Autoregulation of glomerular filtration rate in patients with type 2 diabetes during isradipine therapy

Comparison of the Surgical Resection and Infarct 5/6 Nephrectomy Rat Models of Chronic Kidney Disease

The 5/6 nephrectomy rat remnant kidney model is commonly employed to study chronic kidney disease (CKD). This model requires removal of one whole kidney and two-thirds of the other. The two most common ways of producing the remnant kidney are surgical resection of poles, known as the polectomy (Pol) model, or ligation of upper and lower renal arterial branches, resulting in pole infarction (Inf). These models have much in common, but also major phenotypic differences, and thus respectively model unique aspects of human CKD. The purpose of this review is to summarize phenotypic similarities and differences between these two models and their relation to human CKD, while emphasizing their vascular phenotype. In this article we review studies that have evaluated arterial blood pressure, the renin-angiotensin-aldosterone-system (RAAS), autoregulation, nitric oxide, single nephron physiology, angiogenic and anti-angiogenic factors, and capillary rarefaction in these two models. Phenotypic similarities: both models spontaneously develop hallmarks of human CKD including uremia, fibrosis, capillary rarefaction, and progressive renal function decline. They both undergo whole-organ hypertrophy, hyperfiltration of functional nephrons, reduced renal expression of angiogenic factor VEGF, increased renal expression of the anti-angiogenic thrombospondin-1, impaired renal autoregulation, and abnormal vascular nitric oxide physiology. Key phenotypic differences: the Inf model develops rapid-onset, moderate-to-severe systemic hypertension, and the Pol model early normotension followed by mild-to-moderate hypertension. The Inf rat has a markedly more active renin-angiotensin-aldosterone-system. Comparison of these two models facilitates understanding of how they can be utilized for studying CKD pathophysiology (e.g., RAAS dependent or independent pathology).

In vitro investigation of the impact of pulsatile blood flow on the vascular architecture of decellularized porcine kidneys

A method was established using a scaffold-bioreactor system to examine the impact pulsatile blood flow has on the decellularized porcine kidney vascular architecture and functionality. These scaffolds were subjected to continuous arterial perfusion of whole blood at normal physiological (650 ml/min and 500 ml/min) and pathophysiological (200 ml/min) rates to examine dynamic changes in venous outflow and micro-/macrovascular structure and patency. Scaffolds subjected to normal arterial perfusion rates observed drops in venous outflow over 24 h. These reductions rose from roughly 40% after 12 h to 60% after 24 h. There were no apparent signs of clotting at the renal artery, renal vein, and ureter. In comparison, venous flow rates decreased by 80% to 100% across the 24 h in acellular scaffolds hypoperfused at a rate of 200 ml/min. These kidneys also appeared intact on the surface after perfusion. However, they presented several arterial, venous, and ureteral clots. Fluoroscopic angiography confirmed substantial alterations to normal arterial branching patterns and patency, as well as parenchymal damage. Scanning electron microscopy revealed that pulsatile blood perfusion significantly disrupted glomerular microarchitecture. This study provides new insight into circumstances that limit scaffold viability and a simplified model to analyze conditions needed to prepare more durable scaffolds for long-term transplantation.

Shunaoxin pills improve the antihypertensive effect of nifedipine and alleviate its renal lipotoxicity in spontaneous hypertension rats

Shunaoxin pills (SNX) have been used to treat cerebrovascular diseases in China since 2005. Hypertension is a major risk factor for cerebrovascular disease. This study aimed to explore the synergistic antihypertensive effect of SNX and nifedipine and whether SNX could alleviate nifedipine-induced renal lipotoxicity. During administration, systolic blood pressure was measured weekly. After 5 weeks administration, we examined pathological changes of kidney, renal function, the lipid metabolism index, and adipogenesis genes expression in the kidney tissues, and explored its underlying mechanism. Finally, network pharmacology was used for supplement and verification. As a result, SNX improved the antihypertensive effect of nifedipine and apparently improved nifedipine-induced renal pathological changes, dyslipidemia and the levels of adipogenesis gene expression in kidney tissues. SNX reduced the levels of interleukin-6 and interleukin-1β in renal tissues, down-regulated the production of malondialdehyde, and increased superoxide dismutase activity and the protein expression of heme oxygenase-1 in kidney tissues. Network pharmacology also showed that SNX could improve nifedipine-induced renal lipotoxicity. The combination of SNX and nifedipine had certain benefits in the treatment of hypertension.

