Effect of angiotensin II on the renal response to amino acid in rats

Functional Reserve of the Kidney

An exploration of the normal limits of physiologic responses and how these responses are lost when the kidney is injured rarely occurs in clinical practice. However, the differences between "resting" and "stressed" responses identify an adaptive reactiveness that is diminished before baseline function is impaired. This functional reserve is important in the evaluation of prognosis and progression of kidney disease. Here, we discuss stress tests that examine protein-induced hyperfiltration, proximal tubular secretion, urea-selective concentration defects, and acid retention. We discuss diseases in which these tests have been used to diagnose subclinical injury. The study and follow-up of abnormal functional reserve may add considerable understanding to the natural history of CKD.

The Role of Renal Functional Reserve in Predicting Acute Kidney Injury

Renal functional reserve (RFR) is described as the difference between a glomerular filtration rate (GFR) measured at baseline and after protein stimulation. The percent change in GFR after a protein load varies based on differences in experimental conditions, with the use of an oral meat protein stimulus and a creatinine clearance method to quantify GFR showing the greatest RFR. A decline in RFR has been found in numerous patient groups. Recent investigations have suggested that a lower RFR may be associated with an increased risk of acute kidney injury and eventual chronic kidney disease.

Renal hemodynamic effects of glucagon-like peptide-1 agonist are mediated by nitric oxide but not prostaglandin

The incretin hormone, glucagon-like peptide-1 (GLP-1), is known for responding to dietary fat and carbohydrate. It elicits effects on pancreas, gut, and brain to stabilize blood glucose levels. We have previously reported that the GLP-1 agonist, exenatide, vasodilates the kidney and suppresses proximal reabsorption. The present study was undertaken to determine whether the renal effects of exenatide are mediated by nitric oxide (NO) and/or prostaglandins. Inulin clearance (glomerular filtration rate, GFR) and urine flow rate (UV) were measured in anesthetized rats before and during exenatide infusion (1 nmol/h iv). Animals were pretreated with cyclooxygenase (COX) inhibitor (meclofenamate), NO synthase (NOS) inhibitor (N^G-monomethyl-l-arginine, l-NMMA), NO clamp (l-NMMA + sodium nitroprusside), or placebo. Effectiveness of COX inhibition was tested by measuring urinary prostaglandin E₂ (UPGE₂). Effectiveness of NOS blockade and NO clamp was determined by urinary NO degradation products (UNOx). Exenatide increased GFR, UV, UPGE₂, and UNOx. Pretreatment with meclofenamate reduced UPGE₂ by 75% and reduced the effect of exenatide on UPGE₂ by 30% but did not modify the effects of exenatide on GFR or UV. Pretreatment with l-NMMA reduced UNOx and the impact of exenatide on GFR and UV by 50%. Pretreatment by NO clamp did not prevent UNOx from increasing during exenatide but blunted the effects of exenatide on GFR and UV. In conclusion, exenatide is a potent renal vasodilator and diuretic in the rat. These effects of exenatide are insensitive to COX inhibition but are mediated, in part, by NO.

Protein- and diabetes-induced glomerular hyperfiltration: role of glucagon, vasopressin, and urea

A single protein-rich meal (or an infusion of amino acids) is known to increase the glomerular filtration rate (GFR) for a few hours, a phenomenon known as “hyperfiltration.” It is important to understand the factors that initiate this upregulation because it becomes maladaptive in the long term. Several mediators and paracrine factors have been shown to participate in this upregulation, but they are not directly triggered by protein intake. Here, we explain how a rise in glucagon and in vasopressin secretion, directly induced by protein ingestion, might be the initial factors triggering the hepatic and renal events leading to an increase in the GFR. Their effects include metabolic actions in the liver and stimulation of sodium chloride reabsorption in the thick ascending limb. Glucagon is not only a glucoregulatory hormone. It is also important for the excretion of nitrogen end products by stimulating both urea synthesis in the liver (along with gluconeogenesis from amino acids) and urea excretion by the kidney. Vasopressin allows the concentration of nitrogenous end products (urea, ammonia, etc.) and other protein-associated wastes in a hyperosmotic urine, thus allowing a very significant water economy characteristic of all terrestrial mammals. No hyperfiltration occurs in the absence of one or the other hormone. Experimental results suggest that the combined actions of these two hormones, along with the complex intrarenal handling of urea, lead to alter the composition of the tubular fluid at the macula densa and to reduce the intensity of the signal activating the tubuloglomerular feedback control of GFR, thus allowing GFR to raise. Altogether, glucagon, vasopressin, and urea contribute to set up the best compromise between efficient urea excretion and water economy.

