用大鼠脑切片进行β—受体放射配体结合实验的方法学研究

本项研究探素了用氚标的β-受体拮抗剂~3H-双氢心得舒与大鼠中脑上丘平面冰冻切片进行结合实验的条件。实验得出配体与脑切片的最大特异性结合量为17.3±4.5 fmol/mg,组织干重,表观平衡解离常数为5.8±0.6nM,Hill系数为0.93±0.3,结合速率常数为0.112±0.025nM~(-1)·min~(-1),解离速率常数为0.69±0.07min~(-1),心得安与配体竞争结合的IC_(50)为1.00±0.15×10~(-8)M,心得宁的IC~(50)为167±0.22×10~(-8)M,异丙肾上慷素的IC_(50)为1.32±0.3×10~(-6)M。这些结果与用组织匀浆进行结合实验所得结果近似,表明在合适的条件下用冰冻组织切片进行β-受体结合实验是可行的。     

局部麻醉药对平滑肌药理受体系统的影响

作者测定了丁卡因和利多卡因对去甲肾上腺素和氯化钾引起的大鼠离体主动脉收缩反应的影响(在所有主动脉实验中,以3.3×10~(-6)M心得安消除β-受体的影响),并与双苄胺进行比较。同时又研究了丁卡因对氨甲酰胆碱引起的大鼠离体气管收缩反应的影响。低浓度的丁卡因(5×10~(-4)M)和利多卡因(3×     

氚标记强效镇痛剂N-[1-(β-羟基-β-苯乙基)-3-甲基-4-哌啶基]-N-丙酰苯胺的制备以及与阿片受体的结合试验

本文报道以N-[1-(对-溴苯甲酰甲基)-3-甲基-4-哌啶基]-N-丙酰苯胺(Ⅲ)为前体,以PdO/BaSO_4作催化剂,用氚气进行卤—氚置换,氚化还原羰基的反应。反应产物经硅胶纸层析纯化后,用甲基橙比色法定量测定,得到N-{1-[β-羟基-β-氚-β-(对-氚苯基)乙基]-3-甲基-4-哌啶基}-N-丙酰苯胺(Ⅳ,[~3H]F-7302),其比放射性为59 Ci/mM,放化纯度为98％。[~3H]F-7302与小鼠脑内阿片受体的特异性结合在浓度为4.5×10~(-9)M时达到饱和,解离常数Kd=1.25×10~(-9)M,最大结合量B_(max)=93.08×10~(12)M/g蛋白,其特异性结合与非特异性结合比值达10～15。     

氰联吡啶酮的正性肌力作用及其机制的初步探讨

利用心得安制备豚鼠和家兔心衰模型,静注氰联吡啶酮10～15μg/kg,继之以0.5～0.75μg/kg/min恒速静注,可使衰竭心脏Lvdp/dt mex升高约40%,但心率影响不明显。离体豚鼠心室乳头肌和心房实验表明,氰联吡啶酮(0.1～6.4μg/ml)可剂量依赖性地加强心肌收缩力,小剂量(0.1～1.6μg/ml)对心率影响不明显,大剂量时显著加快心率。较高浓度钙离子拮抗剂戊脉安(2×10~(-6)M)可显著减小氰联吡啶酮的正性肌力作用,而β受体阻断剂心得安(5×10~(-7)M)和H_2受体阻断剂西米替丁(5×10~(-3)M)则无此对抗作用。上述结果提示,氰联吡啶酮的正性肌力作用可能也与钙离子经钙通道内流的增加有关,而与β受体、H_2受体无关。     

