Renal Medulla in Hypertension.

Studies have found that blood flow to the renal medulla is an important determinant of pressure-natriuresis and the long-term regulation of arterial pressure. First, a brief review of methods developed enabling the study of the medullary circulation is presented. Second, studies performed in rats are presented showing medullary blood flow plays a vital role in the pressure-natriuresis relationship and thereby in hypertension. Third, it is shown that chronic reduction of medullary blood flow results in hypertension and that enhancement of medullary blood flow reduces hypertension hereditary models of both salt-sensitive rats and salt-resistant forms of hypertension. The key role that medullary nitric oxide production plays in protecting this region from ischemic injury associated with circulating vasoconstrictor agents and reactive oxygen species is presented. The studies cited are largely the work of my students, research fellows, and colleagues with whom I have performed these studies dating from the late 1980s to more recent years.

Renal medulla in hypertension

Abstract: Studies have found that blood flow to the renal medulla is an important determinant of pressure-natriuresis and the long-term regulation of arterial pressure. First, a brief review of methods developed enabling the study of the medullary circulation is presented. Second, studies performed in rats are presented showing medullary blood flow plays a vital role in the pressure-natriuresis relationship and thereby in hypertension. Third, it is shown that chronic reduction of medullary blood flow results in hypertension and that enhancement of medullary blood flow reduces hypertension hereditary models of both salt-sensitive rats and salt-resistant forms of hypertension. The key role that medullary nitric oxide production plays in protecting this region from ischemic injury associated with circulating vasoconstrictor agents and reactive oxygen species is presented. The studies cited are largely the work of my students, research fellows, and colleagues with whom I have performed these studies dating from the late 1980s to more recent years.

Evidence of the Importance of Nox4 in Production of Hypertension in Dahl Salt-Sensitive Rats.

This study reports the consequences of knocking out NADPH (nicotinamide adenine dinucleotide phosphate) oxidase 4 (Nox4) on the development of hypertension and kidney injury in the Dahl salt-sensitive (SS) rat. Zinc finger nuclease injection of single-cell SS embryos was used to create an 8 base-pair frame-shift deletion of Nox4, resulting in a loss of the ≈68 kDa band in Western blot analysis of renal cortical tissue of the knock out of Nox4 in the SS rat (SS(Nox4-/-)) rats. SS(Nox4-/-) rats exhibited a significant reduction of salt-induced hypertension compared with SS rats after 21 days of 4.0% NaCl diet (134±5 versus 151±3 mm Hg in SS) and a significant reduction of albuminuria, tubular casts, and glomerular injury. Optical fluorescence 3-dimensional cryoimaging revealed significantly higher redox ratios (NADH/FAD [reduced nicotinamide adenine dinucleotide/flavin adenine dinucleotide]) in the kidneys of SS(Nox4-/-) rats even when fed the 0.4% NaCl diet, indicating greater levels of mitochondrial electron transport chain metabolic activity and reduced oxidative stress compared with SS rats. Before the development of hypertension, RNA expression levels of Nox subunits Nox2, p67(phox), and p22(phox) were found to be significantly lower (P<0.05) in SS(Nox4-/-) compared with SS rats in the renal cortex. Thus, the mutation of Nox4 seems to modify transcription of several genes in ways that contribute to the protective effects observed in the SS(Nox4-/-) rats. We conclude that the reduced renal injury and attenuated blood pressure response to high salt in the SS(Nox4-/-) rat could be the result of multiple pathways, including gene transcription, mitochondrial energetics, oxidative stress, and protein matrix production impacted by the knock out of Nox4.

Null mutation of the nicotinamide adenine dinucleotide phosphate-oxidase subunit p67phox protects the Dahl-S rat from salt-induced reductions in medullary blood flow and glomerular filtration rate.

