Improved Renal Denervation Mitigated Hypertension Induced by Angiotensin II Infusion

Summary

Here, we present a protocol for renal sympathetic denervation (RDN) in mice with hypertension induced by Angiotensin II infusion. The procedure is repeatable, convenient and allows to study the regulatory mechanisms of RDN on hypertension and cardiac hypertrophy.

Abstract

The benefits of renal sympathetic denervation (RDN) on blood pressure have been proved in a large number of clinical trials in recent years. However, the regulatory mechanism of RDN on hypertension remains elusive. Thus, it's essential to establish a simpler RDN model in mice. In this study, osmotic mini pumps filled with Angiotensin II were implanted in 14-week-old C57BL/6 mice. One week after the implantation of the mini-osmotic pump, a modified RDN procedure was performed on bilateral renal arteries of the mice using phenol. Age-sex-matched mice were given saline and served as sham group. Blood pressure was measured at baseline and every week subsequently for 21 days. Then, renal artery, abdominal aorta and heart were collected for histological examination using H&E and Masson staining. In this study, we present a simple, practical, repeatable, and standardized RDN model, which can control hypertension and alleviate cardiac hypertrophy. The technique can denervate peripheral renal sympathetic nerves without renal artery damage. Compared to previous models, the modified RDN facilitates the study of the pathobiology and pathophysiology of hypertension.

Introduction

Hypertension is a major chronic cardiovascular disease around the world. Uncontrolled hypertension could damage target organs and contribute to heart failure, stroke, and chronic kidney diseases^1,2,3. The prevalence of hypertension has increased from 20% to 31% between 1991 and 2007 in China. The number of adults with hypertension in China could double following a recent revision of the diagnostic criteria for hypertension (130/80 mmHg)⁴. Hypertension can be controlled by medicine, however, approximately 20% of patients are unable to control their hypertension, even when receiving at least three antihypertensive drugs (including one diuretic) at maximally tolerated dose, which may lead to the development of drug-resistant hypertension⁵.

Renal sympathetic denervation (RDN) has been proven to be a potential treatment for hypertension. In 2009, Krum and colleagues reported resistant hypertension treatment using RDN for the first time. It was found that percutaneous renal artery ablation can effectively cause persistent blood pressure reduction in patients⁶. However, the failure of the Symplicity Hypertension 3 (HTN-3) trial impeded the application of RDN⁷, turning RDN into a controversial therapy. Nevertheless, the prospect of RDN have not yet been ruled out. Recent clinical trials, including RADIANCE-HTN SOLO, SPYRAL HTN-OFF MED/ON MED, and SPYRAL HTN-OFF MED Pivotal have confirmed the efficacy of RDN on hypertension^8,9,10,11,12. Thus, more detailed mechanistic research needs to be performed to explore the effects of RDN.

The overall purpose of this study is to demonstrate how RDN in mice can be modified to produce a simpler and more stable surgery. A large number of experiments have studied various approaches of RDN, such as intravascular cryoablation, extracorporeal ultrasound and local application of a chemical or neurotoxin in different animal models^{13,14,15,16,17}. The RDN model generated using chemical ablation with phenol is a well-established experimental model to study the pathogenesis of sympathetic activation on hypertension. This model is generated by chemical corrosion of the renal sympathetic nerves with 10% phenol/ethanol solution using a cotton swab¹⁸. On one hand, the conventional RDN potentially inhibit renal sympathetic activity, which then decreases renin secretion and sodium reabsorption, and increases renal blood flow. On the other hand, it suppresses renin-angiotensin-aldosterone system¹⁹. Thereby, RDN has a beneficial effect on hypertension. However, the chemical ablation generated RDN model lacks ablation criteria and ablation time and the details of the experimental procedure are yet unclear. Also, there are no technical reports available. In this report, we describe a surgical protocol for the generation of RDN model with phenol using weigh paper in Angiotensin II (Ang II) induced hypertension in C57BL/6 mice. We wrap the renal artery with weighing paper containing phenol and unify the ablation time, which helps to establish a more reproducible, reliable RDN model. This experimental model is aimed to evaluate the effect of RDN on hypertension.

Protocol

All animal experimental procedures complied with the relevant ethical Guide for the Care and Use of Laboratory Animals (NIH Publication no. 85-23, revised 2011) and were approved by the committees on animal research of Huadong Hospital affiliated to Fudan University. Fourteen-week-old male C57BL/6 mice (28-30g) were randomly divided into four groups: Sham group, Sham+Ang II group, RDN group, RDN+Ang II group, n = 6 in each group. All animals were maintained under specific closed pathogen-free conditions in a temperature-controlled room at 24 ± 1 °C with a 12h light/dark cycle and free access to standard rodent chow and water ad libitum.

1. Preparation of the operation field

1. Disinfect the operation table with 70% ethanol. Adjust the heating pad temperature to 37 °C.
2. Ensure that all surgical instruments are sterilized before surgery at 121 °C for 30 min or by other methods. This procedure requires micro surgical scissors, two fine straight forceps, two fine curved forceps, hemostatic forceps, sterile gauzes, and weighing papers.

