Microvascular Rarefaction and Hypertension in the Impaired Recovery and Progression of Kidney Disease Following AKI in Preexisting CKD States

Acute kidney injury (AKI) is a major complication in hospitalized patients and is associated with elevated mortality rates. Numerous recent studies indicate that AKI also significantly increases the risk of chronic kidney disease (CKD), end-stage renal disease (ESRD), hypertension, cardiovascular disease, and mortality in those patients who survive AKI. Moreover, the risk of ESRD and mortality after AKI is substantially higher in patients with preexisting CKD. However, the underlying mechanisms by which AKI and CKD interact to promote ESRD remain poorly understood. The recently developed models that superimpose AKI on rodents with preexisting CKD have provided new insights into the pathogenic mechanisms mediating the deleterious interactions between AKI and CKD. These studies show that preexisting CKD impairs recovery from AKI and promotes the development of mechanisms of CKD progression. Specifically, preexisting CKD exacerbates microvascular rarefaction, failed tubular redifferentiation, disruption of cell cycle regulation, hypertension, and proteinuria after AKI. The purpose of this review is to discuss the potential mechanisms by which microvascular rarefaction and hypertension contribute to impaired recovery from AKI and the subsequent progression of renal disease in preexisting CKD states

Severity of Acute Kidney Injury in Mice Associated with Ischemia Duration and Gender

Acute kidney injury (AKI) is a major health burden associated with a 50% mortality rate. Of particular concern, the incidence of AKI has increased dramatically over the last decade. Yet, there is a paucity of available treatments to prevent AKI or to reduce the high rate of AKI-associated mortality. A common cause of AKI, especially in hospital settings, is prolonged decreases in renal blood flow (i.e., renal ischemia). Recent studies have demonstrated that activating the cholinergic anti-inflammatory pathway via vagal stimulation can mitigate AKI severity in rodent models of renal ischemia-reperfusion (IR) injury. While vagal stimulation is not a practical approach to prevent AKI in patients due its invasive nature and numerous side effects, recent studies have identified non-neuronal cholinergic cells within the kidney that could be targeted to reduce the severity of AKI. The overarching goal of this project is to examine the potential role of the renal cholinergic system in modulating the severity of and recovery from AKI in transgenic mice expressing green fluorescent protein (GFP) under control of the choline acetyl-transferase (ChAT) promoter, a protein involved in the synthesis of acetylcholine. The objectives of this study were to develop a clinically relevant model of renal IR-induced AKI in mice by identifying the duration of ischemia required for manifestation of the effects of AKI and to determine whether differences in susceptibility to AKI exists between male and female mice. Initially, male mice underwent 20 (n=3), 22 (n=3), or 25 (n=4) minutes of bilateral renal IR under isoflurane anesthesia with body temperature controlled at 37°C. Ischemia was achieved by careful placement of vascular clamps on the renal artery and vein supplying each kidney. The severity of AKI was determined by measuring serum creatinine (SCr) at 3 days post-AKI. Compared to SCr of mice that were 3 days post-sham AKI (SCr = 0.47 mg/dl, n=2), SCr of male mice from all three ischemia time categories was substantially elevated (SCr \u3e 3 mg/dl, n=10). However, mortality associated with 22 and 25 minutes IR was striking (\u3e90%) making studies of long-term AKI effects difficult. In contrast, 20 minutes IR resulted in AKI manifest by elevated SCr (3.43±0.7 mg/dl, n=3), widespread acute tubular necrosis and a clinically relevant mortality rate of 50%. Next, male (n=10) and female (n=5) mice were subjected to 20 minutes of IR. The mortality rate in male mice (n=10) was 50% (n=10) through 7 days post-AKI; however, all female mice survived. Additional studies showed that female mice had lower SCr 3 days post-AKI (0.63±0.1 mg/dl, n=2) with very modest levels of acute tubular necrosis as compared to the higher SCr (1.92±0.1 mg/dl, n=2) and extensive acute tubular necrosis observed in male mice. The differences observed in AKI severity and mortality rates suggest that female mice are protected against AKI as compared to male mice and future studies will explore the potential role of the renal cholinergic system in contributing to these sex differences in AKI

