Effect of reactive oxygen species and nitric oxide in the neural control of intrarenal haemodynamics in anaesthetized normotensive rats

Neural tone and cardio-renal outcomes in patients with type 2 diabetes mellitus: a review of the literature with a focus on SGLT2 inhibitors

Recent clinical trials involving the systemic effects of sodium-glucose cotransporter 2 inhibitors (SGLT2i) have revealed beneficial outcomes pertaining to the microvascular sequelae of type 2 diabetes mellitus (T2DM) such as nephropathy, as well as macrovascular effects such as major adverse cardiovascular effects (MACE). Such findings have spurred the elevation of these agents to level A-tiers of recommendation within clinical guidelines addressing the management of complicated T2DM. While the mechanisms of SGLTi (-flozin drugs) are still being elucidated, a paucity of data exists within the literature appraising the role of neuromodulation and associated mechanisms in the aforementioned outcome studies. Given the role of the nervous system in orchestrating the pathologic processes that hamper cardio-renal status, insight into this topic offers an expanded perspective on T2DM. In this review we investigate the mechanisms by which SGLTi improve cardio-renal function in T2DM patients with emphases on neural tone and nervous system physiology.

Assessment of Perfusion and Oxygenation of the Human Renal Cortex and Medulla by Quantitative MRI during Handgrip Exercise

BACKGROUND

Renal flow abnormalities are believed to play a central role in the pathogenesis of nephropathy and in primary and secondary hypertension, but are difficult to measure in humans. Handgrip exercise is known to reduce renal arterial flow (RAF) by means of increased renal sympathetic nerve activity.

METHODS

To monitor medullary and cortical oxygenation under handgrip exercise-reduced perfusion, we used contrast- and radiation-free magnetic resonance imaging (MRI) to measure regional changes in renal perfusion and blood oxygenation in ten healthy normotensive individuals during handgrip exercise. We used phase-contrast MRI to measure RAF, arterial spin labeling to measure perfusion, and both changes in transverse relaxation time (T₂*) and dynamic blood oxygenation level-dependent imaging to measure blood oxygenation.

RESULTS

Handgrip exercise induced a significant decrease in RAF. In the renal medulla, this was accompanied by an increase of oxygenation (reflected by an increase in T₂*) despite a significant drop in medullary perfusion; the renal cortex showed a significant decrease in both perfusion and oxygenation. We also found a significant correlation (R²=0.8) between resting systolic BP and the decrease in RAF during handgrip exercise.

CONCLUSIONS

Renal MRI measurements in response to handgrip exercise were consistent with a sympathetically mediated decrease in RAF. In the renal medulla, oxygenation increased despite a reduction in perfusion, which we interpreted as the result of decreased GFR and a subsequently reduced reabsorptive workload. Our results further indicate that the renal flow response's sensitivity to sympathetic activation is correlated with resting BP, even within a normotensive range.

The Effect of Superoxide Dismutase Enzyme Inhibition on Renal Microcirculation of Spontaneously Hypertensive-Stroke Prone and Wistar Rats

A significant factor in the development of hypertension may be excessive vasoconstriction within the renal medulla. This study therefore investigated the role of superoxide dismutase (SOD) in the regulation of renal medullary and cortical blood perfusion (MBP and CBP, respectively) in both stroke-prone spontaneously hypertensive rats (SHRSP) and normotensive Wistar rats. CBP and MBP were measured before and after intra-renal infusion of the SOD inhibitor, diethyldithio-carbamic acid (DETC). Under basal conditions, mean arterial pressure was significantly greater in SHRSP than Wistar rats, but both MBP and heart rate (HR) were significantly lower in SHRSP relative to Wistar rats (P<0.05, n=7 in both groups). Infusion of DETC (2 mg/kg/min) into the cortico-medullary border area of the kidney significantly decreased MBP in the SHRSPs (by 28±3 %, n=7, P<0.05), indicating a greater vasoconstriction within this vascular bed. However, DETC also significantly decreased MBP in Wistar rats to a similar extent (24±4 %, n=7, P<0.05). These results suggest that superoxide anions play a significant role in reducing renal vascular compliance within the renal medulla in both normotensive and hypertensive animals, although the responses are not greater in the hypertensive relative to the control animals.

Effect of tempol and tempol plus catalase on intra-renal haemodynamics in spontaneously hypertensive stroke-prone (SHSP) and Wistar rats

Vasoconstriction within the renal medulla contributes to the development of hypertension. This study investigated the role of reactive oxygen species (ROS) in regulating renal medullary and cortical blood perfusion (MBP and CBP respectively) in both stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar rats. CBP and MBP were measured using a laser-Doppler flow meter before and after intra-renal infusion of tempol, the superoxide dismutase (SOD) mimetic or tempol plus catalase, the hydrogen peroxide-degrading enzyme. Tempol infusion significantly elevated blood perfusion within the renal medulla (MBP) in both SHRSP (by 43 ± 7%, P < 0.001) and Wistar rats (by 17 ± 2%, P < 0.05) but the magnitude of the increase was significantly greater in the SHRSP (P < 0.01). When the enzyme catalase and tempol were co-infused, MBP was again significantly increased in SHRSP (by 57 ± 6%, P < 0.001) and Wistar rats (by 33 ± 6%, P < 0.001), with a significantly greater increase in perfusion being induced in the SHRSP relative to the Wistar rats (P < 0.01). Notably, this increase was significantly greater than in those animals infused with tempol alone (P < 0.01). These results suggest that ROS plays a proportionally greater role in reducing renal vascular compliance, particularly within the renal medulla, in normotensive and hypertensive animals, with effects being greater in the hypertensive animals. This supports the hypothesis that SHRSP renal vasculature might be subjected to elevated level of oxidative stress relative to normotensive animals.

