Antagonism of scavenger receptor CD36 by 5A peptide prevents chronic kidney disease progression in mice independent of blood pressure regulation.

Scavenger receptor CD36 participates in lipid metabolism and inflammatory pathways important for cardiovascular disease and chronic kidney disease (CKD). Few pharmacological agents are available to slow the progression of CKD. However, apolipoprotein A-I-mimetic peptide 5A antagonizes CD36 in vitro. To test the efficacy of 5A, and to test the role of CD36 during CKD, we compared wild-type to CD36 knockout mice and wild-type mice treated with 5A, in a progressive CKD model that resembles human disease. Knockout and 5A-treated wild-type mice were protected from CKD progression without changes in blood pressure and had reductions in cardiovascular risk surrogate markers that are associated with CKD. Treatment with 5A did not further protect CD36 knockout mice from CKD progression, implicating CD36 as its main site of action. In a separate model of kidney fibrosis, 5A-treated wild-type mice had less macrophage infiltration and interstitial fibrosis. Peptide 5A exerted anti-inflammatory effects in the kidney and decreased renal expression of inflammasome genes. Thus, CD36 is a new therapeutic target for CKD and its associated cardiovascular risk factors. Peptide 5A may be a promising new agent to slow CKD progression.

Low plasma renin and reduced renin secretory responses to acute stimuli in conscious COX-2-deficient mice.

In the current experiments, we determined the response of plasma renin concentration (PRC) to acute intraperitoneal administration of furosemide (40 mg/kg), hydralazine (2 mg/kg), isoproterenol (10 mg/kg), candesartan (50 microg), or quinaprilate (50 microg) in conscious wild-type (WT) and cyclooxygenase (COX)-2-/- mice on three different genetic backgrounds (mixed, C57BL/6, 129J). PRC was measured in plasma obtained by tail vein puncture. Basal PRC was significantly lower in COX-2-/- than WT mice independent of genetic background (51, 10, and 17% of WT in mixed, 129J, and C57BL/6). All five acute interventions caused significant increases of PRC in both COX-2+/+ and -/- mice, but the response was consistently less in COX-2-deficient mice (e.g., DeltaPRC in ng ANG I x ml(-1) x h(-1) caused by furosemide, isoproterenol, hydralazine, quinaprilate, or candesartan 4,699 +/- 544, 3,534 +/- 957, 2,522 +/- 369, 9,453 +/- 1,705, 66,455 +/- 21,938 in 129J WT, and 201 +/- 78, 869 +/- 275, 140 +/- 71, 902 +/- 304, 2,660 +/- 954 in 129J COX-2-/-). A low-NaCl diet and enalapril for 1 wk caused a 14-fold elevation of PRC in COX-2-/- mice and was associated with a greatly increased PRC response to acute furosemide (DeltaPRC 201 +/- 78 before and 15,984 +/- 2,397 after low Na/enalapril). As measured by radiotelemetry, blood pressure and heart rate responses to furosemide, hydralazine, isoproterenol, candesartan, or quinaprilate were not different between COX-2 genotypes. In conclusion, chronic absence of COX-2 reduces renin expression, release, and PRC and is associated with a reduced ability to alter PRC during acute stimulation regardless of the nature of the stimulus. COX-2 activity does not appear to be a mandatory and specific requirement for furosemide-stimulated renin secretion.

Adenosine as a mediator of macula densa-dependent inhibition of renin secretion.

Adenosine acting through A(1) adenosine receptors (A1AR) has been shown previously to be required for the vasoconstriction elicited by high luminal NaCl concentrations at the macula densa (MD). The present experiments were performed to investigate a possible role of A1AR in MD control of renin secretion in conscious wild-type (WT) and A1AR-deficient mice. The intravenous injection of NaCl (5% body wt) reduced plasma renin concentration (PRC; ng ANG I x ml(-1) x h(-1)) from 1,479 +/- 129 to 711 +/- 77 (P < 0.0001; n = 18) in WT mice but did not significantly change PRC in A1AR-/- mice (1,352 +/- 168 during control vs. 1,744 +/- 294 following NaCl; P = 0.19; n = 17). NaCl injections also caused a significant reduction in PRC in beta(1)/beta(2)-adrenergic receptor-/- mice (298 +/- 47 vs. 183 +/- 42; P = 0.03; n = 6). Injections of isotonic NaHCO(3) (5% body wt) elicited significant increases in PRC in both WT and A1AR-/- mice. NaCl as well as NaHCO(3) injections were accompanied by transient increases in blood pressure, heart rate, and activity that were similar in WT and A1AR-/- mice. The increase in PRC caused by an intraperitoneal injection of furosemide (40 mg/kg) was comparable in WT and A1AR-/- mice, and it was accompanied by similar transient increases in blood pressure, heart rate, and activity. Similarly, the stimulation of PRC caused by hydralazine was the same in WT and A1AR-/- mice. We conclude that the inhibition of renin secretion in response to an increase in NaCl at the MD requires A1AR and therefore appears to be adenosine dependent, whereas the stimulation of renin secretion during reductions in MD NaCl transport or arterial pressure does not require functional A1AR.