虎杖苷对LPS作用下心肌细胞β-肾上腺素能受体的调节作用

目的　探讨LPS对心肌细胞 β肾上腺素能受体 (β AR)功能的影响以及虎杖苷 (PD)的防治效果。方法　体外培养心肌细胞 ,利用放射配体竞争结合法检测 β AR数量和亲和力 ,同时应用流式细胞术检测 β AR数量。 结果　正常心肌细胞膜上的 β AR最大结合容量 (Bmax)为 (4 14± 1 4 1)fmol·1× 10 5cells-1,解离常数Kd 值为 (4 0 2± 0 5 9)nmol·L-1;LPS刺激后 ,受体Bmax减小为 1 78± 0 12fmol·(1×10 5cells) -1,亲和力下降为 (2 3 88± 2 34)nmol·L-1;当用PD处理后 ,最大结合容量和亲和常数都恢复至接近正常 ,分别为 (3 37± 0 36 )fmol·(1× 10 5cells) -1和 (3 4 4± 0 4 4 )nmol·L-1;单纯用PD处理细胞Bmax和Kd 都与正常组差异无显著性 ,分别为 (2 76± 0 32 )fmol· 1× 10 5cells) -1和(4 6 3± 1 5 4 )nmol·L-1。流式细胞术检测 β AR数量变化与放射配体结合法检测结果一致。结论　LPS可直接引起心肌细胞β AR数量下调、亲和力降低 ,虎杖苷可调节 β AR的功能 ,使其维持正常水平。     

大鼠心肌质膜[~3H]forskolin结合部位的特征(英文)

近年来从植物研究中发现的一种二萜衍生物(forskolin)是一种新型作用于心血管系统的药物。质膜中forskolin结合部位被认为是腺苷酸环化酶(EC4.6.1.1.)的催化亚单位。我们已经证明了它对大鼠心肌质膜腺苷酸环化酶的激活作用呈剂量效应相关。本文用〔1,2-~3H〕forskolin研究了大鼠心肌质膜for-skolin结合部位的特征。实验结果表明:〔~3H〕forskolin结合量和膜蛋白呈线性相关;其特异结合是快速的、可饱和的、可逆的、依赖温度的;结合反应的平衡解离常数(Ka)为0.21±SD 0.08μmol/L,其最大结合浓度为3.3±SD1.2 pmol/mg蛋白,Hill系数为1.07±SD0.05;37℃时缔合速率常数为0.0013(nmol/L)~(-1)·min~(-1),30 min达到平衡状态;心肌质膜结合的〔1,2-~3H〕forskolin被forskolin取代的解离速率常数:37℃时为0.22 min~(-1)(t_(1/2)=3.2 min),0℃时为0.03 min~(-1)(t_(1/2)=25.8min),IC_(50)为1.6μmol/L。本文实验结果提示:心肌质膜〔~3H〕forskolin结合分析可以作为研究激素和药物对心肌腺苷酸环化酶催化亚基作用的良好模型。     

左旋咪唑对大鼠脑组织咪唑啉2受体-配体结合试验的影响

目的研究左旋咪唑(LMS)对大鼠脑组织咪唑啉2受体(I_2R)的影响。方法采用受体-配体竞争结合试验,观察LMS与受体的结合能力;采用受体-配体饱和试验,测定LMS处理大鼠和对照组大鼠脑内I_2R最大受体结合容量(B_(max))和平衡解离常数(K_d)变化。结果LMS抑制了放射性配基[~3H]idazoxan与大鼠脑组织内I_2R的结合,其半数抑制浓度(IC_(50))为4.04×10~(-7)mol·L~(-1)。LMS长期处理大鼠,脑内I_2R亲和力上调63%。结论LMS能与I_2R结合,长期应用LMS能通过刺激I_2R而改变了受体亲和力。     

PGE和惕柯烷对胃酸分泌的抑制

虽然从理论上已经知道惕柯烷(Tigli-anes)作用于前列腺素(PG)受体,但对于它们的药理作用却了解甚少。本研究用离体大鼠胃粘膜标本比较了惕柯烷(大戟科)和PGE_2的抗分泌作用。PGE_2(2×10~(-6))在30分钟的接触后能抑制离体大鼠粘膜对组胺(5×10~(-5)M)的反应,此后离体组织的反应能力几乎完全恢复。12-脱氧巴豆醇苯乙酸酯(12-Deoxy-phorbolphenylace-tate)(12-DOPP)和它的C-20-醋酸酯(12-DOPPA)在浓度为3×10~(-7)M下亦产     

白叶藤素对大鼠离体输精管α-受体的阻断作用

白叶藤素是萝藦科自叶藤属植物Cryp-tolepis sanguinolenta(Lindl.schlecter)根中分离得到的3种生物碱之一。为了研究白叶藤素对血管及血管外平滑肌的作用,本文探讨了该生物碱的α-肾上腺素受体拮抗作用和在离体大鼠输精管中此种拮抗作用的可能机理。去甲肾上腺素(3.0×10~(-7)～10~(-6)M)对离体大鼠输精管的作用存在着浓度-效应之     

