Angiotensin-Converting Enzyme 2 in the Rostral Ventrolateral Medulla Regulates Cholinergic Signaling and Cardiovascular and Sympathetic Responses in Hypertensive Rats

Yu Deng, Xing Tan, Miao-Ling Li, Wei-Zhong Wang, Yang-Kai Wang. Neurosci Bull. 2018

The rostral ventrolateral medulla (RVLM) is an area in the brainstem that regulates sympathetic nerve activity associated with cardiovascular mechanisms.  Angiotensin is a hormone that causes vasoconstriction which leads to high blood pressure. This study focuses on the angiotensin-converting enzyme 2 (ACE2) that converts angiotensin into a much less reactive form of angiotensin. Therefore, ACE2 can contribute to the lowering of blood pressure and help therapeutically with hypertensive patients. This study examines how hypertension is affected by high levels of ACE2 in the RVLM.

Higher concentrations of ACh were observed in the RVLM of spontaneously hypertensive rats (SHRs) when compared to wild type rats and VAChT, a vesicular ACh transporter, levels were also shown to be increased in SHRs compared to wild type. Atropine was then injected into the RVLM to show that ACh is acting as a major neurotransmitter. Before the atropine was injected, SHRs exhibited higher blood pressures and heart rates than the wild type rats. The atropine injection caused a significantly larger decrease in blood pressure and nerve activity in the SHRs. Both the wild type and SHRs exhibited a very similar decrease in heart rate.

To deliver the human ACE2 into mice models, ACE2 lentiviruses were created. A lentivirus is a type of retrovirus which inserts a piece of DNA into the host’s genome. A control lentivirus was also created that has all the same sequences, but left out the one coding for the human ACE2. The lentiviruses were then injected directly into the RVLM. The results of the lentivirus injection showed that the expression of ACE2 was significantly higher in the Lenti-ACE2 SHRs compared to the Lenti-control SHRs four weeks after the injection. The Lenti-ACE2 SHRs exhibited decreased blood pressure and heart rate three weeks after the injection. Urine samples were taken to test the levels of norepinephrine (NE), which showed that the SHRs given Lenti-ACE2 exhibited a 72% decrease in NE compared to the SNRs given Lenti-control.

Concentrations of ACh in the extracellular fluid of the RVLM were then tested using microdialysis. Four weeks after the injection, SNRs given Lenti-ACE2 had decreased concentrations of ACh and VAChT, the vesicular ACh transporter. Atropine was also injected into the SHRs that received Lenti-control or Lenti-ACE2. Blood pressure and nerve activity was shown to have a smaller decrease in the Lenti-ACE2 rats, while heart rate remained very similar between both groups. 

In conclusion, increased levels of ACE2 in the RVLM were shown to decrease hypertensive effects in rats. High levels of ACh and the a ACh transporter were found in the SHRs, but when the ACE2 lentivirus was injected, ACh levels decreased along with blood pressure and nerve activity. Urine tests showed that NE also decreased in the rats given the ACE2 lentivirus, which shows there is less excitation releasing the neurotransmitter NE. The authors discussed how there has not been significant evidence that there is any change in the muscarinic receptors during hypertension and more research needs to be done.

Paul M