Influence of genetic background and gender on hypertension and renal failure in COX-2-deficient mice.

The present study was undertaken to determine whether the severity of renal failure or hypertension in homozygous cyclooxygenase (COX)-2-deficient (COX-2-/-) mice affected by genetic background or gender. COX-2 deletion was introduced into three congenic genetic backgrounds, 129/Sv (129/COX-2-/-), C57/BL6 (C57/COX-2-/-), and BALB/c (BALB/COX-2-/-), by backcrossing the original mixed-background knockout mice with the respective inbred strains for 9 or 10 generations. Evaluation of the severity of hypertension and renal failure was performed in knockout and wild-type mice at the age of 2.5-3.5 mo. Blood pressure measured by tail-cuff plethysmography was significantly elevated in the male 129/COX-2-/- mice (165.8 +/- 9.2 vs. 116 +/- 5.1 mmHg, P < 0.05), and to a much lesser extent in the female 129/COX-2-/- mice (127.4 +/- 3.3 vs. 102.4 +/- 3.3), whereas it was unchanged in the C57- or BALB/COX-2-/- mice regardless of gender. Urinary excretion of albumin, determined by EIA, was remarkably increased in the 129/COX-2-/- (16.4 +/- 4.1 vs. 0.16 +/- 0.043 mg albumin/mg creatinine, P < 0.001), and to a lesser extent in the male C57/COX-2-/- mice (0.595 +/- 0.416 vs. 0.068 +/- 0.019). Albumin excretion was not elevated in the male BALB/COX-2-/- or in female COX-2-/- mice on any of the three genetic backgrounds. Histological analysis showed abundant protein casts, dilated tubules, and infiltration of inflammatory cells in the male 129/COX-2-/- mice, but not in COX-2-/- mice in other strains or gender. However, the presence of small glomeruli in the nephrogenic zone was observed in all strains of COX-2 knockout mice, regardless of genetic background and gender. Therefore, we conclude that the severity of hypertension and renal failure in COX-2-deficient mice is influenced by genetic background and gender, whereas the incomplete maturation of outer cortical nephrons appears to be independent of genetic background effects.

Effect of carbonic anhydrase inhibition on GFR and renal hemodynamics in adenosine-1 receptor-deficient mice.

The reduction of glomerular filtration rate (GFR) caused by inhibitors of carbonic anhydrase (CA) is thought to be initiated by activation of the tubuloglomerular feedback (TGF) mechanism. We determined the effect of the CA inhibitor benzolamide (Bz) on renal hemodynamics in adenosine-1 receptor (A1AR) knockout mice that have been shown previously to lack a TGF response. In A1AR(+/+) mice, Bz (150 microg plus 2 microg/min) reduced RBF by 19.8% (from 829+/-42 to 666+/-44 microl/min; n=7), and GFR by 19.8% (from 396+/-43 to 324+/-46 microl/min; n=9, P=0.001). In A1AR(-/-) mice, RBF fell by 15.9 % (from 809+/-24 to 680+/-40 microl/min; n=7), and GFR by 21.1% (from 358+/-27 to 287+/-32 microl/min; n=10, P=0.0003; NS compared with A1AR(+/+)). The absence of TGF responses both before and during Bz infusion in A1AR(-/-) mice was confirmed by micropuncture. Following angiotensin II-receptor blockade with candesartan, Bz did not alter RBF (1.4+/-0.2 vs. 1.4+/-0.15 ml/min in A1AR(+/+), and 1.4+/-0.22 vs. 1.39+/-0.2 ml/min in A1AR(-/-); n=5/genotype) while GFR changed by -8.9 % in A1AR(+/+) mice ( n=7), and by -1% in A1AR(-/-) mice ( n=9; NS compared with A1AR(+/+)). Bz caused a significant rise of plasma renin concentration in both A1AR(+/+) and A1AR(-/-) mice. Our data show that the absence of a functional TGF mechanism does not prevent the reduction in GFR or RBF caused by CA inhibition. Acute angiotensin II receptor blockade, on the other hand, diminishes the effect of CA inhibition on GFR and RBF. The causes for the GFR reduction appear to be complex and include an effect of the renin-angiotensin system.

Inhibition of nNOS expression in the macula densa by COX-2-derived prostaglandin E(2).