多巴胺D1受体在Sf9昆虫细胞中的表达及左旋氯代斯阔任对重组D1受体的激动作用

目的:在Sf9昆虫细胞中表达D_1受体,并研究左旋氯代斯阔任对重组D_1受体的激动作用。方法:构建含D_1受体cDNA的重组杆状病毒,以其感染Sf9昆虫细胞得到D_1受体表达。[~3H]SCH23390受体结合检测重组D_1受体的药理特性。[~3H]SCH23390受体结合和cAMP测定实验检测左旋氯代斯阔任对重组D_1受体的激动作用。结果:在Sf9昆虫细胞中成功表达D_1受体,[~3H]SCH23390与重组D_1受体最大结合量(B_(max))为(0.94±0.06)nmol/g蛋白,[~3H]SCH23390与重组D_1受体的结合解离常数(K_d)为(1.9±0.3)nmol/L,其药理特性与小牛纹状体脑匀浆所得结果一致。左旋氯代斯阔任对重组D_1受体有高亲和力,解离常数K_i为(6.3±1.4)nmol/L;并剂量依赖地引起胞内cAMP增加,EC_(50)为0.72μmol/L(95％可信限为0.67-0.77μmol/L),表现出D_1激动作用。结论:在杆状病毒/昆虫细胞Sf9中,成功建立了D_1受体异源表达系统。在细胞分子水平,直接证实了左旋氯代斯阔任对D_1受体的激动作用。     

3H]glycogenolysis in brain slices mediated by beta-adrenoceptors: comparison of physiological response and [3H]dihydroalprenolol binding parameters

The subclass of beta-adrenergic receptors mediating glycogenolysis in slices from cerebral cortex of the mouse, incubated in the presence of [3H]glucose, was identified by comparing the relative potencies of agonists and the inhibition constants of antagonists to those found on reference systems. (+/-)Isoprenaline, (-)adrenaline and (-)noradrenaline produced a concentration-related glycogenolysis with Kact values of 2.2 x 10(-8) M, 2.8 x 10(-7) M and 3.6 x 10(-7) M, respectively. Zinterol, a selective beta 2-adrenergic agonist, did not produce any glycogenolytic response even in a large concentration. Salbutamol, a predominantly beta 2-adrenergic receptor agonist, elicited in a very large concentration (10(-4) M) less than 40% glycogenolysis, an effect which was not related to stimulation of beta-adrenergic receptors. The predominantly beta 1-adrenergic receptor antagonists, practolol and metoprolol shifted the concentration-response curve to noradrenaline to the right, with apparent Ki values of 8.0 x 10(-7) M and 7.6 x 10(-8) M, respectively, close to those reported in the rat heart. These various data indicate that the glycogenolytic response is selectively mediated by beta 1-adrenergic receptors. Under experimental conditions which were strictly identical to those used to measure glycogenolysis, a saturable binding of [3H]dihydroalprenolol to the slices occurred with Kd and Bmax values consistent with corresponding values on cell free preparations. Whereas the Ki values of antagonists were similar on the two systems, the Kact values of agonists on glycogenolysis were 10 times less than the Ki values for the binding of [3H]dihydroalprenolol. This suggests that the maximal glycogenolytic response is elicited for a partial receptor occupancy.     

Binding of [3H]CB 34, a selective ligand for peripheral benzodiazepine receptors, to rat brain membranes.

The 2-phenyl-imidazo[1,2-a]pyridine derivative CB 34 is a ligand for peripheral benzodiazepine receptors. The binding of [3H]CB 34 to rat cerebrocortical membranes was characterized. Specific binding was rapid, reversible, saturable and of high affinity. Kinetic analysis yielded association and dissociation rate constants of 0.2x10(8) M(-1) min(-1) and 0.29 min(-1), respectively. Saturation binding experiments revealed a single class of binding sites with a total binding capacity of 188+/-8 fmol/mg protein and an apparent dissociation constant of 0.19+/-0.02 nM. Specific [3H]CB 34 binding was inhibited by ligands selective for peripheral benzodiazepine receptors, whereas, with the exception of flunitrazepam and diazepam, ligands for central benzodiazepine receptors were inactive. Of the brain regions examined, the density of the [3H]CB 34-binding sites was greatest in the hypothalamus and lowest in the cerebral cortex. [3H]CB 34 is thus a potent and selective ligand for peripheral benzodiazepine receptors and should be proven useful for studies of the roles of these receptors.     

