Miralles A, Olmos G, Sastre M, Barturen F, Martin I, Garcia-Sevilla J A
Department of Fundamental Biology and Health Sciences, University of the Balearic Islands, Spain.
J Pharmacol Exp Ther. 1993 Mar;264(3):1187-97.
The alpha-2 adrenoceptor antagonist idazoxan has been shown to also recognize with high affinity nonadrenoceptor sites (I2-imidazoline sites). In contrast, the 2-methoxy derivative of idazoxan, 2-methoxy idazoxan (RX821002), binds almost exclusively to alpha-2 adrenoceptors. The purpose of this study was to assess and extend the pharmacological characterization of I2-imidazoline sites and alpha-2 adrenoceptors in the human and rat brains. Competition studies with several imidazoli(di)ne/guanidine drugs and other nonrelated structures were performed in cortical membranes against [3H]idazoxan (4 nM in the presence of 10(-6) M I-epinephrine to prevent binding to alpha-2 adrenoceptors) or [3H]RX821002 (1 nM). Drugs such as cirazoline, guanoxan, naphazoline, tolazoline, clonidine, bromoxidine (UK 14,304) and phenylbiguanide displaced [3H]idazoxan from two distinct binding sites, which suggested the existence of two affinity states for I2-imidazoline sites that were not modulated by MgCl2 or the nucleotide analog guanylyl-5'-imido-diphosphate. Binding affinities at the low-affinity site (KiL) were consistently more than 2 orders of magnitude lower than binding affinities at the high-affinity site (KiH), and there was a good correlation between KiH and KiL values for a given drug in the human (r = 0.89) and rat (r = 0.92) brains. For 18 to 22 drugs, the Ki values in the human brain correlated well with the corresponding Ki values in the rat brain both for I2-imidazoline sites (r = 0.94) and alpha-2 adrenoceptors (r = 0.97). However, the Ki values for I2-imidazoline sites did not correlate with the Ki values for alpha-2 adrenoceptors in human and rat brains. The order of drug potency for the I2-imidazoline sites was: guanoxan (1.3 nM) approximately cirazoline > idazoxan approximately naphazoline > clonidine > phentolamine > RX821002 > (8aR, 12aS, 13aS)-3-methoxy-12-methanesulfonyl-5,6,8a,9,10,11,12,12a,13,13a- decahydro-8H-isoquino[2,1-g]-naphthyridine (RS 15385-197) (> 10 microM). In contrast, the potencies at the alpha-2 adrenoceptor were: RS 1538-197 (0.3 nM) > RX821002 > clonidine > phentolamine > idazoxan approximately naphazoline > guanoxan approximately cirazoline (307 nM). The results demonstrate that I2-imidazoline sites (labeled by [3H]idazoxan) and alpha-2 adrenoceptors are different pharmacological entities with similar characteristics in the human and rat brains. In both species, I2-imidazoline sites are markedly heterogeneous in nature.
α2肾上腺素能受体拮抗剂咪唑克生已被证明还能以高亲和力识别非肾上腺素能受体位点（I2-咪唑啉位点）。相比之下，咪唑克生的2-甲氧基衍生物2-甲氧基咪唑克生（RX821002）几乎只与α2肾上腺素能受体结合。本研究的目的是评估并扩展人及大鼠脑中I2-咪唑啉位点和α2肾上腺素能受体的药理学特征。使用几种咪唑啉/胍类药物和其他不相关结构在皮质膜中进行竞争研究，以对抗[3H]咪唑克生（在10^(-6) M I-肾上腺素存在下为4 nM，以防止与α2肾上腺素能受体结合）或[3H]RX821002（1 nM）。西拉唑啉、胍那佐、萘甲唑啉、妥拉唑啉、可乐定、溴莫尼定（UK 14,304）和苯乙双胍等药物从两个不同的结合位点取代了[3H]咪唑克生，这表明I2-咪唑啉位点存在两种亲和力状态，且不受MgCl2或核苷酸类似物鸟苷-5'-亚氨基二磷酸调节。低亲和力位点（KiL）的结合亲和力始终比高亲和力位点（KiH）的结合亲和力低2个以上数量级，并且对于人（r = 0.89）和大鼠（r = 0.92）脑中的给定药物，KiH和KiL值之间存在良好的相关性。对于18至22种药物，人脑中的Ki值与大鼠脑中I2-咪唑啉位点（r = 0.94）和α2肾上腺素能受体（r = 0.97）的相应Ki值均具有良好的相关性。然而，人及大鼠脑中I2-咪唑啉位点的Ki值与α2肾上腺素能受体的Ki值不相关。I2-咪唑啉位点的药物效价顺序为：胍那佐（1.3 nM）≈西拉唑啉＞咪唑克生≈萘甲唑啉＞可乐定＞酚妥拉明＞RX821002＞(8aR, 12aS, 13aS)-3-甲氧基-12-甲磺酰基-5,6,8a,9,10,11,12,12a,13,13a-十氢-8H-异喹啉并[2,1-g]-萘啶（RS 15385-197）（＞10 μM）。相比之下，α2肾上腺素能受体的效价为：RS 1538-197（0.3 nM）＞RX821002＞可乐定＞酚妥拉明＞咪唑克生≈萘甲唑啉＞胍那佐≈西拉唑啉（307 nM）。结果表明，I2-咪唑啉位点（由[3H]咪唑克生标记）和α2肾上腺素能受体是不同的药理学实体，在人及大鼠脑中具有相似的特征。在这两个物种中，I2-咪唑啉位点本质上明显具有异质性。
