periods (p = ns). Creatinine clearance overestimated inulin clearance by 15-32% but there were no differences between control glucose and mannitol periods. PAH clearance fell from control values of 595 +/- 29 to 340 +/- 22 ml/min during hyperglycemia (p less than .05). During osmotic diuresis with mannitol PAH, clearance rose to control values. In vitro studies excluded the possibility that conjugation between glucose and PAH can explain the clearance results. These results in normal subjects documenting renal vasoconstriction with hyperglycemia are of particular interest in view of recent experimental data suggesting that failure to vasoconstrict characterizes the hemodynamics of early diabetes.

Role of enhanced glomerular synthesis of thromboxane A2 in progressive kidney disease

Normotensive rats of the Milan strain (MNS) spontaneously develop focal glomerulosclerosis. In order to explore the contribution of glomerular thromboxane (TX) A2 synthesis to the development of the disease, we have characterized the time course of renal functional and biochemical changes, and their modification by long-term treatment with a TX-synthase inhibitor. Oral administration (150 mg.kg-1 from 1 to 14 months of age) of FCE 22178 suppressed enhanced glomerular TXB2 production at all experimental times (mean inhibition 80%) and proteinuria (varying between 27.1 and 73.0%) while preserving renal blood flow and glomerular filtration rate. These effects of TX-synthase inhibition were seen in the absence of any statistically significant changes in systemic blood pressure. Moreover, FCE 22178 had no antihypertensive effects in hypertensive rats of the Milan strain (MHS) nor in spontaneously hypertensive rats (SHR). Treatment also prevented the age-related hypoalbuminemia and hyperlipidemia observed in control MNS and significantly (P less than 0.01) reduced glomerular histologic damage, as demonstrated by light microscopy studies and measurement of sclerotic area. We conclude that: 1) MNS rats provide an animal model of long-lasting proteinuria characterized by an age-related increase in glomerular TXB2 production paralleled by progressive loss of renal structural integrity and function and by a secondary dyslipidemia; 2) pharmacological inhibition of glomerular TX-synthase attenuates the structural as well as the functional expression of kidney disease, without a primary effect on systemic blood pressure. These data are suggestive of an important modulating role of TXA2 in the progression of MNS renal disease.

Renal vascular responsiveness to arachidonic acid in experimental diabetes

1. Isolated perfused kidneys from diabetic rats (duration 4-6 and 20-24 weeks) were more sensitive to the vasoconstrictor effects of arachidonic acid than kidneys from age-matched control rats. Sensitivity diminished with age in both control and diabetic groups. 2. The enhanced vasoconstrictor effect of arachidonic acid in diabetic rat kidneys was associated with increased conversion to prostaglandins. 3. The renal vasoconstrictor response to arachidonic acid in both groups was reduced by thromboxane A2/prostaglandin H2 receptor antagonism but not by inhibition of thromboxane synthase. 4. Diabetic rat kidneys were also more sensitive to the vasoconstrictor effects of the endoperoxide analogue, U46619, while vasoconstrictor responses to phenylephrine were not markedly different from those of control rat kidneys. 5. In conclusion, prostaglandin endoperoxides appear to mediate arachidonic acid-induced vasoconstriction in diabetic and control rat kidneys. The enhanced renal vasoconstrictor response to arachidonic acid in diabetic rats results from increased sensitivity to endoperoxides and increased formation of endoperoxides from arachidonic acid.

Studies of renal autoregulation in pancreatectomized and streptozotocin diabetic rats

We studied renal autoregulation in pancreatectomized Munich-Wistar diabetic rats and in their sham-operated controls. In a second experiment we studied renal autoregulation in untreated and insulin treated streptozotocin diabetic Munich-Wistar rats and their nondiabetic controls. In the first experiment the diabetic rats had higher baseline renal blood flows (RBF). There was a fall in renal vascular resistance (RVR) and sustained RBF in both diabetic and control rats as renal perfusion pressures (RPP) was reduced from 130 and 110 mm Hg. As RPP was reduced from 110 and 80 mm Hg, there was no significant Change in RVR in control rats and RBF began to fall. Below RPP of 80 mm Hg RVR rose and RBF fell sharply in these rats. In contrast, there was a progressive fall in RVR as RPP was lowered to 60 mm Hg in the diabetic rats and, thus, RBF was much better sustained in these animals. In the second experiment the plasma glucose level was 502 +/- 52 mg/dl (X +/- SD) in the untreated diabetic rats and only modestly reduced to 411 +/- 49 mg/dl in the insulin treated animals. Untreated streptozotocin diabetic rats had moderately reduced and insulin-treated diabetic rats had mildly reduced baseline RVR and RBF. However, in these animals as in the pancreatectomized rats the increases in RVR noted in control rats at subautoregulatory RPPs were not seen. Thus, regardless of whether baseline RBFs were increased or decreased, diabetic rats sustained RBF at markedly reduced RPPs far more efficiently than did nondiabetic rats. The pathogenesis of these abnormalities in diabetic rats was not learned in these studies.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased renal TXA2 synthesis in diabetes mellitus: simultaneous determination of urinary TXB2 and 2,3-dinor-TXB2

