Inhibition of gastric acid secretion by a glycoprotein isolated from human urine (human urinary gastric inhibitor).

The gastric antisecretory activity of an inhibitor newly isolated from human urine (Human Urinary Gastric Inhibitor or HUGI) has been studied. HUGI was given intravenously and its activity determined in the following test systems: gastric secretion in the rat with pyloric ligation; gastric secretion in the dog with a Heidenhain pouch stimulated with pentagastrin, histamine and a protein meal; acid secretion by the isolated gastric mucosa of the rat; gastrointestinal motility; bile flow and gall-bladder tone and arterial and venous blood pressure and heart rate. HUGI was found to have marked activity only in the pyloric-ligated rats and in the dogs with Heidenhain pouches stimulated by a protein meal. Particularly in the dog, HUGI (0.1 to 6.4 micrograms/kg, i.v) markedly inhibited gastric secretion, dose-dependently and without changing the plasma gastrin concentration. Negative results were obtained in the other tests, but these results serve to demonstrate that HUGI is an inhibitor well-differentiated from other glycoproteins or peptides with gastric antisecretory activity, such as urogastrone and GIP. The results obtained to date are not sufficient to allow the mechanism of action of HUGI to be defined.     

Adenylate cyclase of the dog gastric mucosa: Stimulation by histamine and inhibition by metiamide

Summary The activity of the non-stimulated, basal adenylate cyclase of the dog gastric mucosa is reduced by the histamine H₂-receptor antagonist metiamide but not by the histamine H₁-receptor antagonist mepyramine. Histamine activates the adenylate cyclase only slightly. In the presence of 10^–5 M metiamide a concentration-dependent stimulation of the enzyme by histamine was found. These data indicate that endogenous histamine in dog gastric mucosal homogenate is contributing at least in part to what is measured as basal adenylate cyclase activity. This effect is mediated by H₂-receptor excitation and in earlier studies has prevented the demonstration of a stimulatory effect of exogenous histamine on this enzyme.Supported by a grant from the Deutsche Forschungsgemeinschaft     

参灵方对药源性胃黏膜损伤大鼠Bcl-2和Bax表达的影响

目的研究预先给予参灵方对佐剂性关节炎(adjuvant arthritis,AA)大鼠药源性胃黏膜损伤的保护作用。方法建立AA大鼠模型,造模成功后随机分为参灵方小、大剂量组、模型组、雷尼替丁组,每组10只,分别灌胃治疗(雷尼替丁组给予30mg·kg-1·d-1;参灵方2组分别给予参灵方3.5g·kg-1·d-1、7g·kg-1·d-1,28d后采用皮下注射消炎痛进一步诱导药源性胃黏膜损伤的动物模型,光学显微镜观察胃黏膜组织病理变化。免疫组织化学ABC法对大鼠胃黏膜组织凋亡相关蛋白Bcl-2、Bax的表达进行细胞计算机医学图像分析及统计学处理。结果参灵方可明显抑制消炎痛致AA大鼠损伤胃黏膜的细胞凋亡,诱导损伤的胃黏膜细胞增殖,上调Bcl-2表达、下调Bax表达,与模型组比较,差异有统计学意义(P<0.05)。结论参灵方抑制细胞凋亡,对AA大鼠药源性胃黏膜损伤有保护作用。     

Structural and functional characterization of gastric mucosa and central nervous system in histamine H2 receptor-null mice