Autonomic and Cholinergic Mechanisms Mediating Cardiovascular and Temperature Effects of Donepezil in Conscious Mice

Donepezil is a centrally acting acetylcholinesterase (AChE) inhibitor with therapeutic potential in inflammatory diseases; however, the underlying autonomic and cholinergic mechanisms remain unclear. Here, we assessed effects of donepezil on mean arterial pressure (MAP), heart rate (HR), HR variability, and body temperature in conscious adult male C57BL/6 mice to investigate the autonomic pathways involved. Central versus peripheral cholinergic effects of donepezil were assessed using pharmacological approaches including comparison with the peripherally acting AChE inhibitor, neostigmine. Drug treatments included donepezil (2.5 or 5 mg/kg sc), neostigmine methyl sulfate (80 or 240 μg/kg ip), atropine sulfate (5 mg/kg ip), atropine methyl bromide (5 mg/ kg ip), or saline. Donepezil, at 2.5 and 5 mg/kg, decreased HR by 36 ± 4% and 44 ± 3% compared with saline (n = 10, P \u3c 0.001). Donepezil, at 2.5 and 5 mg/kg, decreased temperature by 13 ± 2% and 22 ± 2% compared with saline (n = 6, P \u3c 0.001). Modest (P \u3c 0.001) increases in MAP were observed with donepezil after peak bradycardia occurred. Atropine sulfate and atropine methyl bromide blocked bradycardic responses to donepezil, but only atropine sulfate attenuated hypothermia. The pressor response to donepezil was similar in mice coadministered atropine sulfate; however, coadministration of atropine methyl bromide potentiated the increase in MAP. Neostigmine did not alter HR or temperature, but did result in early increases in MAP. Despite the marked bradycardia, donepezil did not increase normalized high-frequency HR variability. We conclude that donepezil causes marked bradycardia and hypothermia in conscious mice via the activation of muscarinic receptors while concurrently increasing MAP via autonomic and cholinergic pathways that remain to be elucidated

Autoregulatory Efficiency Assessment in Kidneys Using Deep Learning

A convolutional deep neural network is employed to assess renal autoregulation using time series of arterial blood pressure and blood flow rate measurements in conscious rats. The network is trained using representative data samples from rats with intact autoregulation and rats whose autoregulation is impaired by the calcium channel blocker amlodipine. Network performance is evaluated using test data of the types used for training, but also with data from other models for autoregulatory impairment, including different calcium channel blockers and also renal mass reduction. The network is shown to provide effective classification for impairments from calcium channel blockers. However, the assessment of autoregulation when impaired by renal mass reduction was not as clear, evidencing a different signature in the hemodynamic data for that impairment model. When calcium channel blockers were given to those animals, however, the classification again was effective

Renal injury in angiotensin II+l-NAME-induced hypertensive rats is independent of elevated blood pressure

The balance between angiotensin II (ANG II) and nitric oxide plays an important role in renal function and is thought to contribute to the progression of renal injury in experimental hypertension. In the present study, we investigated the extent of blood pressure (BP)-dependent and BP-independent pathways of renal injury following 2 wk of hypertension produced by intravenous infusion of ANG II (5 ng·kg−1·min−1)+Nω-nitro-l-arginine methyl ester (l-NAME; 1.4 μg·kg−1·min−1) in male Sprague-Dawley rats. An aortic balloon occluder was positioned between the renal arteries to maintain (24 h/day) BP to the left kidney (servo-controlled) at baseline levels, whereas the right kidney (uncontrolled) was chronically exposed to elevated BP. Over the 14-day experimental protocol, the average BP to uncontrolled kidneys (152.7 ± 1.8 mmHg) was significantly elevated compared with servo-controlled (113.0 ± 0.2 mmHg) kidneys and kidneys from sham rats (108.3 ± 0.1 mmHg). ANG II+l-NAME infusion led to renal injury that was focal in nature and mainly confined to the outer medulla. Despite the differences in BP between servo-controlled and uncontrolled kidneys, there was a similar ∼3.5-fold increase in renal outer medullary tubular injury, ∼2-fold increase in outer medullary interstitial fibrosis, ∼2-fold increase in outer medullary macrophage infiltration, and a significant increase in renal oxidative stress, all of which are indicative of BP-independent mediated pathways. The results of this study have important implications regarding the pathogenesis of renal injury in various experimental models of hypertension and provide novel insights regarding the variable association observed between hypertension and renal injury in some human populations