Muscarinic receptor binding in the guinea pig urinary bladder

The muscarinic receptors in the guinea pig urinary bladder and the longitudinal muscle of the guinea pig ileum were studied by means of a receptor-binding technique. 1-Quinuclidinyl [phenyl 4-3H]benzilate ((-)3H-QNB) was employed as radio-ligand and the separation of bound from free (-) 3H-QNB was performed by microcentrifugation. Under conditions of equilibrium (-)3H-QNB was specifically bound with high affinity to a limited number of sites, 0.32 and 1.62 pmol/mg protein in the bladder and ileum, respectively. The binding appeared to represent a single population of non-interacting binding sites. The apparent dissociation constants were 2.6 x 10(-10) M in the bladder and 1.2 x 10(-9) M in the ileum, whereas the KD-values, estimated by extrapolation to an infinitely low receptor concentration were 1.1 x 10(-10) M (bladder) and 3.1 x 10(-10) M (ileum). The binding of (-)3H-QNB appears to represent an interaction with muscarinic receptors, as it was effectively inhibited by muscarinic antagonists and agonists, but not by a variety of non-cholinergic drugs.     

Pharmacological and functional characterization of bradykinin receptors in canine cultured tracheal epithelial cells.

1. A direct [3H]-bradykinin ([3H]-BK) binding assay has been used to characterize the BK receptors in canine cultured tracheal epithelial cells (TECs). Based on receptor binding assay, TECs have specific, saturable, high-affinity binding sites for [3H]-BK. 2. The specific [3H]-BK binding was time- and temperature-dependent. Equilibrium of association of [3H]-BK with the BK receptors was attained within 30 min at room temperature and 1 h at 4 degrees C, respectively. 3. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 1.5 +/- 0.2 nM and a maximum receptor density (Bmax) of 53.2 +/- 5.2 fmol mg-1 protein. The Hill coefficient for [3H]-BK binding was 1.00 +/- 0.02. The association (K1) and dissociation (K-1) rate constants were (7.6 +/- 1.1) x 10(6) M-1 min-1 and (9.2 +/- 1.5) x 10 M-3 min-1, respectively. KD, calculated from the ratio of K-1 and K1, was 1.2 +/- 0.3 nM, a value close to that calculated from Scatchard plots of binding isotherms. 4. Neither a B1 receptor selective agonist (des-Arg9-BK, 0.1 nM - 10 microM) nor antagonist ([Leu8, des-Arg9]-BK, 0.1 nM - 10 microM) significantly inhibited [3H]-BK binding to TECs, which excludes the presence of B1 receptors in canine TECs. 5. The specific binding of [3H]-BK to canine TECs was inhibited by the B2 receptor selective antagonists ([D-Arg0, Hyp3, Thi5, D-Tic7, Oic8]-BK (Hoe 140, 0.1 nM-10 microM) and [D-Arg0, Hyp3, Thi5.8, D-Phe7]-BK, 0.1 nM - 10 microM) and agonists (BK and kallidin, 0.1 nM-10 microM) with a best fit by a one-binding site model. The order of potency for the inhibition of [3H]-BK binding was kallidin = BK = Hoe 140 > [D-Arg0, Hyp3, Thi5,8, D-Phe7]-BK. 6. BK and kallidin significantly induced concentration-dependent accumulation of IPs with a half-maximal response (EC50) at 17.6 +/- 3.5 and 26.6 +/- 5.3 nM, respectively, while the B1-selective agonist, des-Arg9-BK did not stimulate IPs accumulation and the B1-selective antagonist [Leu8, des-Arg9]-BK did not inhibit BK-induced IPs accumulation. Two B2-selective antagonists, Hoe 140 and [D-Arg0, Hyp3, Thi5,8, D-Phe7]-BK, inhibited BK-stimulated IPs accumulation with apparent pKB values of 8.8 +/- 0.3 and 7.0 +/- 0.3, respectively. 7. It is concluded that the pharmacological characteristics of the BK receptors in canine cultured TECs are primarily of the B2 receptor subtype which might regulate the function of tracheal epithelium through the activation of this receptor subtype coupling to PI hydrolysis.     

Pharmacological, radioligand binding, and electrophysiological characteristics of FPL 64176, a novel nondihydropyridine Ca2+ channel activator, in cardiac and vascular preparations.