To examine the physiological role of the histamine H(2) receptor, histamine H(2) receptor-null mice were generated by homologous recombination. Histamine H(2) receptor-null mice, which developed normally and were fertile and healthy into adulthood, exhibited markedly enlarged stomachs and marked hypergastrinemia. The former was due to hyperplasia of gastric gland cells (small-sized parietal cells, enterochromaffin-like cells and mucous neck cells which were rich in mucin), but not of gastric surface mucous cells, which were not increased in number as compared with those in wild-type mice despite the marked hypergastrinemia. Basal gastric pH was slightly but significantly higher in histamine H(2) receptor-null mice. Although carbachol but not gastrin induced in vivo gastric acid production in histamine H(2) receptor-null mice, gastric pH was elevated by both muscarinic M(3) and gastrin antagonists. Thus, both gastrin and muscarinic receptors appear to be directly involved in maintaining gastric pH in histamine H(2) receptor-null mice. Interestingly, gastric glands from wild-type mice treated with an extremely high dose of subcutaneous lansoprazole (10 mg/kg body weight) for 3 months were very similar to those from histamine H(2) receptor-null mice. Except for hyperplasia of gastric surface mucous cells, the findings for gastric glands from lansoprazole-treated wild-type mice were almost identical to those from gastric glands from histamine H(2) receptor-null mice. Therefore, it is possible that the abnormal gastric glands in histamine H(2) receptor-null mice are secondary to the severe impairment of gastric acid production, induced by the histamine H(2) receptor disruption causing marked hypergastrinemia. Analyses of the central nervous system (CNS) of histamine H(2) receptor-null mice revealed these mice to be different from wild-type mice in terms of spontaneous locomotor activity and higher thresholds for electrically induced convulsions. Taken together, these results suggest that (1) gastrin receptors are functional in parietal cells in histamine H(2) receptor-null mice, (2) abnormal gastric glands in histamine H(2) receptor-null mice may be secondary to severe impairment of gastric acid production and secretion and (3) histamine H(2) receptors are functional in the central nervous system.     

Inhibition of adenylate cyclase and ATPase activities from rat liver plasma membrane by hexachlorophene.

Adenylate cyclase activity from a rat liver plasma membrane preparation purified according to Neville's procedure was inhibited in vitro by low concentrations (1 μM to 0.1 mM) of hexachlorophene. Complete inhibition was obtained with 1 mM hexachlorophene. Similar effects were observed whether the activity was assayed in the presence of 10 μM GTP. 0.1 μm glucagon. 10 mM NaF or without any addition. The effect of hexachlorophene was not due to inhibition of the regenerating system present in the incubation medium, since the effect of the drug was preserved in its absence. The inhibition brought about by hexachlorophene was not reversed by increasing the concentration of the substrate ATP-Mg. The inhibition was immediate and irreversible spontaneously: it was not affected by the simultaneous addition of 2-mercaptoethanol. Hexachlorophene inhibited also the “total” Mg²⁺-ATPase, as well as its Na⁺, K⁺-dependent fraction. These results suggest that the interaction of hexachlorophene with the plasma membrane, as reflected in the inhibition of two major enzymes activities, might play a key role in the toxicity of the drug.     

Effect of benzydamine,a topically administered NSAID onin vitro prostanoid synthesis by human and rat gastric mucosa and rat kidney,aorta and urinary bladder: Lack of effect on cyclooxygenase but potent inhibition of receptor-linked prostanoid synthesis

Orally administered NSAIDs are notorious for their frequent and severe side-effects in the gastrointestinal tract and kidney, whereas topically administered NSAIDs may avoid these untoward effects. Since NSAID-induced side-effects are largely prostaglandin (PG)-mediated, the effects of the topically administered NSAID, benzydamine, onin vitro PGI₂ and PGE₂ synthesis by the rat and human gastric mucosa and rat kidney slices was investigated. The effect on receptor-linked PG synthesis in the isolated rat aorta (adrenergic) and urinary bladder (cholinergic) was also investigated since NSAIDs may disrupt mobilization of calcium therein. Benzydamine was a very weak inhibitor of spontaneous PGI₂ and PGE₂ synthesis by human and rat gastric mucosa and rat kidney. In contrast, benzydamine was a potent inhibitor of noradrenaline- and carbachol-stimulated (but not arachidonate- or trauma-stimulated) PGI₂ synthesis. It is concluded that: a) benzydamine is unlikely to elicit cyclooxygenase-mediated side-effects on the gastrointestinal tract or kidney, b) the anti-inflammatory action of benzydamine is mediated by disruption of calcium linked to receptor-PG synthesis coupling, and c) calcium-dependent inflammatory processes may also be affected by benzydamine.     

β2-Adrenoceptors mediate inhibition of carbachol-induced contraction and cAMP generation in isolated smooth muscle cells from rabbit gastric antrum