Functional renal reserve capacity in different stages of chronic kidney disease

AIM

There is conflict in published reports on the extent of availability of the functional renal reserve (RR) in healthy adults and in various stages of chronic kidney disease (CKD). The aim of the present study was to determine the RR in various stages of CKD.

METHODS

Baseline glomerular filtration rate (GFR) and 'stimulated GFR' following amino acid infusion were measured in 25 volunteers and 100 patients at various stages of CKD by measuring plasma clearance of Tc99m diethyl triamine pentaacetic acid. Any obtained difference between stimulated and basal GFR was considered as RR and expressed as percentage.

RESULTS

The mean renal reserve was 23.4% in the healthy control group, 19.08% in CKD stage 1, 15.4% in CKD stage 2, 8.9% in CKD stage 3 and 6.7% in CKD stage 4, respectively.

CONCLUSION

Renal reserve falls relentlessly with progression of CKD from 23.4% in normal to 6.7% in stage 4 CKD. However, RR may also get completely exhausted even with a normal or with a minimal decline basal GFR. Kidneys may retain some RR even up to the GFR level of 15 mL/min.

L-arginine-induced glomerular hyperfiltration response: the roles of insulin and ANG II

Infusion of L-arginine produces an increase in glomerular filtration via kidney vasodilation, correlating with increased kidney excretion of nitric oxide (NO) metabolites, but the specific underlying mechanisms are unknown. We utilized clearance and micropuncture techniques to examine the whole kidney glomerular filtration rate (GFR) and single nephron GFR (SNGFR) responses to 1) L-arginine (ARG), 2) ARG+octreotide (OCT) to block insulin release, 3) ARG+OCT+insulin (INS) infusion to duplicate ARG-induced insulin levels, and 4) losartan (LOS), an angiotensin AT-1 receptor blocker, +ARG+OCT. ARG infusion increased GFR, while increasing insulin levels. OCT coinfusion prevented this increase in GFR, but with insulin infusion to duplicate ARG induced rise in insulin, the GFR response was restored. Identical insulin levels in the absence of ARG had no effect on GFR. In contrast to ARG infusion alone, coinfusion of OCT with ARG reduced proximal tubular fractional and absolute reabsorption potentially activating tubuloglomerular feedback. Losartan infusion, in addition to ARG and OCT (LOS+ARG+OCT), restored the increase in both SNGFR and proximal tubular reabsorption, without increasing insulin levels. In conclusion, 1) hyperfiltration responses to ARG require the concurrent, modest, permissive increase in insulin; 2) inhibition of insulin release after ARG reduces proximal reabsorption and prevents the hyperfiltration response; and 3) inhibition of ANG II activity restores the hyperfiltration response, maintains parallel increases in proximal reabsorption, and overrides the arginine/octreotide actions.

Effects on kidney filtration rate by agmatine requires activation of ryanodine channels for nitric oxide generation

Agmatine, decarboxylated arginine, is produced in the kidney and can increase nephron and kidney filtration rate via renal vasodilatation and increases in plasma flow. This increase in filtration rate after agmatine is prevented by administration of nitric oxide synthase (NOS) inhibitors. In endothelial cells, agmatine-stimulated nitrite production is accompanied by induction of cytosolic calcium. NOS activity requires calcium for activation; however, the source of this calcium remains unknown. Ryanodine receptor (RyR) calcium-activated calcium release channels are present in the kidney cortex, and we evaluated if RyR contributes to the agmatine response. Agmatine microperfused into Bowman's space reversibly increases nephron filtration rate (SNGFR) by approximately 30%. cADP-ribose (cADPR) regulates RyR channel activity. Concurrent infusion of agmatine with the cADPR blocker 8-bromo-cADPR (2 microM) prevents the increase in filtration rate. Furthermore, direct activation of the RyR channel with ryanodine at agonist concentrations (5 microM) increases SNGFR, and, like agmatine, this increase is prevented by administration of N(G)-monomethyl-l-arginine, a nonselective NOS blocker. We demonstrate that agmatine does not elicit ADPR cyclase activity in vascular smooth muscle membranes and does not directly affect RyR calcium channel responses using sea urchin egg homogenates. These results imply interplay between endothelial cell cADPR/RyR/Ca(2+)/NO and the cADPR/RyR/Ca(2+) pathways in vascular smooth muscle cells in arterioles in the regulation of kidney filtration rate. In conclusion, we show that agmatine-induced effects require activation of cADPR and RyR calcium release channels for NO generation, vasodilation, and increased filtration rate.