The present study was designed to determine urinary excretion of kallikrein(KAL)-kinin as well as prostaglandin (PG) E2, TXB2 and 2,3-dinor-TXB2, a major urinary metabolite of TXA2 synthesized in platelets, by specific RIAs in patients with diabetes mellitus (DM). KAL or kinin excretion in 26 type II DM did not differ from control values obtained in 18 age-matched healthy subjects (C), although DM with HbA1 greater than 11% excreted less KAL. Urinary PGE2 excretion (7.6 +/- 2.8 ng/mg creatinine, mean +/- SE) was significantly lower in DM compared to C (17.5 +/- 3.9, p less than 0.05), while DM excreted more TXB2 (0.57 +/- 0.09, p less than 0.01) and 2,3-dinor-TXB2 (0.56 +/- 0.12, N.S.) than C (0.19 +/- 0.02 or 0.33 +/- 0.01). DM with or without mild proteinuria demonstrated lower PGE2, but higher TXB2 and 2,3-dinor-TXB2 excretion. A positive correlation of TXB2/2,3-dinor-TXB2 with proteinuria was observed in this group. However, in DM with massive proteinuria over 500 micrograms/mg creatinine, TXB2 and 2,3-dinor-TXB2 excretion decreased to levels almost identical to C. As a whole, a ratio of TXB2 to PGE2 or 2,3-dinor-TXB2 in DM was significantly higher than in C. The results suggest that a relative preponderance of TXB2 to 2,3-dinor-TXB2 may indicate an augmented renal, in addition to platelet, TXA2 synthesis. An excessive vasoconstrictive and proaggregatory TXA2 renal synthesis, concomitant with a decrease in vasodilatory and antiaggregatory PGE2, may have profound effects on renal functions such as protein excretion in DM.

Evidence against a role for prostaglandins in sustaining renal hyperfiltration in type 1 diabetes mellitus

The acute effect of inhibition of prostaglandin synthesis with indomethacin 50 mg by mouth on renal haemodynamics has been examined in 8 male Type 1 diabetic patients with elevated glomerular filtration rate who were free of diabetic complications. Renal plasma flow and glomerular filtration rate were measured by constant infusion of para-amino hippurate and polyfructosan, the patients being studied at normoglycaemia. Urinary excretion of prostaglandin E2 fell from 885 +/- 160 to 345 +/- 115 pg min-1, and excretion of 6-keto-prostaglandin F1 alpha fell from 489 +/- 77 to 283 +/- 50 pg min-1. There was no change in renal plasma flow or glomerular filtration rate following indomethacin. The results do not support the hypothesis that increased renal prostaglandin synthesis is a cause of hyperfiltration in diabetes mellitus.

Sorbinil suppresses glomerular prostaglandin production in the streptozotocin diabetic rat

Previous studies have suggested a link between hyperfiltration and enhanced polyol pathway activity in the streptozotocin diabetic rat. In the present study we examined the relationship between glomerular sorbitol content, a measure of polyol pathway activity and glomerular filtration rate (GFR), as a function of plasma glucose and time after induction of diabetes. GFR is increased by 1 to 2 weeks in the untreated streptozotocin diabetic rat but falls to values equal to or below control by 2 months. Treatment of diabetic rats with a low dose of insulin to achieve moderate hyperglycemia results in the maintenance of elevated GFR for 2 months. Glomerular sorbitol content in the 1- to 2-week diabetic rats was not significantly different from values in glomeruli from control rats at 1 to 2 weeks but was 11-fold higher than control by 2 months in the untreated diabetic rat. Treatment of diabetic rats with insulin to achieve moderate hyperglycemia resulted in values for glomerular sorbitol content that were not different from control. Thus, elevated GFR was not associated with elevated glomerular sorbitol content in the 1- to 2-week diabetic rat and was dissociated from elevated glomerular sorbitol content in the 2-month diabetic rat. Treatment of 1- to 2-week diabetic rats with sorbinil prevented the rise in GFR observed at this time despite the fact that sorbitol content of glomeruli was not elevated. These results suggested that sorbinil was reducing GFR in the diabetic rat by a mechanism other than aldose-reductase inhibition. The synthesis of vasodilatory prostaglandins by isolated glomeruli and the activity of phospholipase A2 in the particulate cell fraction of glomerular homogenates is higher in 1- to 2-week diabetic rats compared with controls, a finding that may contribute to the elevated GFR in these rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Increased urinary prostaglandin excretion in galactose-fed rats