Intrarenal bradykinin elicits reno-renal reflex sympatho-excitation and renal nerve-dependent fluid retention

AIMS

The renal sensory nerves are importantly involved in the sympathetic regulation of cardiovascular and renal function. Two reno-renal reflexes are recognized, one in which activation of renal sensory nerves elicits a renal sympatho-inhibition, and one which causes a renal sympatho-excitation and about which little is known. This study investigated the role of bradykinin (BK) in engaging an excitatory reno-renal reflex.

METHODS

Rats were anaesthetized (chloralose/urethane) and prepared for the measurement of renal function or renal sympathetic nerve activity (RSNA). BK was infused into the cortico-medullary border of the ipsilateral kidney and the impact on contralateral renal function and RSNA evaluated.

RESULTS

Intrarenal infusion of BK at 3 × 10(-9) and 6 × 10(-9) g L(-1) had no effect on mean arterial pressure, at 104 ± 5 mmHg or glomerular filtration rate in either the ipsilateral or contralateral kidneys, at 4.31 ± 0.45 mL min(-1) kg(-1) . At the highest dose of BK, fractional sodium excretion (FENa) was 1.47% in the ipsilateral kidney and was significantly lower, at 0.64% (P < 0.05) in the contralateral kidney but this difference did not occur following ipsilateral renal denervation. Ipsilateral intrarenal infusion of BK at 3 × 10(-9) , 6 × 10(-9) and 1.2 × 10(-8) g L(-1) elicited dose-related increases (P < 0.05) in contralateral RSNA, reaching some 78% at the highest dose, but these responses were prevented by ipsilateral renal denervation.

CONCLUSIONS

Intrarenal infusion of BK produced an excitatory reno-renal reflex which was expressed as a renal nerve-dependent antinatriuresis in the contralateral kidney. The findings suggest that inflammatory mediators such as BK may be important in initiating a sympatho-excitation associated with renal and cardiovascular diseases.

How bold is blood oxygenation level-dependent (BOLD) magnetic resonance imaging of the kidney? Opportunities, challenges and future directions

Renal tissue hypoperfusion and hypoxia are key elements in the pathophysiology of acute kidney injury and its progression to chronic kidney disease. Yet, in vivo assessment of renal haemodynamics and tissue oxygenation remains a challenge. Many of the established approaches are invasive, hence not applicable in humans. Blood oxygenation level-dependent (BOLD) magnetic resonance imaging (MRI) offers an alternative. BOLD-MRI is non-invasive and indicative of renal tissue oxygenation. Nonetheless, recent (pre-) clinical studies revived the question as to how bold renal BOLD-MRI really is. This review aimed to deliver some answers. It is designed to inspire the renal physiology, nephrology and imaging communities to foster explorations into the assessment of renal oxygenation and haemodynamics by exploiting the powers of MRI. For this purpose, the specifics of renal oxygenation and perfusion are outlined. The fundamentals of BOLD-MRI are summarized. The link between tissue oxygenation and the oxygenation-sensitive MR biomarker T2∗ is outlined. The merits and limitations of renal BOLD-MRI in animal and human studies are surveyed together with their clinical implications. Explorations into detailing the relation between renal T2∗ and renal tissue partial pressure of oxygen (pO2 ) are discussed with a focus on factors confounding the T2∗ vs. tissue pO2 relation. Multi-modality in vivo approaches suitable for detailing the role of the confounding factors that govern T2∗ are considered. A schematic approach describing the link between renal perfusion, oxygenation, tissue compartments and renal T2∗ is proposed. Future directions of MRI assessment of renal oxygenation and perfusion are explored.

Endothelial cationic amino acid transporter-1 overexpression can prevent oxidative stress and increases in arterial pressure in response to superoxide dismutase inhibition in mice

AIM

Oxidative stress may play an important role in the pathogenesis of hypertension. The aim of our study is to examine whether increased expression of the predominant endothelial l-arginine transporter, cationic amino acid transporter-1 (CAT1), can prevent oxidative stress-induced hypertension.

METHODS

Wild-type mice (WT; n = 9) and endothelial CAT1 overexpressing (CAT+) mice (n = 6) had telemetry probes implanted for the measurement of mean arterial pressure (MAP), heart rate (HR) and locomotor activity. Minipumps were implanted for infusion of the superoxide dismutase inhibitor diethyldithiocarbamic acid (DETCA; 30 mg kg(-1) day(-1) ; 14 days) or its saline vehicle. Baseline levels of MAP, HR and locomotor activity were determined before and during chronic DETCA administration. Mice were then killed, and their plasma and kidneys collected for analysis of F2 -isoprostane levels.

RESULTS

Basal MAP was less in CAT+ (92 ± 2 mmHg; n = 6) than in WT (98 ± 2 mmHg; n = 9; P < 0.001). During DETCA infusion, MAP was increased in WT (by 4.2 ± 0.5%; P < 0.001) but not in CAT+, when compared to appropriate controls (PDETCA*genotype = 0.006). DETCA infusion increased total plasma F2 -isoprostane levels (by 67 ± 11%; P = 0.05) in WT but not in CAT+. Total renal F2 -isoprostane levels were greater during DETCA infusion in WT (by 72%; P < 0.001), but not in CAT+, compared to appropriate controls.

CONCLUSION

Augmented endothelial l-arginine transport attenuated the prohypertensive effects of systemic and renal oxidative stress, suggesting that manipulation of endothelial CAT1 may provide a new therapeutic approach for the treatment of cardiovascular disease associated with oxidative stress.