Renal Mass Reduction Increases the Response to Exogenous Insulin Independent of Acid-Base Status or Plasma Insulin Levels in Rats

Impairments in insulin sensitivity can occur in patients with chronic kidney disease (CKD). Correction of metabolic acidosis has been associated with improved insulin sensitivity in CKD, suggesting that metabolic acidosis may directly promote insulin resistance. Despite this, the effect of acid or alkali loading on insulin sensitivity in a rodent model of CKD (remnant kidney) has not been directly investigated. Such studies could better define the relationship between blood pH and insulin sensitivity. We hypothesized that in remnant kidney rats, acid or alkali loading would promote loss of pH homeostasis and consequently decrease insulin sensitivity. To test this hypothesis, we determined the impact of alkali (2 wk) or acid (5-7 days) loading on plasma electrolytes, acid-base balance, and insulin sensitivity in either sham control rats, 2/3 nephrectomized rats, or 5/6 nephrectomized rats. Rats with 5/6 nephrectomy had the greatest response to insulin followed by rats with 2/3 nephrectomy and sham control rats. We found that treatment with 0.1 M sodium bicarbonate solution in drinking water had no effect on insulin sensitivity. Acid loading with 0.1 M ammonium chloride resulted in significant reductions in pH and plasma bicarbonate. However, acidosis did not significantly impair insulin sensitivity. Similar effects were observed in Zucker obese rats with 5/6 nephrectomy. The effect of renal mass reduction on insulin sensitivity could not be explained by reduced insulin clearance or increased plasma insulin levels. We found that renal mass reduction alone increases sensitivity to exogenous insulin in rats and that this is not acutely reversed by the development of acidosis

Active hyperemia and vascular conductance differ between men and women for an isometric fatiguing contraction

To understand the role of muscle perfusion in the sex differences of muscle fatigue, we compared the time to task failure, postcontraction (active) hyperemia, and vascular conductance for an isometric fatiguing contraction performed by young men and women with the handgrip muscles at 20% of maximal voluntary contraction (MVC) force. In study 1, the men (n = 16) were stronger than the women (n = 18), and study 2, the men (n = 7) and women (n = 7) were matched for strength. Isometric contractions were sustained during two sessions: 1) until the target force could no longer be achieved or 2) for 4 min. For both studies, blood flow and vascular conductance were similar for the men and women at rest and after 10 min of occlusion, and at task failure for the fatiguing contraction estimated using forearm venous occlusion plethysmography. In study 1, the time to task failure was longer for the women (11.4 ± 2.8 min) than for the men (8.4 ± 2.4 min; P = 0.003). However, at the end of the 4-min contraction, active hyperemia and vascular conductance were greater for the men than the women (99 vs. 70% peak blood flow; P \u3c 0.001). In study 2, the men and women had similar strength and a similar time to failure (8.4 ± 1.6 vs. 8.6 ± 2.3 min). Active hyperemia was greater for the men than the women (86 vs. 64% peak flow; P = 0.038) after the 4-min contraction, as was vascular conductance (80 vs. 57% peak conductance; P = 0.02). Thus the briefer time to failure of men than women for an isometric fatiguing contraction is a function of the greater strength of men but is not dependent on differences in the active hyperemia and vascular conductance