Increased renal production of prostaglandins (PG) may contribute to the hyperfiltration that accompanies early diabetes. It was postulated that a putative metabolic abnormality of diabetes, ie, increased flux through the polyol pathway, stimulates renal PG production and that this phenomenon can be prevented by aldose-reductase inhibition. To test this hypothesis, the effects of polyol accumulation on urinary excretion rates (UER) of PGE2 and 6-keto-PGF1 alpha were studied, using the galactose-fed rat model. UER of PGE2 and 6-keto-PGF1 alpha were measured in three groups of weanling Wistar male rats. Group 1 was maintained on normal chow (n = 6), group 2 was fed chow supplemented with 30% galactose (n = 6), and group 3 received chow supplemented with 30% galactose and 0.7% sorbinil (n = 6). Ten 24-hour urine samples were obtained from each group between 151 and 240 days on the respective diets. UER of PGE2 (P less than .001) and 6-keto-PGF1 alpha (P less than .01) were higher in group 2 than in group 1. UER of PGE2 (NS) and 6-keto-PGF1 alpha (NS), respectively, were similar in groups 1 and 3. These data indicate that flux through the polyol pathway modulates the UER of PGE2 and 6-keto-PGF1 alpha. This phenomenon may contribute to the glomerular hyperfiltration of early diabetes.

Protein kinase C is activated in glomeruli from streptozotocin diabetic rats. Possible mediation by glucose

Glomerular inositol content and the turnover of polyphosphoinositides was reduced by 58% in 1-2 wk streptozotocin diabetic rats. Addition of inositol to the incubation medium increased polyphosphoinositide turnover in glomeruli from diabetic rats to control values. Despite the reduction in inositol content and polyphosphoinositide turnover, protein kinase C was activated in glomeruli from diabetic rats, as assessed by an increase in the percentage of enzyme activity associated with the particulate cell fraction. Total protein kinase C activity was not different between glomeruli from control and diabetic rats. Treatment of diabetic rats with insulin to achieve near euglycemia prevented the increase in particulate protein kinase C. Moreover, incubation of glomeruli from control rats with glucose (100-1,000 mg/dl) resulted in a progressive increase in labeled diacylglycerol production and in the percentage of protein kinase C activity which was associated with the particulate fraction. These results support a role for hyperglycemia per se in the enhanced state of activation of protein kinase C seen in glomeruli from diabetic rats. Glucose did not appear to increase diacylglycerol by stimulating inositol phospholipid hydrolysis in glomeruli. Other pathways for diacylglycerol production, including de novo synthesis and phospholipase C mediated hydrolysis of phosphatidylcholine or phosphatidyl-inositol-glycan are not excluded.

Glomerular hyperfiltration and urinary prostaglandins in type 1 diabetes mellitus

In order to determine whether glomerular hyperfiltration in diabetes is related to renal prostaglandin production we have studied the urinary excretion of PGE2, 6-keto-PGF1 alpha, and TXB2 in two sex, age and duration of diabetes matched groups of 9 and 10 Type 1 diabetic patients with either normal (mean 121, range 105-129 ml min-1 1.73 m-2) or supranormal glomerular filtration rate (154, 135-206 ml min-1 1.73 m-2). A group of 15 matched healthy volunteers served as control subjects. Urine was collected overnight for an uninterrupted period of at least 6 h. All studies in the patients were performed during insulin-induced sustained euglycaemia to prevent the confounding effect of variable degrees of blood glucose control on urinary prostaglandin excretion. Blood pressure was normal in all subjects. Urinary excretion of 6-keto-PGF1 alpha was significantly higher in the patients with glomerular hyperfiltration (median 17.1, range 4.5-33.6 ng h-1) than in those without (8.8, 1.5-13.8 ng h-1; p less than 0.05) or in normal control subjects (9.6, 5.2-15.5 ng h-1; p less than 0.05). No significant differences were found in the excretion rates of PGE2 and TXB2 between the three groups. Under conditions of controlled plasma glucose and insulin concentrations the urinary excretion of 6-keto-PGF1 alpha, the stable breakdown product of PGI2, a compound of endothelial, possibly glomerular, origin was elevated only in the diabetic patients with glomerular hyperfiltration.

Control of glomerular hypertension by insulin administration in diabetic rats

Micropuncture studies were performed in Munich Wistar rats made diabetic with streptozotocin and in normal control rats. Diabetic rats received daily ultralente insulin to maintain moderate hyperglycemia (approximately 300 mg/dl). Group 1 diabetic rats studied after routine micropuncture preparation exhibited elevation of the single nephron glomerular filtration rate (SNGFR) due to increases in the glomerular transcapillary hydraulic pressure difference and glomerular plasma flow rate. In group 2 diabetic rats infusion of insulin to achieve acute blood glucose control normalized the glomerular transcapillary pressure gradient while increasing the glomerular ultrafiltration coefficient, so that SNGFR remained elevated. Persistent elevation of SNGFR despite normalization of the transcapillary pressure gradient was also observed in group 3 diabetic rats infused with insulin plus sufficient dextrose to maintain hyperglycemia. These studies indicate that glomerular capillary hypertension in diabetes is an acutely reversible consequence of insulin deficiency and not the result of renal hypertrophy.

