Philip R. Griffiths, Stephen J. Lolait, Louise E. Pearce, Fiona D. McBryde, Julian F.R. Paton, Anne-Marie O’Carroll. Frontiers in Physiology (October 2018)
Apelin is a neuropeptide found in many organs, including the brain. It is thought to regulate the intake of food, water, and the release of vasopressin. Previous studies have shown that apelin receptors play a role in heart failure, hypertension, and heart diseases. Apelin has been shown to be increased in the Rostral Ventrolateral Medulla (RVLM) in hypertensive rats, which is the main control center for sympathetic activity. This study focuses on if the apelin receptor plays a role in the progression of hypertension in two different models and if it is also part of the development stage of hypertension.
Thirty-six Wistar Kyoto (WKY) rats, sixteen spontaneous hypertensive rats (SHRs), and twenty younger SHRs were used in these studies. To generate the L-NAME model of hypertension, six WKY rats were given a daily dose of a nitric oxide synthase inhibitor, which increased sympathoexcitation and leads to hypertension. Lentiviruses with an apelin receptor gene knockout were created and injected into the RVLM. Control lentiviruses without the knockout were also created and injected directly into the RVLM. Injections of an apelin receptor agonist were used to test the knockout of the gene and then glutamate injections confirmed RVLM location.
Researchers first looked at the micropunches of the RVLM and found that the protein levels of apelin and the receptor gene were significantly higher in SHRs, when compared to the control rats. When observing cardiovascular effects, researchers started by measuring the baseline blood pressure and observed it was significantly higher in the SHRs. Microinjections of apelin-13 were then given directly into the RVLM, which exhibited a significant increase in MABP and systolic blood pressure in both SHRs and the controls. When rats were injected with a saline solution in the place of the apelin-13, no increases were observed. The increases in blood pressure were much greater in the SHRs compared to the controls. When an apelin receptor antagonist was injected, only the increases from the apelin-13 cancelled out, but no further decrease occurred. Researchers then used a lentivirus vector to knockout the apelin receptor. The lentivirus delivery was confirmed using immunofluorescence and observing the expression within the RVLM 25 days after they were injected. The lentivirus was shown to decrease expression of the receptor by about 65% at day 25. Apelin-13 was then injected into the knockout and control rats. The control lentivirus rats exhibited a significant increase in blood pressure, while the knockout rats had no increase. Using injections of glutamate to also confirm the pipette was in the correct place, similar blood pressure increases in the lentivirus knockout and the control rats were observed.
There were no significant differences in heart rate or blood pressure in the normotensive control rats given the lentivirus knockout or lentivirus control. Similarly, the blood pressure drops caused by an injection of hexamethonium exhibited no significant difference. The SHRs and the L-NAME treated rats exhibited higher blood pressures at the beginning of the experiments, which is expected with those models, though the respiration rate and the heart rate were similar to the normotensive rats. SHRs and the L-NAME treated models exhibited no significant difference in heart rate, blood pressure, hexamethonium- induced blood pressure drop, and body weight between the lentivirus knockout and lentivirus control rats. When the RVLM was injected with apelin-13 25 days after the lentivirus delivery, SHRs and L-NAME treated rats with the knockout exhibited a decrease in the apelin receptor gene and blood pressure, when compared to the rats treated with the lentivirus control.
Researchers then tested if a SHR was injected with apelin-13 when it was still young, would hypertension would develop? In SHRs that only received the lentivirus knockout or control, blood pressure remained very similar after 10 weeks. In SHRs that received the lentivirus and an injection of apelin-13, blood pressure was observed to slightly decrease in rats given the lentivirus knockout and increase in rats with the lentivirus control.
In conclusion, although the mechanisms are still not fully understood, it appears that the apelin receptor does not have a role in the development of high blood pressure or hypertension. These results also suggest that if the apelin receptor gene is knocked out in SHRs, the onset of hypertension is not slowed or prevented. Injections of the receptor agonist were shown to increase blood pressure and when the agonist and a receptor antagonist were given after one another, the agonist induced increases were eliminated. When just the antagonist was given to the SHRs, there was no change in blood pressure, which suggest these receptors do not play a role in the regulation of high blood pressure in hypertensive rat models.
-Paul M