The regulation of inhibition of carbachol-induced contraction by β-adrenoceptors of rabbit gastric antrum has been investigated. A functional assay using isolated smooth muscle cells (ISMC) was used to study the effect of β-adrenergic agonists and antagonists on carbacholcontracted ISMC and cAMP generation. The nonselective β-adrenergic agonist isoproterenol caused concentration-related inhibition of carbachol-induced contraction associated with a significant increase in cellular cAMP. The EC₅₀ for the effect of isoproterenol on cell inhibition of carbachol-induced contraction was closely related to the EC₅₀ for cAMP formation; the two curves were superimposable, indicating a positive correlation between the biological activity (inhibition of carbachol-induced contraction) and the intracellular event (cAMP formation) caused by the activation of β-adrenoceptors. The Kinetic studies demonstrate that maximum inhibition of carbachol-induced contraction and a parallel elevation of intracellular cAMP content were reached at 30 sec of incubation with isoproterenol. The β₂-selective receptor agonist terbutaline induced inhibition of carbachol-induced contraction of carbachol-contracted ISMC and cAMP generation. However, the relatively β₁-selective receptor agonist norepinephrine had no significant effect. Propranolol, a nonselective β- adrenoceptor antagonist (β₁ and β₂) caused a significant inhibition of the carbacholβ₂ induced contraction and rise in cAMP induced by isoproterenol and terbutaline, while the β₁-selective adrenergic receptorantagonist metoprolol, even at higher concentration, was inactive. These data demonstrate that there is a correlation between inhibition of carbachol-induced contraction and cAMP generation upon activation of the β₂-adrenoreceptors in the ISMC of rabbit gastric antrum. © 1992 Wiley-Liss, Inc.     

二烯丙基二硫下调LIMK1抑制人胃癌MGC803细胞迁移与侵袭

目的研究二烯丙基二硫(diallyl disulfide,DADS)对人胃癌MGC803细胞迁移侵袭及LIMK1表达的影响。方法MTT、划痕愈合和侵袭实验分别检测DADS对MGC803细胞增殖、迁移与侵袭能力的作用;RT-PCR、Western blot与免疫细胞化学检测LIMK1表达。结果 MTT显示,30 mg.L-1DADS作用MGC803细胞24、48、72、96 h后,增殖抑制率分别为31.8%、66.1%、83.6%、89.2%,呈明显的时间依赖关系(P<0.05)。划痕实验显示,10、20、30、40、50 mg.L-1DADS分别作用MGC803细胞,划痕愈合明显慢于对照组,呈剂量依赖关系(P<0.05)。侵袭实验显示,10、20、30、40、50 mg.L-1DADS作用MGC803细胞24 h后,穿过基质胶的细胞数分别为(35.8±3.74)、(34.1±2.02)、(31.7±4.81)、(17.2±3.08)、(13.2±3.36)个,较对照组(39.5±2.99)个数明显减少,呈剂量依赖性(P<0.05)。RT-PCR显示,30 mg.L-1DADS与MGC803细胞作用48 h后,LIMIK1mRNA明显下调(P<0.05)。Western blot与免疫细胞化学检测显示,30 mg.L-1DADS作用MGC803细胞6、12、24、48h后,LIMK1蛋白的表达水平呈时间依赖性下降(P<0.05)。结论 DADS可抑制人胃癌MGC803细胞迁移与侵袭能力,其机制可能与下调LIMK1有关。     

Inhibition of pentagastrin-stimulated gastric acid secretion and plasma levels of the new histamine H2-receptor antagonist ramixotidine dihydrochloride (CM 57755) in human volunteers

Summary The new competitive histamine H₂-receptor antagonist, ramixotidine 2 HCl (CM 57755), has been tested in healthy male volunteers for its ability to inhibit pentagastrin-stimulated gastric acid secretion. In the first study, in 8 subjects, pentagastrin 6 µg·kg^–1 was injected s.c., 90 min after the following 4 oral treatments given in random order at weekly intervals: placebo, 100, 200 and 400 mg CM 57755. Gastric contents were collected over 15-min periods during the 2 h after pentagastrin stimulation. In a second, similar study, 8 subjects received placebo, 0.5 and 1.0 g CM 57755 and 800 mg cimetidine, 120 min before a 2 h i.v. infusion of 6 µg·kg^–1·h^–1 pentagastrin. Cumulative gastric secretion in placebo-treated subjects was 46±14 and 62±11 mmol H⁺·2 h^–1 (mean±SD), respectively, in the first and second studies. It was significantly reduced only after 400 mg CM 57755 in the first study. In the second study either dose of CM 57755 and cimetidine caused a significant reduction in gastric acid secretion. Average plasma levels of ramixotidine were dose-related after 0.2 and 1.0 g and ranged from 0.3 and 1.6 µg/ml, respectively, at 60 min to 0.5 and 3.7 µg/ml at 180 min. The peak cimetidine level averaged 3.6 µg/ml at 150 min. Individual CM 57755 plasma levels throughout the test period were fairly consistent with the inhibition of cumulative gastric acid secretion scored concurrently in each subject. No subjective side-effects attributable to the treatments were reported, and no abnormal findings were seen in the ECG or in laboratory tests.     