Purinoceptors, renal microvascular function and hypertension

Proper renal blood flow (RBF) and glomerular filtration rate (GFR) are critical for maintaining normal blood pressure, kidney function and water and electrolyte homeostasis. The renal microvasculature expresses a multitude of receptors mediating vasodilation and vasoconstriction, which can influence glomerular blood flow and capillary pressure. Despite this, RBF and GFR remain quite stable when arterial pressure fluctuates because of the autoregulatory mechanism. ATP and adenosine participate in autoregulatory control of RBF and GFR via activation of two different purinoceptor families (P1 and P2). Purinoceptors are widely expressed in renal microvasculature and tubules. Emerging data show altered purinoceptor signaling in hypertension-associated kidney injury, diabetic nephropathy, sepsis, ischemia-reperfusion induced acute kidney injury and polycystic kidney disease. In this brief review, we highlight recent studies and new insights on purinoceptors regulating renal microvascular function and renal hemodynamics. We also address the mechanisms underlying renal microvascular injury and impaired renal autoregulation, focusing on purinoceptor signaling and hypertension-induced renal microvascular dysfunction. Interested readers are directed to several excellent and comprehensive reviews that recently covered the topics of renal autoregulation, and nucleotides in kidney function under physiological and pathophysiological conditions (Inscho 2009, Navar et al. 2008, Carlstrom et al. 2015, Vallon et al. 2020).

Nifedipine Modulates Renal Lipogenesis via the AMPK-SREBP Transcriptional Pathway

Lipid accumulation in renal cells has been implicated in the pathogenesis of obesity-related kidney disease, and lipotoxicity in the kidney can be a surrogate marker for renal failure or renal fibrosis. Fatty acid oxidation provides energy to renal tubular cells. Ca²⁺ is required for mitochondrial ATP production and to decrease reactive oxygen species (ROS). However, how nifedipine (a calcium channel blocker) affects lipogenesis is unknown. We utilized rat NRK52E cells pre-treated with varying concentrations of nifedipine to examine the activity of lipogenesis enzymes and lipotoxicity. A positive control exposed to oleic acid was used for comparison. Nifedipine was found to activate acetyl Coenzyme A (CoA) synthetase, acetyl CoA carboxylase, long chain fatty acyl CoA elongase, ATP-citrate lyase, and 3-hydroxy-3-methyl-glutaryl-coenzyme A (HMG CoA) reductase, suggesting elevated production of cholesterol and phospholipids. Nifedipine exposure induced a vast accumulation of cytosolic free fatty acids (FFA) and stimulated the production of reactive oxygen species, upregulated CD36 and KIM-1 (kidney injury molecule-1) expression, inhibited p-AMPK activity, and triggered the expression of SREBP-1/2 and lipin-1, underscoring the potential of nifedipine to induce lipotoxicity with renal damage. To our knowledge, this is the first report demonstrating nifedipine-induced lipid accumulation in the kidney.

Renal autoregulation and blood pressure management in circulatory shock

The importance of personalized blood pressure management is well recognized. Because renal pressure–flow relationships may vary among patients, understanding how renal autoregulation may influence blood pressure control is essential. However, much remains uncertain regarding the determinants of renal autoregulation in circulatory shock, including the influence of comorbidities and the effects of vasopressor treatment. We review published studies on renal autoregulation relevant to the management of acutely ill patients with shock. We delineate the main signaling pathways of renal autoregulation, discuss how it can be assessed, and describe the renal autoregulatory alterations associated with chronic disease and with shock.