It is well established that cyclooxygenase-2 (COX-2) and the neuronal form of nitric oxide synthase (nNOS) are coexpressed in macula densa cells and that the expression of both enzymes is stimulated in a number of high-renin states. To further explore the role of nNOS and COX-2 in renin secretion, we determined plasma renin activity in mice deficient in nNOS or COX-2. Plasma renin activity was significantly reduced in nNOS -/- mice on a mixed genetic background and in COX-2 -/- mice on either BALB/c or C57/BL6 congenic backgrounds. In additional studies, we accumulated evidence to show an inhibitory influence of PGE(2) on nNOS expression. In a cultured macula densa cell line, PGE(2) significantly reduced nNOS mRNA expression, as quantified by real-time RT-PCR. In COX-2 -/- mice, nNOS mRNA expression in the kidney, determined by real-time RT-PCR, was upregulated throughout the postnatal periods, ranging from postnatal day (PND) 3 to PND 60. The induction of nNOS protein expression and NOS activity in COX-2 -/- mice was localized to macula densa cells using immunohistochemistry and NADPH-diaphorase staining methods, respectively. Therefore, these findings reveal that the absence of either COX-2 or nNOS is associated with suppressed renin secretion. Furthermore, the inhibitory effect of PGE(2) on nNOS mRNA expression indicates a novel interaction between NO and prostaglandin-mediated pathways of renin regulation.

Feedback control of glomerular vascular tone in neuronal nitric oxide synthase knockout mice.

For further elucidation of the role of neuronal nitric oxide synthase (nNOS) in macula densa (MD) cells, experiments were performed in anesthetized nNOS knockout mice (nNOS -/-). At comparable levels of arterial BP, renal blood flow was not significantly different between nNOS +/+ and nNOS -/- (1.7 +/- 0.2 versus 1.4 +/- 0.1 ml/min), and autoregulation of renal blood flow was maintained to a pressure level of approximately 85 mmHg in both groups of mice (n = 6 in each group). The fall in proximal tubular stop-flow pressure in response to an increase in loop of Henle perfusion rate from 0 to 30 nl/min was comparable in nNOS +/+ and -/- mice (40.7 +/- 1.6 to 32 +/- 2 mmHg versus 40.6 +/- 1.6 to 31.6 +/- 2 mmHg; not significant; n = 13 versus 18 nephrons). Luminal application of the nonselective NOS inhibitor nitro-L-arginine (10(-3) and 10(-2) M) enhanced the perfusion-dependent fall in stop-flow pressure in nNOS +/+ (7 +/- 1 to 13 +/- 2 mmHg; P < 0.05) but not in nNOS -/- (7 +/- 1 to 8 +/- 1 mmHg; not significant) mice. nNOS -/- mice exhibited a lower nephron filtration rate, compared with nNOS +/+, during free-flow collections from early distal tubules (influence of MD intact, 7 +/- 0.7 versus 10.9 +/- 1 nl/min; P = 0.002) but not from late proximal tubule (influence of MD minimized, 10.1 +/- 1 versus 11.7 +/- 1 nl/min; not significant; n = 16 nephrons). Distal Cl concentration and fractional absorption of fluid or chloride up to the early distal tubule was not different between nNOS -/- and +/+ mice. The data indicate that nNOS in MD tonically attenuates the GFR-lowering influence of ambient luminal NaCl, which may serve to increase the fluid and electrolyte load to the distal tubule, consistent with a role of MD nNOS in tubuloglomerular feedback resetting.

Angiotensin II blockade causes acute renal failure in eNOS-deficient mice.

Compared with wild-type mice, adult endothelial nitric oxide synthase (eNOS) knockout mice (eight months of age) have increased blood pressure (BP) (126±9 mmHg vs. 100±4 mmHg), and an increased renal vascular resistance (155±16 vs. 65±4 mmHg.min/ml). Renal vascular resistance responses to i.v. administration of noradrenaline were markedly enhanced in eNOS knockout mice. Glomerular filtration rate (GFR) of anaesthetised eNOS -/- mice was 324±57 µl/min gKW, significantly lower than the GFR of 761±126 µl/min.gKW in wild-type mice. AT1-receptor blockade with i.v. candesartan (1-1.5 mg/kg) reduced arterial blood pressure and renal vascular resistance, and increased renal blood flow (RBF) to about the same extent in wild-type and eNOS -/- mice. Candesartan did not alter GFR in wild-type mice (761±126 vs. 720±95 µl/min.gKW), but caused a marked decrease in GFR in eNOS -/- mice (324.5±75.2 vs. 77±18 µl/min.gKW). A similar reduction in GFR of eNOS deficient mice was also caused by angiotensin-converting enzyme (ACE) inhibition. Afferent arteriolar granularity, a measure of renal renin expression, was found to be reduced in eNOS -/- compared with wild-type mice. In chronically eNOS-deficient mice, angiotensin II (Ang II) is critical for maintaining glomerular filtration pressure and GFR, presumably through its effect on efferent arteriolar tone.