Protein intake regulates the vasodilatory function of the kidney and NMDA receptor expression

Glycine infusion in normal rats causes an increase in renal plasma flow and glomerular filtration rate (GFR). Although the renal response to glycine infusion is well characterized, the mechanism initiating this vasodilation is unknown. We recently observed functionally active N-methyl-d-aspartate (NMDA) receptors in the kidney, located primarily in tubular structures. The mechanisms regulating activity of the NMDA receptor within the kidney are also unknown, as is its normal day-to-day functional role. Therefore, we hypothesize that dietary protein may impact the functional response to glycine infusion in both untreated rats and rats pretreated with angiotensin-converting enzyme (ACE) inhibitor and, furthermore, that renal NMDA receptors may be involved in the glycine response. Surprisingly, 2 wk of low-protein diet (8% protein vs. 21% protein in control diet) totally inhibited the glycine-induced vasodilation and GFR response. Associated with the absence of renal vasodilation, a significant reduction in proximal tubular reabsorption was observed during glycine infusion in low-protein-diet rats. In contrast to the disease models previously studied in our laboratory, administration of ACE inhibitors did not restore the glycine response in rats treated with low-protein diet. Western blots of normal- and low-protein-diet kidneys demonstrate that the newly described renal NMDA receptor is downregulated in rats fed a low-protein diet. Low-protein feeding results in loss of glycine-induced vasodilation and GFR responses associated with decreased renal NMDA receptor expression. Kidney NMDA receptor expression is conditioned by protein intake, and this receptor may play an important role in the kidney vasodilatory response to glycine infusion and protein feeding in rats.

Renal functional reserve is impaired in patients with systemic sclerosis without clinical signs of kidney involvement

OBJECTIVE

To evaluate the functional response of the kidney to an amino acid challenge (the so called renal functional reserve (RFR)) in patients with systemic sclerosis (SSc) with no clinical sign of renal involvement.

METHODS

Before and after an intravenous amino acid load (Freamine III Baxter, 8.5% solution, 4.16 ml/min for two hours), glomerular filtration rate (GFR, as creatinine clearance), effective renal plasma flow (ERPF, as para-aminohyppurate clearance), and calculated total renal vascular resistance (TRVR) were measured in 21 patients with SSc with apparently normal renal function and 10 normal controls.

RESULTS

In basal conditions, patients had lower ERPF (403.5 (SD 43.8) v 496.4 (SD 71.3) ml/min, p<0.0002) and higher TRVR (10 822 (SD 2044) v 8874 (SD 1639) dyne/sxcm(-5), p<0.014) than controls. The RFR, evaluated as the percentage increase of GFR after the amino acid load, was significantly reduced in patients with SSc (SSc +1.9 (SD18.6)%, controls +34.8 (SD 13.9)%; p<0.0002). However, the response of patients was not uniform. Multiple regression analysis showed that the RFR was inversely dependent on the patients' mean arterial pressure at admission and basal GFR (R(2)=65%, p<0.0001).

CONCLUSIONS

Most patients with SSc cannot increase renal filtration under the challenge of a protein overload. This defective renal response to the amino acid load test sustains the concept of the prevalence of vasoconstrictor over vasodilating factors in the kidney of these patients.

Effects of glycine on glomerular filtration rate and segmental tubular handling of sodium in conscious rats

1. Infusion of the amino acid glycine leads to an increase in effective renal plasma flow (ERPF) and glomerular filtration rate (GFR) by a mechanism that possibly involves stimulation of nitric oxide (NO). Because NO also increases proximal tubular fluid output (Vprox) by inhibition of proximal tubular Na+ reabsorption and modulation of the tubuloglomerular feedback system, we hypothesized that glycine would increase Vprox as measured by lithium clearance (CLi). 2. In the first series of experiments, the effect of glycine infusion (4 mg/min) was examined in conscious, unstressed, chronically catheterized rats. In an additional series of experiments, the effect of glycine was examined under similar conditions in rats pretreated with a NO synthase (NOS) inhibitor (NG-nitro-L-arginine methyl ester (L-NAME), 2.5 microg/min). 3. Glycine significantly increased ERPF (from 3268 to 4018 microL/min per 100 g bodyweight (BW)), GFR (from 874 to 1009 microL/min per 100 g BW), CLi (from 275 to 461 microL/min per 100 g BW) and Na+ clearance (CNa; from 2.9 to 14.0 microL/min per 100 g BW). Fractional excretion of lithium (FELi; from 32 to 46%) and CNa/CLi (from 0.99 to 2.99%) also rose, indicating inhibition of proximal and distal nephron Na+ reabsorption, respectively. In the rats pretreated with L-NAME, similar haemodynamic and tubular responses to glycine infusion were seen, suggesting that the effects were not mediated by NO. 4. We conclude, that glycine increases ERPF and GFR and it also inhibits proximal and distal nephron Na+ reabsorption leading to an increase in CLi and CNa. There was no indication that any of these effects were mediated by NO.