Inhibition of cytochrome P450 2E1 by propofol in human and porcine liver microsomes

While almost anesthetics are metabolized by the cytochrome P450 (CYP) 3A4, some major volatile ones such as halothane and sevoflurane are metabolized by CYP2E1 in humans. To determine whether 2,6-diisopropylphenol (propofol), a widely used intravenous anesthetic agent, known to inhibit CYP3A4 and CYP1A2, also inhibits CYP2E1, 6-OH hydroxylation of chlorzoxazone, a prototypical CYP2E1 substrate, was estimated using two pools of human microsomes and one pool of porcine microsomes from seven livers. Basal human enzyme activities were characterized by a V(max) of 1426+/-230 and 288+/-29 pmol min(-1)mg(-1) protein and a K(m) of 122+/-47 and 149+/-42 microM, while the corresponding porcine activities were associated with a V(max) of 352+/-42 pmol min(-1)mg(-1) protein and a K(m) of 167+/-38 microM. A competitive inhibition of CYP2E1 by propofol was observed with low inhibition constants in the therapeutic range in both porcine (19 microM) and human (48 microM) liver microsomes. These in vitro results suggest that propofol could have a protective effect on toxic metabolite activation of compounds catalyzed by CYP2E1.     

冬凌草甲素诱导胃癌BGC-823细胞凋亡及G2／M细胞周期阻滞作用研究

目的研究冬凌草甲素体外诱导胃癌BGC-823细胞凋亡和细胞周期阻滞的作用，阐明其作用机理，为临床应用提供科学依据。方法用MTT方法测定冬凌草甲素体外抑制BGC-823细胞生长的作用。用共聚焦荧光显微镜和流式细胞仪分别观察诱导凋亡的情况。线粒体膜电位和细胞内钙离子浓度分别用荧光探针标记后流式细胞仪测定。凋亡和细胞周期相关蛋白的表达用Westem blotting进行测定。结果冬凌草甲素对BGC-823细胞的半数生长抑制浓度IC50值为22．21μmol.L^-1，并诱导细胞凋亡，呈现浓度依赖性。此外，能够降低线粒体膜电位，升高细胞内钙离子浓度，激活caspase-3前体。同时，冬凌草甲素能够使得BGC-823细胞阻滞在细胞周期的G2／M期，伴随有cyclin A蛋白的表达下降。在发生凋亡和细胞阻滞之前，观察到p53蛋白的表达增加。结论冬凌草甲素能够诱导BGC-823细胞产生凋亡，并使细胞周期阻滞在G2／M期。其凋亡机理与caspase-3的激活，p53蛋白表达上调及cyclin A表达下调相关。     

二烯丙基二硫对人胃癌MGC803细胞G_2/M期检查点Chk1与Chk2的影响

目的研究二烯丙基二硫(diallyl disulfide,DADS)对人胃癌MGC803细胞G2/M期检查点Chk1与Chk2的影响。方法流式细胞术检测细胞周期改变;Northern blot、Westernblot与免疫细胞化学检测DADS处理MGC803细胞的Chk1与Chk2表达。结果流式细胞术分析显示,30 mg.L-1DADS呈时间依赖性阻滞MGC803细胞在G2/M期(P<0.05);Northern blot检测表明,DADS不同时间作用MGC803细胞后,Chk1与Chk2 mRNA表达与未处理组差异无显著性(P>0.05);免疫细胞化学发现Chk1与Chk2表达与未处理组无明显改变(P>0.05);Western blot DADS在不同时间对MGC803细胞Chk1与Chk2总蛋白表达无改变(P>0.05),而磷酸化的Chk1表达呈时间依赖性增加(P<0.05),但磷酸化的Chk2无明显改变(P>0.05)。结论 DADS阻滞MGC803细胞G2/M期与磷酸化Chk1有关。     

Induction of trefoil factor (TFF)1, TFF2 and TFF3 by hypoxia is mediated by hypoxia inducible factor-1: implications for gastric mucosal healing

Background and purpose:

Mucosal microcirculation is compromised during gastric damage induced by non-steroidal anti-inflammatory drugs, such as aspirin. Consequently, oxygen supply to epithelial cells is decreased. The trefoil factor (TFF) peptides are involved in mechanisms of defence and repair in the gastrointestinal tract but their regulation at sites of gastric injury is unknown.

