Production of arginine by the kidney is impaired in a model of sepsis: early events following LPS.

Lipopolysaccharide (LPS) is used experimentally to elicit the innate physiological responses observed in human sepsis. We have previously shown that LPS causes depletion of plasma arginine before inducible nitric oxide synthase (iNOS) activity, indicating that changes in arginine uptake and/or production rather than enhanced consumption are responsible. Because the kidney is the primary source of circulating arginine and renal failure is a hallmark of septicemia, we determined the time course of changes in arginine metabolism and kidney function relative to iNOS expression. LPS given intravenously to anesthetized rats caused a decrease in mean arterial blood pressure after 120 min that coincided with increased plasma nitric oxide end products (NOx) and iNOS expression in lung and liver. Interestingly, impairment of renal function preceded iNOS activity by 30-60 min and occurred in tandem with decreased renal arginine production. The baseline rate of renal arginine production was approximately 60 micromol.h(-1).kg(-1), corresponding to an apparent plasma half-life of approximately 20 min, and decreased by one-half within 60 min of LPS. Calculations based on the systemic production and clearance show that normally only 5% of kidney arginine output is destined to become nitric oxide and that <25% of LPS-impaired renal production was converted to NOx in the first 4 h. In addition, we provide novel observations indicating that the kidney appears refractory to iNOS induction by LPS because no discernible enhancement of renal NOx production occurred within 4 h, and iNOS expression in the kidney was muted compared with that in liver or lung. These studies demonstrate that the major factor responsible for the rapid decrease in extracellular arginine content following LPS is impaired production by the kidney, a phenomenon that appears linked to reduced renal perfusion.

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.

Effect of acute iNOS inhibition on glomerular function in tubulointerstitial nephritis.

BACKGROUND: Tubulointerstitial nephritis (TIN) is characterized by progressive inflammatory infiltrate of the renal interstitium, induction of cortical tubular inducible nitric oxide synthase (iNOS) and reductions in glomerular filtration rate (GFR). These studies were designed to examine the changes in glomerular hemodynamics 7 and 21 days after induction of TIN and to evaluate the effect of acute iNOS blockade on glomerular function in the early stages of this model. METHODS: TIN was induced by immunizing Brown Norway rats with renal tubular antigen in complete Freund's adjuvant (RTA/CFA). Control rats were immunized with CFA alone. Micropuncture and morphologic studies were performed 7 and 21 days after immunization. RESULTS: Histology revealed minimal peritubular and interstitial inflammation in the RTA/CFA group one week after immunization while extensive interstitial inflammation with few preserved superficial nephron was observed three weeks after RTA/CFA immunization. Micropuncture studies on day 7 in the RTA/CFA group revealed a significant reduction in single nephron GFR due to a profound reduction in nephron plasma flow and in the ultrafiltration coefficient. Studies performed on day 21 revealed that single nephron GFR (SNGFR), nephron plasma flow (SNPF) and the ultrafiltration coefficient had returned to the normal baseline value despite the severe reduction in GFR. To assess the role of increased nitric oxide production secondary to iNOS induction on the glomerular hemodynamic changes observed in the early stages of the disease, the iNOS blocker (l-N(6)-iminoethyl lysine, L-NIL) was administered IV (1 mg/h) in RTA/CFA rats and CFA rats. L-NIL had no effect in CFA rats but produced significant increases in GFR, SNGFR and SNPF in RTA/CFA rats. CONCLUSIONS: These results demonstrate that TIN is associated with a progressive reduction in GFR, which is likely the result of functional vasoconstriction and decreases in the ultrafiltration coefficient in the early stages of the disease and on a significant reduction in the number of functioning nephron in the later stages. Induction of iNOS with increased NO production actively participates in the functional changes observed in the early stages of the disease most likely by inhibiting normal endothelial NOS activity.

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.