Molecular Mechanism Of Selective Antihypertensive Effect Of ATP-sensitive Potassium Channel Opener Iptakalim

Hypertension is a common and severe disease that damage human health, thereare more than1billion hypertensive patients need lifelong medication in the world[1].At present, hypertension has become the primary risk factors for cardiovasculardisease, the prevention and control situation is very grim[2,3]. Hypertension is mainlycaused by the increase in total vascular peripheral resistance[4]. The excessivecontraction of resistance vessels including small arteries and arterioles, leads to theincreasing pressure, which is the basic pathophysiological development andmechanism of hypertension[5]. Therefore, highly selective regulation of resistancevessels is fundamental to the treatment of hypertension[6]. Unfortunately, nearly300kinds of hypertension drugs didn’t show highly selective expansion of resistancesmall arteries and arterioles. Vasodilators as an important class of anti-hypertensivedrugs, such as calcium antagonists, are widely used in the treatment of hypertension.However, they inevitable dilate the aorta and capacity vessels while expand resistancevessels, causing side effects such as rapid heartbeat and water retention of urine[7].Therefore, it is important to find untihypertensive drugs that highly selective dilateresistance vessels.The activation of adenosine triphosphate （ATP）-sensitive potassium channels（KATP） is recognized as an important therapeutic approach to modulating the tone ofthe resistance vasculature. Iptakalim, a recently developed KATPchannel opener with aunique chemical structure[8], has been shown to exert a potent and long-lastingantihypertensive action in animals[10]. Our preclinical pharmacological studies havealso demonstrated that iptakalim protects target organs against hypertensive damage[9],reverses insulin resistance[11], and improves the endothelial dysfunction[12]. However,the exact mechanisms underlying the antihypertensive action of iptakalim are still notfully understood, especially its role in the increasing tension on resistance arteriolesand its effects on different subtypes of KATPchannel[13]. Therefore, the purpose of thispaper is to study the dilative characteristics of Ipt on resistance vessels and to use Iptas a represent for SUR2B/Kir6.1type KATPopenner, for its vasodilatation molecularmechanism[14,15].Our data suggest that the size of mesenteric arterioles correlate with theirsensitivities to iptakalim, with smaller arteriolar diameter having a stronger response. Given the fact that the smaller caliber of arteriole provides a greater contribution tototal peripheral resistance[21], the selective arteriolar dilatation of iptakalim may be theprimary cause to lower blood pressure. Indeed, iptakalim induced increasingly strongvasodilatation with the gradually raising perfusion pressure in arterioles but did notinfluence vascular tone under the physiological perfusion pressure in pressurized rats.These findings were consistent with that iptakalim afforded potently antihypertensiveeffects in patients with essential hypertension but did not affect the level of bloodpressure in normotensive subjects, indicating that the regulation of iptakalim on bloodpressure is hypertensive status-dependent. It has been reported that expression of KATPchannel in vascular endothelial cells or smooth muscle cells are increased by shearstress[24]. Furthermore, the vascular reactivity and KATPfunction are also altered underthe condition of hypertension[25]. These pathological changes may be relative to theselectivity of iptakalim on hypertensive status.By comparing the vasodilatation of iptakalim on endothelium intact and denudedarteriole, we found that endothelium denuded arteriole were partially resistant toiptakalim-induced dilatation, showing an approximate70%deficit in their maximaldilatory response which occurred at the highest dose of iptakalim examined,indicating that vasodilatation induced by iptakalim is endothelium-dependent. PGI₂,NO and EDHF are three key relaxing factor participated in mediatingendothelium-dependent vasodilatation. The activation of endothelial IKCaand SKCaisrequired for EDHF-mediated smooth muscle cell hyperpolarization as well asCa2+-dependent activation of endothelial NO generation[28]. In endothelium intactarterioles, besides I/SKCainhibitors CTX plus apamin and eNOS inhibitor L-NAMEboth significantly attenuated iptakalim-induced vasodilatation respectively, but not byspecific PGI₂blocker indomethacin, implying that endothelial EDHF and NO areinvolved in vasodilatation to iptakalim.Vascular wall express at least4different types of K⁺channels including KATPchannels, voltage-activated K⁺（KV） channels, inward rectifier K⁺（KIR） channels, andthree subtypes of calcium-activated potassium channels （KCa） of large, intermediateand small conductance （BKCa, IKCaand SKCa）, and all of which may be involved inthe regulation of vascular tone[16]. In healthy arteries, BKCachannels are preferentiallyexpressed in vascular smooth muscle cells, while IKCaand SKCaare preferentiallylocated in endothelial cells[27]. We showed in the present study that only specific membrane KATPchannels blocker glibenclamide completely abrogated vasodilatationto iptakalim in endothelium denuded arteriole, suggesting that iptakalim, at leastpartially, through directly activating KATPchannels in smooth muscle cells to producearteriolar dilatation. Together with these pharmacological inhibitions and thatiptakalim-mediated dilatation was almost disappeared in arterioles from Kir6.1-/-mice,we infers iptakalim through opening Kir6.1-type channel but not directly activatingKCachannels to produce endothelium-dependent vasodilatation，which is differentfrom acetycholine, braykinin and substance P.We found that endothelium-denuded arterioles showing an approximately70%deficit in their maximal dilatatory response, which occurred at the highest dose ofiptakalim examined. This indicates that the contribution of its direct action on smoothmuscles was about30%, and iptakalim-induced vasodilatation is mainly endotheliumdependent. K⁺as the only EDHF combined with NO as EDRF are involved in thisendothelial vasodilatation and their contribution is approximately40%and30%respectively. Therefore, we postulated a working hypothesis to explainiptakalim-mediated arteriolar dilatation. Iptakalim opens the SUR2B/Kir6.1-type KATPchannel, thereby permitting plasma membrane K⁺efflux. In part, K⁺causeshyperpolarization, closing voltage-dependent calcium channels （VDCC）, decreasingCa2+entry, and consequent vasodilatation. The other contributor is EDHF inendothelial cells, combined with NO, ultimately relaxing smooth muscles. Themolecular pathways of iptakalim-induced vasodilatation are different from those ofother antihypertensive agents.