Blood pressure and age-related GFR decline in the general population

BACKGROUND

Hypertension is one of the most important causes of end-stage renal disease, but it is unclear whether elevated blood pressure (BP) also accelerates the gradual decline in the glomerular filtration rate (GFR) seen in the general population with increasing age. The reason may be that most studies have considered only baseline BP and not the effects of changes in BP, antihypertensive treatment and other determinants of GFR during follow-up. Additionally, the use of GFR estimated from creatinine or cystatin C instead of measurements of GFR may have biased the results because of influence from non-GFR related confounders. We studied the relationship between BP and GFR decline using time-varying variables in a cohort representative of the general population using measurements of GFR as iohexol clearance.

METHODS

We included 1594 subjects aged 50 to 62 years without baseline diabetes, kidney-, or cardiovascular disease in the Renal Iohexol-clearance Survey in Tromsø 6 (RENIS-T6). GFR, BP, antihypertensive medication and all adjustment variables were ascertained at baseline, and at follow-up after a median observation time of 5.6 years in 1299 persons (81%). The relationship between GFR decline and BP was analyzed in linear mixed models.

RESULTS

The mean (standard deviation) GFR decline rate was 0.95 (2.23) mL/min/year. The percentage of persons with hypertension (systolic BP ≥ 140 mmHg, diastolic BP ≥ 90 mmHg or antihypertensive medication) increased from 42 to 52% between baseline and follow-up. In multivariable adjusted linear mixed models using time-varying independent variables measured at baseline and follow-up, higher systolic and diastolic BP were associated with slower GFR decline rates by 0.10 and 0.20 mL/min/year/10 mmHg, respectively (p < 0.05). The association was stronger in persons on antihypertensive medication than in others (p < 0.05 for the interaction between BP and antihypertensive medication).

CONCLUSIONS

In the medium-term, elevated BP is not associated with accelerated GFR decline in the general middle-aged population. In persons using antihypertensive medication, elevated BP is associated with a paradoxical slower GFR decline. Studies with even longer observation periods are needed to evaluate the ultimate effect of BP on kidney function.

Renal autoregulation in health and disease

Intrarenal autoregulatory mechanisms maintain renal blood flow (RBF) and glomerular filtration rate (GFR) independent of renal perfusion pressure (RPP) over a defined range (80-180 mmHg). Such autoregulation is mediated largely by the myogenic and the macula densa-tubuloglomerular feedback (MD-TGF) responses that regulate preglomerular vasomotor tone primarily of the afferent arteriole. Differences in response times allow separation of these mechanisms in the time and frequency domains. Mechanotransduction initiating the myogenic response requires a sensing mechanism activated by stretch of vascular smooth muscle cells (VSMCs) and coupled to intracellular signaling pathways eliciting plasma membrane depolarization and a rise in cytosolic free calcium concentration ([Ca(2+)]i). Proposed mechanosensors include epithelial sodium channels (ENaC), integrins, and/or transient receptor potential (TRP) channels. Increased [Ca(2+)]i occurs predominantly by Ca(2+) influx through L-type voltage-operated Ca(2+) channels (VOCC). Increased [Ca(2+)]i activates inositol trisphosphate receptors (IP3R) and ryanodine receptors (RyR) to mobilize Ca(2+) from sarcoplasmic reticular stores. Myogenic vasoconstriction is sustained by increased Ca(2+) sensitivity, mediated by protein kinase C and Rho/Rho-kinase that favors a positive balance between myosin light-chain kinase and phosphatase. Increased RPP activates MD-TGF by transducing a signal of epithelial MD salt reabsorption to adjust afferent arteriolar vasoconstriction. A combination of vascular and tubular mechanisms, novel to the kidney, provides for high autoregulatory efficiency that maintains RBF and GFR, stabilizes sodium excretion, and buffers transmission of RPP to sensitive glomerular capillaries, thereby protecting against hypertensive barotrauma. A unique aspect of the myogenic response in the renal vasculature is modulation of its strength and speed by the MD-TGF and by a connecting tubule glomerular feedback (CT-GF) mechanism. Reactive oxygen species and nitric oxide are modulators of myogenic and MD-TGF mechanisms. Attenuated renal autoregulation contributes to renal damage in many, but not all, models of renal, diabetic, and hypertensive diseases. This review provides a summary of our current knowledge regarding underlying mechanisms enabling renal autoregulation in health and disease and methods used for its study.