Experimental approach:

Hypoxia and expression of TFF genes and peptides were measured in the damaged stomach of aspirin-treated rats. In a human gastric cell line (AGS cells), the effects of hypoxia and of hypoxia inducible factor (HIF)-1 (through transient transfection of HIF-1α siRNA or over-expression of HIF-1α) on TFF gene expression were evaluated.

Key results:

Hypoxyprobe immunostaining, up-regulation of TFF2 (1.9-fold) and TFF3 (1.8-fold) and a non-significant increase of TFF1 (1.5-fold) mRNA were observed in the damaged stomach of aspirin-treated rats, compared with control animals. Hypoxia (3% O₂, 16 h) induced mRNA for TFF1 (5.8-fold), TTF2 (9.1-fold) and TFF3 (9.3-fold) in AGS cells, an effect mediated by HIF-1, as transient transfection of HIF-1α siRNA reduced the effects of hypoxia. Over-expression of HIF-1α by transfection in non-hypoxic epithelial cells produced a similar pattern of TFF induction to that observed with hypoxia and transactivated a TFF1 reporter construct.

Conclusions and implications:

Hypoxia inducible factor-1 mediated the induction of TFF gene expression by hypoxia in gastric epithelial cells. Low oxygen levels and up-regulation of TFF gene expression in the damaged stomach of aspirin-treated rats suggest that hypoxia induced expression of TFF genes at sites of gastric injury.     

Thimerosal-induced apoptosis in human SCM1 gastric cancer cells: activation of p38 MAP kinase and caspase-3 pathways without involvement of [Ca2+]i elevation.

Thimerosal is a mercury-containing preservative in some vaccines. The effect of thimerosal on human gastric cancer cells is unknown. This study shows that in cultured human gastric cancer cells (SCM1), thimerosal reduced cell viability in a concentration- and time-dependent manner. Thimerosal caused apoptosis as assessed by propidium iodide-stained cells and caspase-3 activation. Although immunoblotting data revealed that thimerosal could activate the phosphorylation of extracellular signal-regulated kinase, c-Jun NH2-terminal protein kinase, and p38 mitogen-activated protein kinase (p38 MAPK), only SB203580 (a p38 MAPK inhibitor) partially prevented cells from apoptosis. Thimerosal also induced [Ca2+](i) increases via Ca2+ influx from the extracellular space. However, pretreatment with (bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetate)/AM, a Ca2+ chelator, to prevent thimerosal-induced [Ca2+](i) increases did not protect cells from death. The results suggest that in SCM1 cells, thimerosal caused Ca2+-independent apoptosis via phosphorylating p38 MAPK resulting in caspase-3 activation.     

Inhibition of endothelial cell Ca2+ entry and transient receptor potential channels by Sigma-1 receptor ligands

Background and Purpose

The Sigma-1 receptor (Sig1R) impacts on calcium ion signalling and has a plethora of ligands. This study investigated Sig1R and its ligands in relation to endogenous calcium events of endothelial cells and transient receptor potential (TRP) channels.

Experimental Approach

Intracellular calcium and patch clamp measurements were made from human saphenous vein endothelial cells and HEK 293 cells expressing exogenous human TRPC5, TRPM2 or TRPM3. Sig1R ligands were applied and short interfering RNA was used to deplete Sig1R. TRP channels tagged with fluorescent proteins were used for subcellular localization studies.

Key Results

In endothelial cells, 10–100 μM of the Sig1R antagonist BD1063 inhibited sustained but not transient calcium responses evoked by histamine. The Sig1R agonist 4-IBP and related antagonist BD1047 were also inhibitory. The Sig1R agonist {"type":"entrez-protein","attrs":{"text":"SKF10047","term_id":"1156210965","term_text":"SKF10047"}}SKF10047 had no effect. Sustained calcium entry evoked by VEGF or hydrogen peroxide was also inhibited by BD1063, BD1047 or 4-IBP, but not {"type":"entrez-protein","attrs":{"text":"SKF10047","term_id":"1156210965","term_text":"SKF10047"}}SKF10047. 4-IBP, BD1047 and BD1063 inhibited TRPC5 or TRPM3, but not TRPM2. Inhibitory effects of BD1047 were rapid in onset and readily reversed on washout. {"type":"entrez-protein","attrs":{"text":"SKF10047","term_id":"1156210965","term_text":"SKF10047"}}SKF10047 inhibited TRPC5 but not TRPM3 or TRPM2. Depletion of Sig1R did not prevent the inhibitory actions of BD1063 or BD1047 and Sig1R did not co-localize with TRPC5 or TRPM3.