Renal functional reserve

Renal functional reserve represents the capacity of the kidney to increase its level of operation in response to certain demands. The reserve of glomerular filtration rate and of renal blood flow is discussed from the following points of view: evaluation, measurement, mechanisms involved and significance. Data from the literature are discussed which show (i) that the mechanism of the hyperfiltration seen in the early stage of diabetic nephropathy may be different from the hyperfiltration induced by infusion of amino acids, (ii) that the remaining kidney in healthy kidney donors maintains its functional reserve, and (iii) that the functional reserve is fairly well maintained as long as the glomerular filtration rate is decreased only moderately. The reserve of tubular functional capacity is discussed from the point of view of concentration and dilution and of acidification and alkalinization.

Neutralization of the anionic sites of cultured rat mesangial cells by poly-L-lysine

Changes in eicosanoid synthesis are a feature of the functional response of rat glomerular epithelial cells to neutralization of the cell surface polyanion. In order to ascertain if this property is common to other glomerular cell types, we have investigated the effect of neutralization of the negatively charged sites of cultured rat mesangial cells by poly-L-lysine on prostaglandin E2 (PGE2) production. Addition of poly-L-lysine (10 micrograms/ml) to the cells stimulated PGE2 synthesis after an initial latency of approximately two minutes, reaching maximum levels at 60 minutes. Poly-L-lysine also progressively increased cytosolic free calcium ([Ca2+]i) in fura-2 loaded monolayers with a similar initial lag time. Poly-L-lysine-evoked PGE2 synthesis and [Ca2+]i increases were dose dependent and prevented by addition of the polyanions, heparin and albumin. Additionally, heparin was also capable of reversing the effect of poly-L-lysine on [Ca2+]i. Removal of extracellular Ca2+ prevented the increase of [Ca2+]i and PGE2 synthesis. Increased PGE2 synthesis following neutralization of mesangial cell anionic sites may play a role in the hemodynamic dysfunction and cellular derangements of glomerular inflammation.

Glomerular prostaglandin production in diabetic rats with renovascular hypertension

A rat model combining two-kidney, one-clip (2K1C) renovascular hypertension and streptozotocin-induced diabetes mellitus was used to assess the pathogenetic significance of vasodilator prostaglandins in diabetic glomerular injury. Glomeruli isolated from normotensive diabetic rats produced greater than normal amounts of PGE2 and 6-keto PGF1 alpha under in vitro incubation conditions. In 2K1C hypertensive-diabetic rats, glomeruli from unclipped kidneys (which are prone to accelerated diabetic glomerular injury) produced similarly elevated amounts of PGE2 and 6-keto PGF1 alpha, which significantly exceeded the levels produced by glomeruli from clipped kidneys (which are relatively protected from glomerular injury), despite exposure to a similar diabetic environment. In contrast, glomeruli from both unclipped and clipped kidneys of 2K1C hypertensive-non-diabetic rats produced normal amounts of PGE2 and 6-keto PGF1 alpha. These results suggests a correlation between vasodilator prostaglandin metabolism and susceptibility to diabetic glomerular injury, and illustrate that enhanced glomerular prostaglandin production is not an invariable metabolic consequence of hyperglycemia or insulin deficiency. The data also demonstrate that hemodynamic as well as metabolic factors may influence glomerular prostaglandin metabolism in experimental diabetes mellitus.

Elevated urinary prostaglandin excretion and the effect of indomethacin on renal function in incipient diabetic nephropathy