Null mutations in the p67(phox) subunit of nicotinamide adenine dinucleotide phosphate-oxidase confer protection from salt sensitivity on Dahl salt-sensitive rats. Here, we track the sequential changes in medullary blood flow (MBF), glomerular filtration rate (GFR), urinary protein, and mean arterial pressure in SSp67(phox) null rats and wild-type littermates during 21 days of 4.0% NaCl high-salt (HS) diet. Optical fibers were implanted in the renal medulla and MBF was measured in conscious rats by laser Doppler flowmetry. Separate groups of rats were prepared with femoral venous catheters and GFR was measured by the transcutaneous assessment of fluorescein isothiocyanate-sinistrin disappearance curves. Mean arterial blood pressure was measured by telemetry. In wild-type rats, HS caused a rapid reduction in MBF, which was significantly lower than control values by HS day-6. Reduced MBF was associated with a progressive increase in mean arterial pressure, averaging 170±5 mm Hg by HS salt day-21. A significant reduction in GFR was evident on day-14 HS, after the onset of hypertension and reduced MBF. In contrast, HS had no significant effect on MBF in SSp67(phox) null rats and the pressor response to sodium was blunted, averaging 150±3 mm Hg on day-21 HS. GFR was maintained throughout the study and proteinuria was reduced. In summary, when p67(phox) is not functional in the salt-sensitive rats, HS does not cause reduced MBF and salt-sensitive hypertension is attenuated, and consequently renal injury is reduced and GFR is maintained.

Maternal diet during gestation and lactation modifies the severity of salt-induced hypertension and renal injury in Dahl salt-sensitive rats.

Environmental exposure of parents or early in life may affect disease development in adults. We found that hypertension and renal injury induced by a high-salt diet were substantially attenuated in Dahl SS/JrHsdMcwiCrl (SS/Crl) rats that had been maintained for many generations on the grain-based 5L2F diet compared with SS/JrHsdMcwi rats (SS/Mcw) maintained on the casein-based AIN-76A diet (mean arterial pressure, 116±9 versus 154±25 mm Hg; urinary albumin excretion, 23±12 versus 170±80 mg/d). RNAseq analysis of the renal outer medulla identified 129 and 82 genes responding to a high-salt diet uniquely in SS/Mcw and SS/Crl rats, respectively, along with minor genetic differences between the SS substrains. The 129 genes responding to salt in the SS/Mcw strain included numerous genes with homologs associated with hypertension, cardiovascular disease, or renal disease in human. To narrow the critical window of exposure, we performed embryo-transfer experiments in which single-cell embryos from 1 colony (SS/Mcw or SS/Crl) were transferred to surrogate mothers from the other colony, with parents and surrogate mothers maintained on their respective original diet. All offspring were fed the AIN-76A diet after weaning. Salt-induced hypertension and renal injury were substantially exacerbated in rats developed from SS/Crl embryos transferred to SS/Mcw surrogate mothers. Conversely, salt-induced hypertension and renal injury were significantly attenuated in rats developed from SS/Mcw embryos transferred to SS/Crl surrogate mothers. Together, the data suggest that maternal diet during the gestational-lactational period has substantial effects on the development of salt-induced hypertension and renal injury in adult SS rats.

Progression of glomerular filtration rate reduction determined in conscious Dahl salt-sensitive hypertensive rats.

Sequential changes in glomerular filtration rate during development of hypertension in the conscious Dahl salt-sensitive rats were determined using a new method for measurement. Using a miniaturized device, disappearance curves of fluorescein isothiocyanate-sinistrin were measured by transcutaneous excitation and real-time detection of the emitted light through the skin. Rats with implanted femoral venous catheters (dye injection and sampling) and carotid catheters (mean arterial pressure by telemetry) were studied, while maintained on a 0.4% NaCl diet and on days 2, 5, 7, 14, and 21 after switching to 4.0% (high-salt [HS]) diet. A separate group of rats were maintained on 0.4% for 21 days as a time control. Mean arterial pressure rose progressively from the last day of 0.4% (130±2 mm Hg) reaching significance by day 5 of HS and averaged 162±7 mm Hg by day 21. Urine albumin excretion was significantly elevated (×3) by day 7 of HS in Dahl salt-sensitive rats. Glomerular filtration rate reduced on day 14 of HS falling from 1.53±0.06 mL/min per 100 g body weight to 1.27±0.04. By day 21, glomerular filtration rate had fallen 28% to 1.1±0.04 mL/min per 100 g (t(1/2) 28.4±1.1 minute.) No significant reductions of creatinine clearance were observed throughout the study in response to HS demonstrating the insensitivity of creatinine clearance measurements even with creatinine measured using mass spectrometry. We conclude that the observed reduction of glomerular filtration rate was a consequence and not a cause of the hypertension and that this noninvasive approach could be used in these conscious Dahl salt-sensitive rats for a longitudinal assessment of renal function.