Potential risks of calcium channel blockers in chronic kidney disease

Antihypertensive therapy remains the most effective strategy for slowing the progression of chronic kidney disease (CKD). However, in proteinuric nephropathies, calcium channel blockers (CCBs) are less effective than other antihypertensives unless normotension is achieved. This is because the glomerular capillaries, rather than larger vessels, are the primary site of hypertensive injury in proteinuric nephropathies. CCBs impair renal autoregulation, which protects glomerular capillaries against the transmission of systemic pressures. CCBs' renoprotective inferiority in the comparator group likely accounts for the greater renoprotection observed with renin-angiotensin system blockade rather than blood pressure (BP)-independent renoprotective superiority. Nevertheless, CKD patients are at greater absolute risk for cardiovascular events rather than end-stage renal disease. Therefore, if the needed BP reductions cannot be achieved with other agents, it may be appropriate to use CCBs because of their antihypertensive effectiveness, provided care is taken to ensure normotension and to closely monitor proteinuria and renal disease progression.

Elevations in serum creatinine with RAAS blockade: why isn't it a sign of kidney injury?

PURPOSE OF REVIEW

The aim of this article is to review the pertinent physiology and pathophysiology of the renin-angiotensin-aldosterone system (RAAS), summarize the proven beneficial cardiovascular and renal effects of RAAS blockade, examine clinical situations in which RAAS blockade may induce reductions in glomerular filtration rate, and explore why increases in serum creatinine in the setting of angiotensin-converting enzyme inhibitor (ACEi) or angiotensin receptor blocker (ARB) therapy do not necessarily signify the presence of clinically relevant kidney failure.

RECENT FINDINGS

RAAS inhibition appears to reduce the likelihood of atrial fibrillation. RAAS inhibition leads to improved insulin sensitivity and glycemic control, but does not appear to prevent diabetes. The beneficial effects of ACEi/ARB therapy extend to those with significant renal disease. Combination ACEi/ARB is safe, and reduces proteinuria more than either agent alone in patients with macroalbuminuric nephropathy. Acute deteriorations in renal function that result from RAAS inhibition are usually reversible.

SUMMARY

RAAS blockade exerts potent hemodynamic, antihypertensive, and antiinflammatory effects, and slows progression of kidney disease beyond that due to lowering of blood pressure. The benefit extends to those with advanced disease. In spite of established benefit, ACEi and ARB therapy remains underutilized, in part due to concerns about acute deteriorations in renal function that result from interruption of the RAAS.

Hypertension and Kidney Damage

Substantial evidence indicates that hypertension is a major contributor to the development of end-stage renal disease in most patients. However, such risk ranges from being fairly low in essential hypertension to a marked increase in susceptibility to hypertensive injury in patients with chronic kidney disease, including diabetic nephropathy. Studies in experimental animal models using blood pressure radiotelemetry have provided significant insights into the quantitative relationships between blood pressure and renal damage and the importance of protective renal autoregulatory capacity as a determinant of such differences in susceptibility to hypertensive injury. Moreover, such investigations have also emphasized the predominant importance of achieving normotension per se over the selection of particular antihypertensive regimens, including renin-angiotensin system blockade, in slowing the progression of chronic kidney disease.