2. Angiotensin II induced hypertension

1. Provide meloxicam (0.5 mg/kg, SC) to the C57BL/6 mice shortly before anesthesia induction. Then, anesthetize the mice using sodium pentobarbital injection as previously described^20,21. Isoflurane can also be used, if preferred. Confirm anesthesia depth with a negative toe pinch reflex.
2. Remove the hair on the back with a shaver. Apply vet ointment to the eyes to prevent dryness while under anesthesia.
3. Place the animal on an operating table in the dorsal position. Swab and wipe the shaved area with povidone-iodine followed by three wipes with 70% ethanol.
4. Make a 1 cm incision using a sterile scalpel blade, perpendicular to the tail, behind the ear over the shoulder blade of the front leg.
5. Use a sterile hemostat to make a subcutaneous tunnel under the skin and create a pocket for the pump²². Insert an osmotic pump filled with Angiotensin II (1,000 ng/kg/min) into the pocket gently. Ensure that there is enough free space to suture the wound without stretching the skin.
6. Suture the muscle with interrupted 6-0 Vicryl sutures and close the skin with interrupted 4-0 nylon sutures. Swab and wipe the wound site with povidone-iodine. Perform the same surgery with equal volume of saline for the control group.
7. Place all the surgical instruments into a sterilizer for 10 s and replace the sterile gloves between surgeries. Monitor all mice until fully recovered.
8. Closely monitor and observe wound healing in mice at least twice a day during the first week and once every day subsequently, including redness, swelling, and infection. Perform dissection immediately if the mice die during Ang II infusion.
9. Measure blood pressure at baseline and every week after Ang II infusion with the tail-cuff plethysmography method²³ in conscious mice. Ensure that the blood pressure measurement experiments are conducted in a quiet area, at 22 ± 2 °C, where mice are acclimatized for 1 h before the experiment begins. Habituate mice for at least 5 consecutive days before baseline blood pressure measurements^23,24.

3. Bilateral renal denervation

1. Select the mice with elevated blood pressure (BP) ≥140/90mmHg or 25% increase in systolic BP/diastolic BP, 1 week after the Ang II infusion.
2. Record animal weight before surgery and choose animals with a minimum weight of 24 g for renal denervation surgery.
3. Anesthetize the mice using sodium pentobarbital. Confirm anesthesia depth with a negative toe pinch reflex.
4. Remove the hair on the abdomen with a shaver. Perform this procedure carefully and thoroughly to avoid any surgical contamination.
5. Place the mice on the operating table, keeping the abdomen up and fix its limbs with tape. Disinfect abdominal skin with povidone-iodine followed by three wipes with 70% ethanol.
6. Make a 2 cm ventral midline abdominal incision using a scalpel blade. Pull back the intestine with gauze soaked in 37 °C saline to expose the left renal artery. Carefully yet bluntly dissect the fat away from the renal artery using curved tweezers. (Figure 1A-C).
7. Cut the weighing paper into a rectangle of the same size as the renal artery with sterile sharp scissors. For reference, cut the weighing paper in the same size as shown by the dotted line in Figure 1C.
   NOTE: It is a critical part of the surgery, try to cut several pieces of the weighing paper at a time to keep the same shape.
8. Dip the weighing paper in 10% phenol/ethanol solution for at least 30 s. Cover the surface of the left renal artery and wrap the vessel with the weighing paper, keep for 2 min (Figure 1D). Use gauze to protect the surrounding tissues to avoid the weighing paper from touching the surrounding kidney and intestine.
   ​NOTE: The phenol solution is stable in plastic tubes but not in glass vials. Therefore, the solution must be freshly prepared for every experiment¹⁸.
9. Perform the same procedure for the right renal artery. Perform the sham surgery with weighing paper immersed in saline.
10. Reposition muscles into their initial position and close the peritoneum with 6-0 Vicryl sutures in an interrupted suture. Then, close the skin with interrupted 4-0 nylon sutures. Monitor all mice until fully recovered.

4. Post-operative care

1. Apply povidone-iodine to the incision and place the animal in a warmed electric blanket for recovery and post-operative monitoring.
2. Monitor the mice twice a day to assess for redness, swelling, and pain or abdominal infection. Provide meloxicam (0.5 mg/kg, SC) to all mice about 1 h before and 24 h after the RDN procedure.

Results

Statistics
All data are expressed as mean ± standard deviation. One-way ANOVA was used for experiments with three or more conditions followed by Bonferroni posthoc tests for comparisons between individual groups. Consider a p-value equal or less than 0.05 as significant. A commercial software was used to perform all statistical analysis.

Increase in blood pressure induced by Ang II was attenuated after RDN
Significant increase in systolic BP (SBP) was ...

Discussion

Whether RDN could lower blood pressure has become controversial since the publication of the negative result of the symplicity HTN-3 trial^7,25. However, the several clinical trials and animal experiments have demonstrated positive and effective results of RDN on hypertensive humans and animals^{9,10,11,12,13}

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Angiotensin II	Sangon Biotech	CAS:4474-91-3	To make a hypertensive animol model
Anti-Tyrosine Hydroxylase antibody	Abcam	ab137869	To evaluate the expression of TH of renal nerves
Blood Pressure Analysis	Visitech Systems	BP-2000	Measure the blood pressure of mice
Mini-osmotic pump	DURECT Corporation	CA 95014	To fill with Angiotensin II
Norepinephrine ELISA Kit	Abcam	ab287789	to measure renal norepinephrine levels
Phenol	Sangon Biotech	CAS:108-95-2	Damage the renal sympathetic nerve
Weighing paper	Sangon Biotech	F512112	To destroy renal nerve with weighing paper immersed with phenol; https://www.sangon.com/productDetail?productInfo.code=F512112.