Conclusions and Implications

The data suggest that two types of Sig1R ligand (BD1047/BD1063 and 4-IBP) are inhibitors of receptor- or chemically activated calcium entry channels, acting relatively directly and independently of the Sig1R. Chemical foundations for TRP channel inhibitors are suggested.     

Inhibition of rat hepatic CYP2E1 by quinacrine: molecular modeling investigation and effects on 4-(methyl nitrosamino)-1-(3-pyridyl)-1-butanone (NNK)-induced mutagenicity

Increased activity of CYP2E1 has been associated with increased risk of chemically-mediated cancers, through enhanced activation of a variety of procarcinogens. In this context, inhibition of CYP2E1 is potentially of significance in xenobiotic toxicity. The aim of the present study was to test the hypothesis that quinacrine inhibits hepatic CYP2E1. For this purpose, disulfiram (75 mg/kg i.p) as an inhibitor and isoniazid (100 mg/kg i.p) as an inducer of CYP2E1, as well as quinacrine (50 mg/kg i.p) were administered to Wistar rats and the hepatic activity of CYP2E1 was measured. The expression of CYP2E1 was further assessed by Western blot analysis. As expected, disulfiram inhibited, while isoniazid induced the activity and expression of the enzyme. Interestingly, treatment with quinacrine resulted in a significant decrease of CYP2E1 activity and expression. To investigate any similarities in the inhibition of CYP2E1 by quinacrine and disulfiram, molecular modeling techniques were adopted and revealed that quinacrine molecule anchors inside the same binding pocket of the protein where disulfiram is also attached. Finally, as assessed by the sister chromatid exchanges (SCE) assay, quinacrine was demonstrated to reduce the mutagenic effects of the tobacco-specific N-nitrosamine 4-(methyl nitrosamino)-1-(3-pyridyl)-1-butanone (NNK), which is known to be converted to active mutagen in the liver principally through CYP2E1. We suggest that these antimutagenic effects of quinacrine could be possibly attributed, at least in part, to its ability to block the bioactivation of NNK, mainly by the inhibition of CYP2E1. Our results, even preliminary, indicate that quinacrine as an inhibitor of CYP2E1 might be protective against chemically-induced toxicities such as NNK-induced mutagenicity.     

Molecular modelling of human CYP2E1 by homology with the CYP102 haemoprotein domain: investigation of the interactions of substrates and inhibitors within the putative active site of the human CYP2E1 isoform

1. The construction of a three-dimensional model of human CYP2E1 is reported. It is based on homology with the haemoprotein domain of the unusual bacterial P450, CYP102, which is of known crystal structure. 2. Interactive docking of a number of human CYP2E1 substrates is consistent with their known positions of CYP2E1-mediated metabolism, where specific interactions with key active site amino acid side-chains appear to rationalize the binding and orientation of substrate molecules. 3. Amino acid residues within the putative active site of human CYP2E1, including those associated with the binding of substrates and inhibitors, are shown to correspond with those identified by site-directed mutagenesis experiments conducted on CYP2 family isoforms, and they are known to affect substrate metabolism regioselectivity. 4. Consequently, it was found that the CYP2E1 active site exhibits complementarity with the structural characteristics of known substrates and inhibitors of this enzyme, including their relatively low molecular weights and disposition of hydrogen bond-forming groups.     

Inhibition by digoxin and SC4453 of (Na + K)-ATPase prepared from human heart, guinea-pig heart and guinea-pig brain

SC4453 is a digoxin analogue with a pyridazine instead of a lactone ring on C_17β. SC4453 was compared with digoxin with respect to inhibition of (Na⁺ + K⁺)-ATPase prepared from human heart, guinea-pig heart and guinea-pig brain. SC4453 was slightly less potent than digoxin but showed a similar sensitivity to K⁺. As for cardenolides, species differences in sensitivity to SC4453 were accounted for by differences in the rate of dissociation from the receptors. These observations confirm that the human heart is one of the tissues most sensitive to cardiac glycosides.