Zhiqiang Hu, Zhilin Wu, Jie Gao, Qi Jia, Na Li, Yeling Ouyang, Shanglong Yao, Xiangdong Chen.
Molecular Pharmacology (2018)
Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are intermembrane proteins that act as voltage gated channels expressed throughout the heart and central nervous system. HCN channels have been shown to be inhibited by the anesthetic Propofol. Propofol is a general anesthetic that has been shown to have side effects such as bradycardia and hypotension. Previous research has suggested that Propofol inhibits vasomotor pressor neurons in the rostral ventrolateral medulla (RVLM), which could lead to the side effects. In this study, researchers tested how HCN1 and HCN2 channels in the RVLM play a role in mediating the effects of Propofol.
Mice were kept under a 12 hour light/dark cycle and had free access to food and water. Mouse blood pressure was measured by a tail cuff while they were awake. All infusions of Propofol were given through the tail vein using a catheter. For microinjection trials, mice were anesthetized using sodium pentobarbital and warming pads were used to maintain body temperature. To access the RVLM, the two occipital bones were removed and then microinjections were given with multibarrelled micropipettes. Quantitative real-time polymerase chain reaction (qRT-PCR) was used to test protein expression levels in different aged mice
While mice that were not under anesthetic, baseline blood pressure was not different between wild type and HCN1 KO mice, but heart rate in the HCN KO mice was significantly lower than the wild type. When Propofol was given continuously for 10 minutes, blood pressure exhibited a significantly larger decreased in wild type mice when compared to HCN KO mice. Heart rate was also shown to decrease more in the wild type mice during the Propofol infusions. Researchers then observed responses to RVLM microinjections of both Propofol and ZD-7288, the HCN channel blocker. Blood pressure and heart rate was shown to significantly decrease after the microinjections. When the RVLM was injected with ZD-7288 shortly before Propofol, the decrease in blood pressure and heart rate was not as severe as a solo Propofol injection. The inhibitory effects of HCN channels were then tested using voltage and current clamps. Propofol was shown to inhibit HCN channel current and cause an increase in resistance in the RVLM neurons, but had no effect on the resistance in the RVLM neurons after ZD-7288 eliminated the current. ZD-7288 was shown to decrease the firing rate of RVLM neurons. Researchers then used Real-time PCR and western blotting to see if aging has any effect on HCN1 and HCN2 protein channels in RVLM neurons. HCN1 expression was shown to significantly increase from 2-3 weeks of age to 8-10 weeks of age and then again from 8-10 weeks to 40-60 weeks of age. HCN2 expression was only shown to significantly increase from 2-3 weeks old age to 40-60 weeks.
In conclusion, blood pressure and heart rate were not different in wild type and HCN KO mice, but once Propofol was given, heart rate and blood pressure showed a larger decrease in wild type mice. ZD-7288 given before Propofol was shown to attenuate the decrease in blood pressure and heart rate. Propofol reduced the firing rate and caused hyperpolarization, but after ZD-7288 was given before Propofol, the firing rate was reduced and hyperpolarization did not occur. Lastly, HCN channel protein expression was shown to increase as age increases. The researchers discuss that these results could lead to the creation of new anesthetics that have fewer side effects on the cardiovascular system. Since most drugs have a long list of side effects, more research must be done to determine how these drugs are effecting the peripheral parts of the body and not just the singular location that the drug in designed to act on.
-Paul M
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