Progression of renal disease: renoprotective specificity of renin-angiotensin system blockade

Recent guidelines for management of patients with chronic kidney disease recommend both lower optimal BP targets and agents that block the renin-angiotensin system (RAS) for specific additional BP-independent renoprotection. Although there are other compelling rationales to use RAS blockade in patients with chronic kidney disease, including its antihypertensive effectiveness and ability to counteract the adverse effects of diuretics, a critical review of the available scientific evidence suggests that the specificity of renoprotection that is provided by RAS blockade has been greatly overemphasized. Little evidence of truly BP-independent renoprotection is observed in experimental animal models when ambient BP is assessed adequately by chronic continuous BP radiotelemetry. Although the clinical trial evidence is somewhat stronger, nevertheless, even when interpreted favorably, the absolute magnitude of the BP-independent component of the renoprotection that is observed with RAS blockade is much smaller than what is due to its antihypertensive effects.

Morphometric evidence for impairment of renal autoregulation in advanced essential hypertension

A morphometric study was performed on 22 renal biopsies from hypertensive patients with proteinuria and/or azotemia, with no evidence of other renal disease. These results were compared with our earlier study of normotensive aging kidneys. Afferent arterioles in hypertensive kidneys showed a significant increase in lumen diameter (15.7+/-4.9 vs 13.4+/-4.7 microm, P=0.0007) and wall area (1234+/-769 vs 998+/-445 microm(2), P=0.037), due primarily to shift in the distribution of arteriolar types, from predominantly normal toward predominantly hyaline arterioles in hypertension. Glomeruli were divided into four basic types: normal, hypertrophic, focal segmental glomerulosclerosis (FSGS) type, and sclerosing. Overall, glomeruli in hypertensive kidneys were much larger than in normotensive aging kidneys, for example, total capillary area (16 247+/-10 681 vs 11 624+/-5702 microm(2), P<0.00001). This increase was due primarily to an increase in size of each type, for example, for hypertrophic glomeruli: total capillary area (22 205+/-10 426 vs 15 349+/-4577 microm(2), P=0.0038). There was an excellent correlation between arteriolar lumen diameter and mean glomerular capillary area for hypertrophic/FSGS-type glomeruli (r=0.4778, P=0.0013), such that as arteriolar diameter increases the mean glomerular capillary area increases, consistent with loss of autoregulation. The morphologic correlates of loss of autoregulation, with afferent arteriolar dilatation and increase in glomerular capillary size, glomerular hypertrophy, and subsequent FSGS, are present on a focal basis in aging kidneys and, much more extensively, although still focally, in hypertensive kidneys.

Hypertensive renal damage: insights from animal models and clinical relevance

Investigations using chronic blood pressure (BP) radiotelemetry in conscious animals have provided substantial insights into the pathophysiology of hypertensive renal damage. Normal renal autoregulation protects the renal microvasculature from significant injury in most patients with primary hypertension, unless BP exceeds a certain threshold, when malignant nephrosclerosis develops. However, if autoregulation is impaired, as in chronic renal disease and/or diabetes models, the threshold for renal damage is lowered and glomerulosclerosis (GS) increases linearly with increasing BP. Modest BP reductions are predicted to prevent malignant nephrosclerosis, but prevention of GS in patients with diabetes and chronic renal disease requires that BP be lowered well into the normotensive range, as recognized in the currently recommended BP goals. When BP load is accurately assessed in these experimental models, renal protection is proportional to the achieved BP reductions, and there is little evidence of BP-independent protection, even with agents that block the renin-angiotensin system (RAS). Recent clinical data also suggest that achieving lower BP targets might be vastly more important than the choice of therapeutic regimens. Nevertheless, because aggressive diuretic use is usually necessary to achieve such BP goals, RAS blockade should be included as initial therapy both for antihypertensive synergy and to minimize the potassium and magnesium depletion associated with diuretics.