Increased renal medullary oxidative stress produces hypertension.

The present study examined whether chronic increased oxidative stress within the medulla of the kidney lowers medullary blood flow and leads to hypertension. Optical fibers were implanted into the renal cortex and medulla of uninephrectomized Sprague-Dawley rats (Harlan Sprague-Dawley, Madison, Wis) for the daily measurement of blood flow to these regions using laser-Doppler flowmetry techniques, while arterial pressure was measured from an indwelling aortic catheter. A renal medullary interstitial catheter was implanted for the continuous delivery of the superoxide dismutase (SOD) inhibitor, diethyldithiocarbamic acid (DETC), at a dose of 7.5 mg/kg/d. Renal interstitial superoxide (O(2)(-)) levels were determined by perfusing an O(2)(-) sensitive fluorescent dye, dihydroethidium, through a microdialysis probe implanted into the medulla. Urine samples (24 hours) were collected for measurements of isoprostane excretion. The results indicate that medullary DETC infusions increased tissue O(2)(-) concentrations in the renal medulla (93.4 +/- 22.3,n=8, saline and 867.3 +/- 260.2, n=8, DETC; fluorescence units) and increased urinary 8-isoprostane excretion (4.1 +/- 0.4 ng/d, n=9, saline and 8.8 +/- 1.6 ng/d, n=10, DETC). Mean arterial pressure increased 24 hours after the start of intrarenal DETC infusion and remained nearly 20 mm Hg above control pressure throughout the 5 days of medullary SOD inhibition. During chronic medullary DETC infusion, medullary blood flow was significantly reduced (42.7%), whereas cortical blood flow was unchanged. Intravenous infusion of the same dose of DETC produced no changes in renal medullary or cortical blood flow or arterial blood pressure. The present experiments indicate that an increase in superoxide concentration within the renal medulla selectively reduces medullary blood flow resulting in chronic hypertension.

Increased renal medullary H2O2 leads to hypertension.

We have recently reported that exaggerated oxidative stress in the renal medulla due to superoxide dismutase inhibition resulted in a reduction of renal medullary blood flow and sustained hypertension. The present study tested the hypothesis that selective scavenging of O2*- in the renal medulla would prevent hypertension associated with this exaggerated oxidative stress. An indwelling, aortic catheter was implanted in nonnephrectomized Sprague-Dawley rats for daily measurement of arterial blood pressure, and a renal medullary interstitial catheter was implanted for continuous delivery of the superoxide dismutase inhibitor diethyldithiocarbamic acid (DETC, 7.5 mg x kg(-1) x d(-1)) and a chemical superoxide dismutase mimetic, 4-hydroxytetramethyl piperidine-1-oxyl (TEMPOL, 10 mg. kg-1. d-1). Renal medullary interstitial infusion of TEMPOL completely blocked DETC-induced accumulation of O2*- in the renal medulla, as measured by the conversion rate of dihydroethidium to ethidium in the dialysate and by urinary excretion of 8-isoprostanes. However, TEMPOL infusion failed to prevent DETC-induced hypertension, unless catalase (5 mg x kg(-1) d(-1)) was coinfused. Direct infusion of H2O2 into the renal medulla resulted in increases of mean arterial pressure from 115+/-2.5 to 131+/-2.1 mm Hg, which was similar to that observed in rats receiving the medullary infusion of both TEMPOL and DETC. The results indicate that sufficient catalase activity in the renal medulla is a prerequisite for the antihypertensive action of TEMPOL and that accumulated H2O2 in the renal medulla associated with exaggerated oxidative stress might have a hypertensive consequence.