The pharmacological, radioligand binding, and electrophysiological properties of FPL 64176, a new nondihydropyridine Ca2+ channel activator, were studied in rat tail artery, cardiac membranes, and A7r5 smooth muscle cells. FPL 64176 induced a contractile response, with an EC50 value of 2.11 x 10(-7) M. The maximum tension response to FPL 64176 was approximately 2-fold higher than that to (S)-Bay K 8644. FPL 64176 showed no significant inhibitory activity at concentrations up to 10(-5) M. The Ca2+ channel antagonists nifedipine, verapamil and diltiazem noncompetitively antagonized and completely relaxed the responses induced by FPL 64176. IC50 values of these three drugs were 5.22 x 10(-9), 1.31 x 10(-7), and 1.95 x 10(-7) M, respectively, for relaxing submaximum contractile responses to FPL 64176 (5 x 10(-7) M). The washout time for FPL 64176 was about 40 min, which was much longer than that for (S)-Bay K 8644 (within 1 min). FPL 64176 weakly inhibited (+)-[3H]PN 200-110, [3H]D888, and [3H]TA-3090 binding in rat cardiac membranes, with IC50 values of 1.04 x 10(-5) M and 7.03 x 10(-6) M for inhibition of (+)-[3H]PN 200-110 and [3H]TA-3090 binding, respectively, and with 23% inhibition of [3H]D888 binding at a FPL 64176 concentration of 1 x 10(-5) M. Dissociation kinetics of the three radioligands were allosterically accelerated by FPL 64176. Electrophysiological studies on the A7r5 smooth muscle cell line directly confirmed a large (approximately 14-fold) stimulatory effect on L-type Ca2+ current amplitude. The results suggest that FPL 64176 is a new type of Ca2+ channel activator with higher efficacy and a mechanism and site of action that are distinct from those for (S)-Bay K 8644.     

Cardiac glycoside receptors in cultured heart cells--I. Characterization of one single class of high affinity receptors in heart muscle cells from chick embryos

Binding of (3H)-ouabain and ouabain-induced inhibition of the sodium pump and of the (Na+ + K+)-ATPase have been characterized in cultured cardiac muscle and non muscle cells, as well as in cardiac cell membranes--all obtained from chick embryos. In both cell types, ouabain binds to a single type of binding sites in a temperature-dependent manner. The association rate but not the dissociation rate, is lowered by K+; specific binding is lost after heat-denaturation of the cells. Binding parameters (association and dissociation rate constants, activation energies for association and dissociation) are similar in muscle and non muscle cells. The dissociation constant of specific ouabain binding is 1.5 X 10(-7)M in cardiac muscle cells, and 1.9 X 10(-7)M in cardiac non muscle cells, the binding capacity being 2.6 and 2.1 pmoles/mg protein respectively. Specific binding of ouabain to the cells is coupled to inhibition of the sodium pump, as can be seen from ouabain-induced inhibition of active (86Rb+ + K+)-uptake, decrease in cellular K+, and increase in cellular Na+ (EC50 = 10(-7)-10(-6)M). The data obtained with cardiac cells are in good agreement with results found for ouabain binding (dissociation constant 4.3 X 10(-7)M) and (Na+ + K+)-ATPase inhibition (EC50 = 1.4 X 10(-6)M) in cardiac cell membranes prepared from the same tissue. Due to the experimental evidence it is concluded that the binding site for ouabain is identical with the cardiac glycoside receptor of these cells. In cardiac non muscle cells, binding of ouabain to its receptor is strictly coupled to inhibition of active K+-transport in a stoichiometric manner. In cardiac muscle cells, however, active K+-transport is inhibited by less than 10% when up to 40% of cardiac glycoside receptors have bound ouabain. It is assumed that this non-stoichiometric coupling of receptor occupancy and sodium pump inhibition in cardiac muscle cells may prevent substantial changes of Na+- and K+-contents in the heart in the presence of therapeutic levels of cardiac glycosides.     