Effect of combining an ACE inhibitor and an angiotensin II receptor blocker on plasma and kidney tissue angiotensin II levels

Increased angiotensin II (AII) activity has been recognized as a risk factor for progression of kidney disease. There is increasing clinical evidence that combining an angiotensin-converting enzyme (ACE) inhibitor with an AII receptor blocker (ARB) reduces proteinuria and blood pressure in patients with renal disease, although the mechanism of this synergistic effect remains poorly defined. This study tested whether the combination of an ACE inhibitor and an ARB reduces plasma AII (AIIp) and kidney tissue AII (AIIk) beyond what is observed with either of these two agents alone. Mean arterial pressure, glomerular filtration rate, AIIp, and AIIk were measured in four groups of Wistar rats after 2 weeks of a low-salt diet and 1 week of treatment with captopril (2.4 mg/d), losartan (1.7 mg/d), combination captopril+losartan (1.7 mg/d of captopril, 0.7 mg/d of losartan), or no treatment (control). Administration of captopril, losartan, and captopril+losartan produced statistically significant reductions in mean arterial pressure (control, 130 +/- 4 mm Hg; captopril, 92 +/- 5 mm Hg; losartan, 88 +/- 4 mm Hg; captopril+losartan, 104 +/- 5 mm Hg) and mild reductions in glomerular filtration rate (control, 3.1 +/- 0.1 mL/min; captopril, 2.2 +/- 0.3 mL/min; losartan, 1.7 +/- 0.3 mL/min; captopril+losartan, 2.3 +/- 0.3 mL/min) when compared with control rats, but no significant differences were observed among the treated groups. Captopril and captopril +losartan reduced AIIp significantly when compared with control (captopril, 43 +/- 8 pg/mL; captopril+losartan, 47 +/- 5 pg/mL; control, 134 pg/mL) and with losartan (99 +/- 2 pg/mL). AIIk values were reduced in captopril (254 +/- 18 pg/g kidney weight) and losartan (292 +/- 33 pg/g kidney weight) when compared with control (1,235 +/- 79 pg/g kidney weight). Captopril+losartan (136 +/- 17 pg/g kidney weight) reduced AIIk to values significantly lower than captopril or losartan alone. Higher doses of captopril (5 mg/d and 7.5 mg/d) or losartan (4 mg/d and 6 mg/d) alone did not reduce AIIk to the levels observed with combination low doses of captopril+losartan. Combining low doses of ACE inhibitor plus ARB reduces AIIk more than higher doses of either agent alone. This reduction in AIIk with ACE inhibitor plus ARB provides a mechanism to understand the synergism of this combination in reducing proteinuria and blood pressure. The reduction in AIIk with ACE inhibitor plus ARB may have important implications in long-term organ protection in hypertension and renal disease.

Effects of amino acids and glucagon on renal hemodynamics in type 1 diabetes

Increased dietary protein and circulating amino acids raise glomerular filtration rate (GFR) and pressure. In diabetes, this glomerular hyperfiltration response is augmented. The purpose of this study was to determine whether glucagon mediates the augmented GFR response to amino acids in diabetes and whether the responses to amino acids and glucagon depend on prostaglandins. Patients with type 1 diabetes mellitus (n = 12) and normal control subjects (n = 12) were studied in a series of six experiments, each on different occasions. Baseline GFR was not significantly increased, but filtration fraction was higher in diabetes. In response to amino acid infusion, GFR increased more and filtration fraction was greater among those with diabetes. Their augmented GFR response to amino acids was not inhibited by octreotide or indomethacin. Participants with diabetes also had enhanced GFR and renal plasma flow responses to glucagon infusion, both of which were inhibited by indomethacin. Glomerular hyperfiltration responses induced by amino acids or glucagon occur by divergent pathways in diabetes; only the response to glucagon is prostaglandin dependent.