We investigated whether the glomerular synthesis of prostaglandins modulates the glomerular filtration rate and urinary albumin excretion in incipient diabetic nephropathy (defined as urinary albumin excretion between 30 and 300 mg/24 h (microalbuminuria) in two out of three sterile ketone-free 24-h urine collections in patients having insulin-dependent diabetes mellitus (IDDM) without hypertension or other kidney disease). The urinary excretion of prostaglandin E2 was significantly elevated in 8 insulin-dependent diabetic patients with incipient nephropathy as compared with 9 normoalbuminuric IDDM patients and 11 healthy controls: 317 (182-1273); 95 (67-225); 132 (54-263) pg/min, respectively (2p less than 0.01). Glomerular filtration rate (single bolus 51Cr-EDTA technique) and albuminuria (radioimmunoassay) were measured twice within 2 weeks in 8 females having IDDM with incipient nephropathy. The study design was a randomized double-blind trial with the patients receiving either indomethacin (150 mg/day) or placebo for 3 days prior to the kidney function studies. Indomethacin treatment induced a significant reduction in urinary prostaglandin E2 excretion (73%) (2p less than 0.01), urinary albumin excretion rate diminished from 207 (63-253) to 87 (49-147) mg/24 h (2p less than 0.01), fractional clearance of albumin declined (70%) (2p less than 0.01). Glomerular filtration rate remained stable (108 (88-133) versus 110 (95-142) ml/min). Blood glucose and blood pressure were comparable during the placebo and indomethacin treatment (12.6 +/- 3 versus 13.4 +/- 5 mmol/l and 122/79 +/- 3/9 versus 122/82 +/- 4/10 mmHg, respectively). Our results suggest that enhanced glomerular synthesis of vasodilating prostaglandins may accelerate microalbuminuria in incipient diabetic nephropathy.

Pathogenesis of diabetic glomerulopathy: hemodynamic considerations

Early stages of diabetes mellitus are characterized by glomerular hyperfiltration in humans and experimental animals. In diabetic rats, single nephron hyperfiltration results from elevations in the glomerular capillary plasma flow rate and hydraulic pressure, which are in turn associated with progressive albuminuria and morphologic injury. Interventions that ameliorate these hemodynamic adaptations afford protection against structural injury. Dietary protein restriction, which lowers glomerular filtration, perfusion, and hydraulic pressure, retards glomerular injury and limits capillary basement membrane thickening in both the glomerular and retinal circulatory beds. Alternatively, selective control of glomerular capillary hypertension using angiotensin I converting enzyme inhibitor therapy limits glomerular injury in this model as well. Each of these interventions is effective even in the absence of improved metabolic control, implying that hemodynamic factors per se are important in this pathogenic process. The pathophysiologic mechanisms of diabetic hyperfiltration remain incompletely elucidated. Recent studies invoke a potential role for atrial natriuretic peptide (ANP). Strict metabolic control abolishes the elevations of glomerular filtration rate and of plasma ANP levels in moderately hyperglycemic diabetic rats. Moreover, infusion of a specific ANP antibody reverses hyperfiltration in diabetic rats. Thus, hyperglycemia-induced chronic volume expansion may trigger ANP release, which in turn contributes to diabetic hyperfiltration. Hemodynamic factors may play an important role in the pathogenesis of extrarenal microangiopathy as well. Elevated peripheral capillary blood flows and/or hydraulic pressure may be found in many peripheral capillaries, in association with thickening of the capillary basement membrane. Dietary protein restriction, which lowers blood flow to many organs, limits retinal as well as glomerular basement membrane thickening in diabetic rats, suggesting that hemodynamically mediated structural injury is a diffuse phenomenon in the diabetic state.

Effects of dietary protein and phosphorus restriction on the progression of chronic renal failure

In recent years, evidence has accumulated suggesting that early dietary intervention in the form of protein restriction can dramatically alter the natural history of chronic renal insufficiency. This article reviews the literature in this area and summarizes the questions that remain for future investigators to answer. Interest in the role of protein intake in renal disease dates back to the early 1900s, when several investigators found that the progression of renal failure was accelerated in rats and rabbits fed high-protein diets. Subsequent work in animal models has demonstrated that nephron loss, resulting from a variety of disease states, is associated with elevated intraglomerular pressures and flows. Several investigators now have postulated that these abnormal hemodynamics, resulting from intrarenal vasodilation and hyperperfusion, damage the glomerulus and may produce further nephron loss independent of the initial renal insult. Dietary protein restriction appears to reduce these pressures and flows towards normal in animals, although the operative mechanism has yet to be identified. Limited studies of dietary protein restriction in human renal disease have suggested a beneficial effect, but carefully controlled prospective studies will be necessary to establish clear therapeutic efficacy.

Diabetes, dietary protein and glomerular hyperfiltration

These discussions are selected from the weekly staff conferences in the Department of Medicine, University of California, San Francisco. Taken from transcriptions, they are prepared by Drs Homer A. Boushey, Professor of Medicine, and David G. Warnock, Associate Professor of Medicine, under the direction of Dr Lloyd H. Smith, Jr, Professor of Medicine and Associate Dean in the School of Medicine. Requests for reprints should be sent to the Department of Medicine, University of California, San Francisco, School of Medicine, San Francisco, CA 94143.