3H-Codeine binding in the guinea pig lower brain stem

Saturable binding of (-)-3H codeine was found in the guinea pig medulla (KD = 5.6 x 10(-7) M, Bmax = 1.4 pmol/mg protein), whereas little stereospecific binding was detected (KD = 4.4 x 10(-5) M). The saturable binding of (-)-3H codeine was slightly enhanced by Na+ and by Mg++ but not by Li++ and Ca++. The enhancement appears to be due to an increase in the number of receptor sites. (-)-3H-Codeine binding was displaced by (-)- and (+)-codeine, morphine, (-)- and (+)-methadone but not by barbiturates. Naloxone, at a high concentration (1 x 10(-5) M), inhibits the binding by only 40%. This agrees with our previously published data which shows that the optical isomers of codeine had significant antitussive effects in the cat, these effects not being antagonized by naloxone. A class of opiate antitussive receptors, which are less naloxone-sensitive and less stereoselective than the mu receptors, is implicated.     

Binding kinetics of quinuclidinyl benzilate and methyl-quinuclidinyl benzilate enantiomers at neuronal (M1), cardiac (M2), and pancreatic (M3) muscarinic receptors

We analyzed the competition kinetics of quinuclidinyl benzilate (QNB) and QNB methiodide enantiomers on human NB-OK1 neuroblastoma (M1), rat cardiac (M2), and rat pancreas (M3) muscarinic binding sites. The association rate constants of the four drugs depended on the receptor subtype studied and were lower with pancreas (M3) (1-9 x 10(5) M-1 sec-1) than with cardiac (M2) (1-5 x 10(6) M-1 sec-1) and NB-OK1 (M1) (1-5 x 10(6) M-1 sec-1) binding sites. At each receptor subtype, we observed no significant difference between the association rate constants of the R- and S-enantiomers of either QNB or QNB methiodide. Receptor stereoselectivity, when present, was associated with differences in unlabeled drug dissociation rate constants. The dissociation rate constant varied much more than the association rate constant, when either (R)-QNB dissociation from the three subtypes (half-life, 77 min to greater than 340 min; best fit, 40 days) or dissociation of the four drugs from each receptor subtype (half-lives varying from 1.4 min to 4 hr at M1 receptors, 1.1 to 77 min at M2 receptors, and 3.5 min to greater than 340 min at M3 receptors were obtained by competition kinetics analysis) was compared.     

Electrophysiological actions of phenytoin on N-methyl-D-aspartate receptor-mediated responses in rat hippocampus in vitro.

1. [3H]-bradykinin was used to characterize the bradykinin receptors associated with canine cultured tracheal smooth muscle cells (TSMCs). Receptor binding assay showed that TSMCs had specific, saturable, high-affinity binding sites for [3H]-bradykinin. 2. The specific [3H]-bradykinin binding increased linearly with increasing cell concentrations. The equilibrium for association of [3H]-bradykinin with the bradykinin receptors was attained within 2 h at 4 degrees C and 1 h at room temperature, respectively. 3. Analysis of binding isotherms yielded an apparent equilibrium dissociation constant (KD) of 2.5 +/- 0.3 nM and a maximum receptor density (Bmax) of 25.1 +/- 0.3 fmol mg-1 protein. The Hill coefficient for [3H]-bradykinin binding was 1.00 +/- 0.02. The association (K1) and dissociation (K-1) rate constants were (8.67 +/- 2.60) x 10(6) M-1 min-1 and 0.024 +/- 0.005 min-1, respectively. KD, calculated from the ratio of K-1 and K1 was 2.8 +/- 0.5 nM, a value close to that of KD calculated from Scatchard plots of binding isotherms. 4. The B1 receptor selective agonist, (des-Arg9-bradykinin, 0.1 nM-10 microM) and antagonist ([Leu8, des-Arg9]-bradykinin, 0.1 nM-10 microM) did not did not inhibit the [3H]-bradykinin binding to TSMCs, which excludes the presence of B1 receptors in canine TSMCs.(ABSTRACT TRUNCATED AT 250 WORDS)     

Tricyclic antidepressants: potent blockade of histamine H1 receptors of guinea pig ileum.

Six tricyclic antidepressants were tested for their ability to antagonize histamine actions at histamine H1 receptors in a bioassay for these receptors (histamine-induced contractions of guinea pig ileum). All compounds were competitive antagonists with equilibrium dissociation constants in the range of 5.6 x 10(-11) M to 1.5 x 10(-7) M. Doxepin hydrochloride and amitriptyline hydrochloride were the most potent compounds of the series and may be the most potent antihistamines known. Antagonism at histamine H1 receptors by these compounds may explain their sedative effects.