Hypertension favors the endothelial non-neuronal cholinergic system

Abstract

This thesis investigates the involvement of the non-neuronal cholinergic system in endothelium-dependent relaxations and the impact of hypertension on the function of this system. In Study1 the contribution of nicotinic receptors (nAChRs) to endothelium-dependent relaxations evoked by acetylcholine was examined. Both muscarinic (mAChRs) and nAChR were expressed in the aortic endothelium of spontaneously hypertensive (SHR)and Wistar-Kyoto rats (WKY). However, isometric tension measurements showed that, the muscarinic antagonist atropine abolished the relaxations to acetylcholine in WKY aortae, but only partially inhibited those in SHR aortae. While the nicotinic antagonist mecamylamine inhibited the remaining response in SHR aortae, it did not significantly affect the response solely in either SHR or WKY preparations. Hence, nAChRs mediate endothelium-dependent relaxations to the acetylcholine only in the SHR aorta and only when mAChRs are inhibited. Nicotine, the prototypical nicotinic agonist, also induced endothelium-dependent relaxations in both SHR and WKY aortae which were due to activation of α7-nAChRsbut not by mecamylamine-sensitive α3-nAChR. The acetylcholine-induced, atropine-insensitive relaxations and that to nicotine both involve the PI3K pathway. Thus, activation of nAChRscan contribute to acetylcholine-induced endothelium-dependent relaxations via PI3K signaling pathway in aortae of hypertensive animals.

Study 2 examined the involvement of non-neuronal cholinergic system in endothelium-dependent relaxations. Isometric tension measurements showed that mild hypothermia (37℃–31℃) induced endothelium-dependent relaxations, which were reduced by atropine, tubocurarine, acetylcholinesterase (enzyme responsible for acetylcholine degradation), bromoacetylcholine (inhibitor of acetylcholine synthesis), hemicholinium-3 (inhibitor of choline uptake) and vesamicol (inhibitor of acetylcholine release) in SHR but not in WKY aortae, indicating that the non-neuronal cholinergic system is involved in mild hypothermia-induced endothelium-dependent relaxations. Compared with WKY, SHR preparations expressed similar levels of acetylcholinesterase and choline acetyltransferase, but lesser vesicular acetylcholine transporter, located mainly in the endothelium. A choline/acetylcholine assay showed that, mild hypothermia increased the uptake of choline by the endothelium of SHR,but not WKY, aortae from extracellular environment for acetylcholine production. To define possible different mechanisms employed by SHR and WKY endothelial cells, the involvement of transient receptor potential (TRP)channels in mild hypothermia-induced response were examined using selective pharmacological

inhibitors of different subtypes of TRP channels, namelyAMTB (TRPM8 antagonist),HC-030031 (TRPA1 antagonist)and HC-067047 (TRPV4 antagonist).The results suggest that both TRPM8 and TRPA1 play a role in the response to mild hypothermia in the WKY aorta; however, in the SHR aortaTRPV4,but not TRPA1, channels are activated by mild hypothermia. Moreover, the observation that the mild hypothermia-induced increases in cyclic guanosine monophosphate (cyclic GMP)and choline uptake were inhibited by HC-030031 in WKY but by HC-067047 in SHR aortae further indicate that in the hypertensive strain compensatory TRPV4 activation can make up for the loss of TRPA1-mediated NO production, and that the endothelial cells of the hypertensive animal utilize TRPV4 channels to activate the production of endogenous acetylcholine in response to mild hypothermia. Taken in conjunction, the results reported in this thesis together suggest that hypertension alters the function of the non-neuronal cholinergic system (e.g. n-AChR sensitivity or acetylcholine production) to modulate endothelium-dependent relaxations.