Effects of indomethacin on kidney function in type 1 (insulin-dependent) diabetic patients with nephropathy

We investigated whether the glomerular synthesis of prostaglandins modulates the glomerular filtration rate and albuminuria in diabetic nephropathy. The urinary excretion of immunoreactive prostaglandin E2 (253 pg/min) was significantly elevated in eight Type 1 (insulin-dependent) diabetic women with nephropathy as compared with nine normoalbuminuric Type 1 diabetic women (95 pg/min) and 11 non-diabetic women (132 pg/min), respectively (p less than 0.01). Glomerular filtration rate (single bolus 51Cr-EDTA technique) and albuminuria (radioimmunoassay) were measured twice within two weeks in the eight Type 1 diabetic women with nephropathy. All eight patients were on a diabetic diet without sodium restriction. The study was performed as a randomized double-blind trial, with the patients receiving either indomethacin (150 mg/day) or placebo for three days prior to the kidney function studies. Indomethacin treatment induced a significant reduction in urinary prostaglandin E2 excretion (73%, p less than 0.01), glomerular filtration rate diminished from 120 +/- 18 to 106 +/- 17 ml/min/1.73 m2 (p less than 0.05), albuminuria declined from 148 to 69 micrograms/min (median and range) (p less than 0.05) and fractional clearance of albumin diminished 42% (p less than 0.05). Blood glucose concentrations were comparable during the placebo and indomethacin treatment, 13.4 +/- 4 versus 14.2 +/- 3 mmol/l, respectively. Our results suggest that glomerular filtration rate in early diabetic nephropathy is dependent on the enhanced glomerular synthesis of vasodilating prostaglandins.

Strict metabolic control and renal function in the streptozotocin diabetic rat

Micropuncture studies were made on insulin-treated streptozotocin diabetic rats two weeks after the induction of diabetes and on age-matched control rats. Kidney size, GFR and single nephron GFR were higher in poorly controlled diabetic rats than in normal animals. Single nephron GFR rose as a result of an increase in the hydraulic pressure difference across the glomerular capillary wall caused mainly by a rise in the glomerular capillary pressure due to a diminished ratio of afferent to efferent arteriolar hydraulic resistances. Furthermore, the intratubular pressure was reduced as a result of a decrease in hydraulic resistance in the loop of Henle. Strict metabolic control prevented these changes. In conclusion, the increase in renal function in experimental diabetes is determined by the degree of metabolic control excluding a potential nephrotoxic effect of streptozotocin per se.

Sequential alterations in glomerular prostaglandin and thromboxane synthesis in diabetic rats: relationship to the hyperfiltration of early diabetes

The present study examined the role of enhanced production of prostaglandin (PG) E2 and 6-keto-PGF1a, the stable metabolite of PGI2, by glomeruli from streptozotocin diabetic rats in the mediation of hyperfiltration. Correlative measurements of insulin clearance (CIn) and glomerular production of PGE2, 6-keto PGF1a and thromboxane (TX) B2 (the stable metabolite of TXA2) were made at two time points, nine to 15 days and 25 to 28 days after streptozotocin. CIn was elevated by 40% to 50% in diabetic rats studied at nine to 15 or 25 to 28 days compared to values in age-matched controls. Basal production of PGE2, 6-keto PGF1a and TXA2 (as reflected by TXB2) and increases in response to A23187 were elevated in glomeruli from nine to 15-day diabetic rats compared to values in control glomeruli. Exogenous arachidonate abolished these differences. Treatment of nine-day diabetic rats with indomethacin (3 mg/kg/d) rapidly (within 24 hours) and reversibly suppressed CIn without altering CIn in control rats. Indomethacin had no effect on plasma glucose in control or diabetic rats. Treatment of nine to 15-day diabetic rats with insulin (10 U/kg/d by osmotic minipump) beginning 24 hours after streptozotocin lowered plasma glucose to values that were not significantly different from control and prevented the rise in CIn. Treatment of diabetic rats with insulin or incubation of glomeruli from untreated diabetic rats with insulin (0.3 mU/mL) for two hours in vitro reduced basal and A23187 induced increases in PGE2, 6-keto PGF1a, and TXB2 to values that were not different from those in control glomeruli.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of pregnancy on plasma renin activity and glomerular synthesis of prostaglandins and thromboxane in rats

Normal pregnancy in women and rats is characterized by an increased glomerular filtration rate. Because prostaglandin glomerular synthesis has been reported to be increased in 2 other circumstances with glomerular hyperfiltration (streptozotocin-induced diabetes and high protein dietary intake in Heymann nephrilis), we studied prostaglandin biosynthesis in glomeruli obtained from pregnant Wistar rats in comparison with non pregnant rats during the estrus cycle. Comparatively to the 3 first phases of the cycle, and wether or not arachidonic acid was present, PGE2 and PGF2 alpha production rates were found significantly higher in diestrus 2 (2-3 fold increase for both) at 15 days of pregnancy (2 fold increase for PGE2, 5-6 fold increase for PGF2 alpha) and at 21 days of pregnancy (5-6 fold increase for both). On the other hand synthesis of TXB2 was not increased during pregnancy nor during diestrus 2. Plasma renin activity (PRA) was increased during pregnancy. It is concluded that in presence of increased PRA, the increased synthesis of PGE2 may be the hormonal factor explaining, at least in part, the hemodynamic mechanism of glomerular hyperfiltration which has been previously described by micropuncture techniques and characterized by increased renal plasma flow, increased glomerular filtration pressure and decreased ultrafiltration coefficient.