Angiotensin receptor blockers in diabetic nephropathy: renal and cardiovascular end points

The activity of the renin-angiotensin-aldosterone system (RAAS) is elevated both in the circulation and in the renal tissue of diabetic and nondiabetic nephropathies. The increased RAAS activity plays an important role in the hemodynamic and nonhemodynamic pathogenetic mechanisms involved in kidney disease. Previous studies have demonstrated that albuminuria is not only a marker of glomerular lesions, but also a progression promoter, and finally a powerful predictor of the long-term beneficial effect of blood pressure-lowering therapy. Randomized crossover and parallel blind studies in patients with diabetic nephropathy have demonstrated that angiotensin II receptor blockers (ARB) induce favorable changes in systemic blood pressure, renal hemodynamics, and proteinuria similar to those induced by angiotensin-converting enzyme (ACE) inhibition. Studies have revealed the optimal renoprotective dose for some ARBs; however, additional dose titration studies are urgently needed to obtain the maximum benefit of this valuable new class of compounds. The combination of ARB and ACE inhibition is well tolerated and even more effective than monotherapy in reducing systemic blood pressure and albuminuria in diabetic nephropathy. In addition, dual RAAS blockade is safe and well tolerated. Impaired autoregulation of glomerular filtration rate (GFR); demonstrated with some blood pressure-lowering agents implies disturbances in the downstream transmission of the systemic blood pressure into the glomerulus, leading to capillary hypertension or hypotension depending of the level of blood pressure. ARB does not interfere with GFR autoregulation in hypertensive diabetic patients. In contrast to previous observational studies with ACE inhibition, long-term treatment with ARB has similar beneficial renoprotective effect on progression of diabetic kidney disease in hypertensive diabetic patients with ACE II and DD genotypes. ARB can prevent/delay development of diabetic nephropathy independently of its beneficial blood pressure-lowering effect in patients with type 2 diabetes and microalbuminuria. Recently, two landmark studies led to the following conclusion: "Losartan and Irbesartan conferred significant renal benefit in patients with type 2 diabetes and nephropathy. This protection is independent of the reduction in blood pressure it causes. The ARB is generally safe and well tolerated." A recent metaanalysis indicates that ARBs reduce cardiovascular events mainly because of reduction in first hospitalization for congestive heart failure in hypertensive type 2 diabetic patients with albuminuria. The studies mentioned here suggest that ARB represents a beneficial treatment of hypertension and proteinuria in incipient and overt diabetic nephropathy.

Update on calcium antagonists and the kidney

PURPOSE OF REVIEW

The treatment of hypertension has been proven to reduce cardiovascular and renal risk. The role of long-acting calcium channel antagonists in the management of hypertension has been confused in the past because of a lack of controlled clinical trials on people with hypertension and in subpopulations including those with diabetes and renal disease. The year 2002 saw the publication of the results of the Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial, the largest ever prospective drug-treatment trial, which involved 33 357 people with hypertension and included a calcium-antagonist group of 9048 individuals. Major publications on blood pressure control in people with kidney disease include the African American Study of Kidney Disease and Hypertension, and publications on people with diabetes include the results of the normotensive arm of the Appropriate Blood Pressure Control in Diabetes trial.

RECENT FINDINGS

The main finding, from the studies reported in the last year, is that blood pressure control can be achieved using one or more of the first-line agents, including diuretics, calcium antagonists and angiotensin-converting enzyme inhibitors. On the basis of the Antihypertensive and Lipid-Lowering Treatment to Prevent Heart Attack Trial, diuretics make clinical and economic sense as initial therapy for those with hypertension. Calcium antagonists are well tolerated and effective and should be considered as the initial drug therapy when diuretics are not tolerated or when multiple drug therapy is indicated. Angiotensin-converting enzyme inhibitors should be used in people with nephropathy, and, in these patients, will nearly always need to be part of multiple drug therapy to achieve blood pressure control. When blood pressure control can be achieved in largely non-nephropathic populations, there is further evidence that the drug class used as initial therapy may not be important. One of the main themes coming from the literature in the last year is that renal function is increasingly being recognized as an important outcome measure and marker of cardiovascular risk.

SUMMARY

The focus in blood pressure management must now be on identifying those with hypertension and bringing their blood pressure to target. For the majority of those with hypertension and renal disease, multiple drug therapy will be required, and, to achieve blood pressure targets, calcium antagonists are an appropriate part of this regimen. Particular attention is needed for nephropathic patients because of their higher risk of progression and the need for combination therapy; this group is likely to be the focus of future research and publications.