Alteration of intracellular cAMP levels and beating rates of cultured chick cardiac cells by Bordetella pertussis adenylate cyclase

Bordetella pertussis, the pathogen responsible for whooping cough, releases a soluble calmodulin-sensitive adenylate cyclase into its culture medium which enters several different types of animal cells and elevates intracellular cAMP. In this study, the influence of B. pertussis adenylate cyclase on intracellular cAMP levels of cultured chick cardiac cells and the beating rates of chick cardiac cell aggregates was examined. Treatment with B. pertussis adenylate cyclase caused up to a 60-fold increase in intracellular cAMP which was significantly greater than that caused by forskolin or isoproterenol. Increases in intracellular cAMP caused by B. pertussis adenylate cyclase were observed within 2 min after treating cells with the enzyme, and binding of calmodulin to the enzyme inhibited these effects. In addition, high concentrations of the enzyme completely inhibited the beating of cardiac cells. However, lower concentrations of the adenylate cyclase accelerated beating rates 30-40% and cardiac cells continued to beat at an accelerated rate for at least 30 min. These data indicate that B. pertussis adenylate cyclase invades chick cardiac cells and catalyzes significant increases in intracellular cAMP. It is proposed that the effect of the enzyme on the beating rates of heart cell aggregates may be due to alteration of intracellular cAMP levels.     

The effects of milrinone and piroximone on intracellular calcium handling in working myocardium from the ferret.

The effects of milrinone and piroximone were compared to those of isoprenaline, dibutyryl adenosine 3':5'-cyclic monophosphate (dibutyryl cyclic AMP), forskolin, isobutylmethylxanthine, increased extracellular calcium [( Ca2+]o) and caffeine in ferret right ventricular papillary muscles that were loaded intracellularly with aequorin, a bioluminescent calcium indicator that emits light when it combines with calcium. The positive inotropic action of each drug, except caffeine, was associated with an increase in the peak amplitude of the aequorin light signal (i.e. intracellular Ca2+ transient) reflecting an increased amount of calcium available for excitation-contraction coupling; the positive inotropic effect of caffeine appears to occur by other mechanisms. The time courses of the aequorin light signal and corresponding tension response were shortened by isoprenaline, forskolin, isobutylmethylxanthine, dibutyryl cyclic AMP, milrinone and piroximone; unchanged by increased [Ca2+]o and prolonged by caffeine, suggesting that the rates of Ca2+ release and uptake by the sarcoplasmic reticulum were respectively increased, unchanged or decreased by these groups of drugs. Relative to changes in [Ca2+]o, the ratio of the peak of the aequorin light signal to the peak of the tension response was increased by isoprenaline, milrinone and piroximone, and decreased by caffeine, indicating that the Ca2+-sensitivity of the myofilaments was respectively decreased, and increased by these drugs. The effects of milrinone and piroximone on the amplitude and time course of the aequorin light signal, as they relate to changes in uptake and release of calcium from the sarcoplasmic reticulum and to changes in the sensitivity of the myofilaments to Ca2+, are consistent with the findings that positive inotropic doses of these agents act by increasing intracellular concentrations of cyclic AMP. Higher doses of milrinone and piroximone produced negative inotropic effects that were characterized by diminution of developed tension but no change or an increase in the amplitude of the aequorin light signal, suggesting a decrease in the sensitivity of the contractile elements to Ca2+. Toxic doses of milrinone, piroximone and isoprenaline were associated with development of a Ca2+-overload state characterized by the presence of after-glimmers, after-contractions and dysrhythmias, and by decreased amplitude of both the aequorin light signal and tension response. The negative inotropic and toxic effects of milrinone and piroximone can be explained only in part by increased intracellular concentrations of cyclic AMP; we suggest that these drugs may have other cardiac actions.     

Beta-adrenoceptor-mediated cAMP accumulation in cardiac cells: effects of nebivolol

The effects of nebivolol, the racemic mixture of the SRRR and RSSS enantiomers, on beta-adrenoceptor-mediated cAMP accumulation in living cardiac cells were compared to those of beta-adrenoceptor antagonists. Serum-free cultivation of cardiac cells from ventricles of 2 to 3-day-old Wistar rats resulted in a population of contractile cardiac cells almost free of mesenchymal non-myocardial cells. Isoproterenol stimulated beta 1- as well as beta 2-adrenoceptor sites. Selective beta 1- and beta 2-receptor site occlusion, in the presence of an appropriate concentration of the selective beta 2-adrenoceptor antagonist, ICI 118-551, or the selective beta 1-adrenoceptor antagonist, CGP 20712-A, showed that the receptor population consisted of mostly the beta 1-adrenergic subtype. The latter could be specifically stimulated by noradrenaline. Nebivolol and d-nebivolol (SRRR) inhibited noradrenaline-induced cAMP accumulation with IC50 values of 22 and 15 nM, respectively. CGP 20712-A was 10 times more active and atenolol was 7 times less active than nebivolol. Both assays, beta-adrenoceptor binding and cAMP accumulation, evidenced beta-adrenoceptor antagonistic properties only for the d-enantiomer of nebivolol (SRRR). 1-Nebivolol (RSSS) showed no beta-adrenergic activity.     