Leukotrienes and other lipoxygenase products of arachidonic acid synthesized in the kidney

Lipoxygenase products are synthesized in the kidney. Rabbit medulla and murine and human glomeruli produce 12- and 15-hydroxy-5,8,10,14-eicosatetraenoic acid (HETE). Minor amounts of leukotrienes are formed under normal conditions, but it is likely that the resident renal cells are capable of synthesizing these metabolites. Rat glomeruli and papillae possess the enzymes necessary to process leukotriene C4 into leukotrienes D4 and E4. However, the enzyme activity of the papillae is masked due to the presence of an inhibitor detected in the 10,000 g supernate of the papillary homogenate. 12-HETE synthesis is markedly increased in glomeruli from rats with nephrotoxic serum nephritis and leukotriene B4 synthesis in glomeruli from rats with cationic bovine gamma-globulin-induced glomerulonephritis. In vivo consequences of the association between the resident glomerular cells and the bone marrow-derived cells have been studied in vitro in co-incubation experiments. Glomeruli release factors that stimulate the cyclo-oxygenase and lipoxygenase pathways in macrophages. Co-incubation of glomeruli, platelets, and polymorphonuclear leukocytes results in the formation of 12,20-diHETE and an excess of 12-HETE. Lipoxygenase products, regardless of their origin, modify the renal functions. Leukotriene C4 binds specifically to rat glomeruli and human cultured glomerular epithelial cells. Leukotrienes C4 or D4 administered in vivo cause renal vasoconstriction and a decline in the glomerular filtration rate. In vitro, these two sulfidopeptide leukotrienes promote epithelial cell proliferation and produce mesangial cell contraction. The lipoxygenase pathway is also implicated in the attachment of macrophages to glomeruli and in the oxidative burst of glomerular mesangial cells during phagocytosis. The future use of specific inhibitors of the synthesis or antagonists of the lipoxygenase products, particularly the leukotrienes, should provide a tool for evaluating the role of these metabolites in renal diseases.

Effect of streptozotocin-induced diabetes on prostaglandin production by rat cerebral microvessels

Diabetes has been shown to result in reduced prostacyclin synthesis by macrovascular tissue in rats and humans. Other studies have shown that plasma levels of 6-keto-prostaglandin-F1 alpha (6-keto-PGF1 alpha) and synthesis of 6-keto-PGF1 alpha by isolated glomeruli are either unchanged or increased in diabetes. Thus, microvascular tissue may respond differently to diabetes than macrovascular tissue. Accordingly, we have studied the effects of streptozotocin-induced diabetes mellitus on prostaglandin synthesis by rat cerebral microvessel (RCMV). Prostaglandin synthesis from both endogenous and exogenous arachidonic acid was determined. The yield of RCMV from diabetic and control animals was similar. Under basal conditions and following stimulation of prostaglandin synthesis by melittin (5 micrograms/ml), RCMV production of prostacyclin (measured as immunoreactive 6-keto-PGF1 alpha) was greater than production of PGE2. Both 6-keto-PGF1 alpha and PGE2 production under basal and stimulated conditions were found to be similar for control and diabetic RCMV. The RCMV converted exogenous [3H]arachidonic acid predominately to PGD2 and to a lesser extent to 6-keto-PGF1 alpha. No significant differences in the conversion of exogenous [3H]arachidonic acid to PGD2 and 6-keto-PGF1 alpha was observed between control and diabetic RCMV. This study suggests that the effect of streptozotocin-induced diabetes mellitus on prostaglandin formation by microvascular endothelium is different from its effect on macrovascular tissue.

Bioconversion of leukotriene C4 by rat glomeruli and papilla

Since leukotriene C4 (LTC4) may be locally synthesized by bone marrow-derived cells infiltrating the kidney in inflammatory renal diseases we examined the in vitro metabolism of exogenously added [3H] LTC4 by rat glomeruli and papilla using radiometric HPLC. Homogenized as well as intact glomeruli converted [3H] LTC4 mainly into [3H] LTE4 (83%) and, at a smaller extent, into [3H] LTD4 (4%). Intact [3H] LTC4 represented 13% of the sum of radioactive leukotrienes. Addition of L-cysteine resulted in accumulation of LTD4. In contrast, there was nearly no conversion of [3H] LTC4 (87% intact) in the presence of homogenized papilla. The metabolism of [3H] LTC4 by the glomeruli was time- and temperature-dependent. The 10,000 g supernatant and pellet of homogenized glomeruli both retained the ability to metabolize [3H] LTC4. The papillary 10,000 g supernatant was inactive, as found for the total homogenate, whereas the papillary 10,000 g pellet separated from its supernatant could transform [3H] LTC4 into its metabolites, LTD4 and LTE4. Addition of increasing amounts of papillary 10,000 g supernatant to homogenized glomeruli progressively protected [3H] LTC4 from its bioconversion. These results demonstrate that both glomeruli and papilla possess the gamma-glutamyl transpeptidase and dipeptidase necessary to process LTC4. However, the enzyme activity of the papilla is unmasked only when the inhibitor present in the 10,000 g supernatant is separated from the enzyme present in the pellet.

Evidence of prostacyclin deficiency in the syndrome of hyporeninemic hypoaldosteronism

Hyporeninemic hypoaldosteronism is an important cause of hyperkalemia and is characterized by low renin secretion. We found that prostacyclin, a potent vasodilator and renin secretagogue, was markedly reduced--as reflected by its stable urinary metabolite 6-keto-prostaglandin F1 alpha--in seven patients with hyporeninemic hypoaldosteronism as compared with seven matched controls with renal insufficiency and as compared with 12 normal volunteers (mean +/- SE, 42 +/- 7 vs. 185 +/- 37 and 164 +/- 20 ng per gram of creatinine, respectively; P less than 0.001). In contrast, renal prostaglandin E2 excretion was similar in all three groups. A low-dose infusion of calcium or norepinephrine (known stimulants of prostacyclin) increased renal prostacyclin release in normal subjects and controls with renal insufficiency. Neither agonist, however, increased the low basal prostacyclin excretion in the patients (49.6 +/- 11 [basal] vs. 62 +/- 20 [norepinephrine] and 47.5 +/- 16 [calcium]; P greater than 0.8). To evaluate the functional importance of the altered prostacyclin production, we studied the responses of renal blood flow and blood pressure to the calcium infusion. The calcium infusion did not alter blood pressure or renal blood flow in the normal subjects or the controls with renal insufficiency. In contrast, the same dose of calcium in the patients with hyporeninemic hypoaldosteronism produced a rise in mean blood pressure (from 91 +/- 6 to 104 +/- 8 mm Hg, P less than 0.05) and a fall in renal blood flow (from 673 +/- 58 to 560 +/- 42 ml per minute per 1.73 m2, P less than 0.05). These results indicate that a deficiency of prostacyclin could explain the low active-renin concentration and altered vasomotor tone seen in hyporeninemic hypoaldosteronism.

Pathogenesis of diabetic microangiopathy. The hemodynamic view

Multiple factors, including altered levels of vasoactive substances, altered vasomotor responsiveness, chronic plasma volume expansion, and tissue hypoxia, contribute to a state of generalized microvascular vasodilatation in early insulin-dependent diabetes mellitus. This vasodilatation, with the consequent elevation in capillary pressures and flows, may be the initiating mechanism leading to both renal and extrarenal diabetic microangiopathy. Sustained hemodynamic actions on the microvasculature, besides directly injuring the capillary wall, promote increased permeability to macromolecules and increased capillary wall proliferation, with consequent thickening of basement membranes and luminal narrowing. These changes eventuate in complete microvascular obstruction and further vasodilatation of less damaged capillaries, thereby ensuring their eventual destruction. The ensuing complications depend on the nature of the surrounding tissue, ranging from reduction of functional reserve, as seen in skeletal muscle, to the devastating functional consequences observed in organs with endarterial circulation such as the kidney and retina.

Predominance of hemodynamic rather than metabolic factors in the pathogenesis of diabetic glomerulopathy

Six groups of Munich-Wistar rats underwent micropuncture study 2-10 weeks and morphologic studies 11-13 months after induction of streptozotocin diabetes or after sham treatment. Diabetic rats received diets containing 6% (group D6), 12% (D12), or 50% protein (D50) and were maintained under similar conditions of moderate hyperglycemia by daily injections of ultralente insulin. Age- and weight-matched normal control rats were also given 6% (Group N6), 12% (N12), or 50% protein (N50). Kidney weight, whole-kidney and single-nephron glomerular filtration rate, glomerular plasma flow, and mean glomerular transcapillary hydraulic pressure difference were higher in D50 rats than in all other groups and predisposed this group to marked and progressive albuminuria. Likewise, histological examination of the kidneys disclosed areas of sclerosis in 19.6% of glomeruli in D50 rats; the frequency of such lesions was less than 2.5% in all other groups. These findings indicate that the metabolic disorder seen in stable, moderately hyperglycemic diabetic rats does not lead to glomerulopathy as long as elevations in glomerular pressures and flows are prevented.