左西孟旦与米力农在心脏瓣膜置换术围术期的心肌保护作用研究

目的分析比较左西孟旦与米力农在心脏瓣膜置换术围术期的心肌保护作用。方法 60例需要在体外循环辅助下进行心脏瓣膜置换的患者随机分为左西孟旦组、米力农组及对照组,每组20例。左西孟旦组使用左西孟旦注射液,米力农组使用米力农注射液。分别在麻醉诱导后,升主动脉开放后1、8 h,术后24、48 h检测患者血浆心肌肌钙蛋白I(cTnI)、乳酸脱氢酶(LDH)及磷酸肌酸激酶同工酶(CK-MB)含量,观察各组患者术后心脏自动复跳率,并记录三组患者术后监护时间。结果与对照组比较,左西孟旦组和米力农组主动脉开放1 h至术后48 h,患者cTnI、LDH与CK-M B明显降低,且左西孟旦组明显低于米力农组,差异有统计学意义(P <0. 05);左西孟旦组和米力农组患者自动复跳率明显高于对照组,且左西孟旦组明显高于米力农组,差异有统计学意义(P <0. 05)。左西孟旦组和米力农组患者术后拔管时间、ICU监护时间均短于对照组,且左西孟旦组短于米力农组,差异有统计学意义(P <0. 05);三组患者术后住院时间比较差异无统计学意义(P> 0. 05)。结论左西孟旦与米力农对心脏瓣膜置换术患者围术期的心肌缺血再灌注损伤均有明显的保护作用,可增强患者术后心肌收缩力,缩短患者康复时间,左西孟旦较米力农效果更为明显。     

紫外线和微波联合照射对大鼠PGE2,cAMP和细胞免疫功能影响的研究

SD大鼠分别照射微波(MW)、长波紫外线(UVA)、短波紫外线(UVC)、UVA +MW和 UVC+MW。结果表明,UVA 或 UVC单独照射均引起血浆前列腺素E_2(PGE_2)、血液淋巴细胞的环磷腺苷酸(cAMP)、酸性α-乙酸萘酯酶阳性细胞(ANAE~+)的分散颗粒型细胞(DG)百分比升高,血液淋巴细胞的ANAE~+百分比、ANAE~+的斑块颗粒型细胞(MG)和MG/DG比值降低;MW 单独照射引起血液淋巴细胞的ANAE~+、MG和MG/DG降低、DG升高,PGE_2 和cAMP无显著变化;UVA+MW或UVC+MW联合照射均引起PGE_2和cAMP明显升高,呈相乘作用;DG升高、ANAE~+、MG和MG/DG降低,呈相加作用。     

Effects of platelet activating factor and ginkgolid B on the cAMP levels in washed rabbit platelets

研究了血小板激活因子（ＰＡＦ）和ＰＡＦ拮抗剂银杏内酯Ｂ对洗涤兔血小板中ｃＡＭＰ含量的作用．结果表明ＰＡＦ（０．１－１．０μｍｏｌ·Ｌ－１）对血小板的基础ｃＡＭＰ水平无影响，但对前列腺素Ｅ１（ＰＧＥ１）２μｍｏｌ·Ｌ－１及４，５－二氢－６－［４－（１Ｈ－咪唑－１－）苯基］－５－甲基－３－（２Ｈ）－哒嗪酮（ＣＩ－９３０）２０μｍｏｌ·Ｌ－１引起的ｃＡＭＰ升高有显著的抑制作用．银杏内酯Ｂ能完全拮抗ＰＡＦ抑制ＰＧＥ１和ＣＩ－９３０升高ｃＡＭＰ的作用，ＩＣ５０分别为４．７和１２．５μｍｏｌ·Ｌ－１．合用磷酸肌酸／磷酸肌酸激酶和阿司匹林对ＰＡＦ和银杏内酯Ｂ的作用均无影响．提示ＰＡＦ对磷酸二酯酶的激活作用及腺苷酸环化酶的抑制作用是ＰＡＦ的直接作用，与其同ＰＡＦ受体结合有关．     

Comparison of the effects of isobutylmethylxanthine and milrinone on ischaemia-induced arrhythmias and platelet aggregation in anaesthetized rabbits.

1. The aim of this study was to compare the effects of the non-selective phosphodiesterase (PDE) inhibitor, isobutylmethylxanthine (IBMX) and the selective PDE III inhibitor, milrinone, in a rabbit model of acute myocardial ischaemia. 2. Coronary artery occlusion caused changes in the ST-segment of the ECG and ectopic activity in all control rabbits. Ventricular fibrillation occurred in 10 out of 14 (71%) of these animals. Pretreatment with IBMX 100 micrograms kg-1 plus 10 micrograms kg-1 min-1, starting 10 min before coronary artery occlusion, reduced ischaemia-induced ST-segment changes and ventricular fibrillation occurred in only 10% of this group (n = 10). A similar dose of milrinone had no antiarrhythmic activity, whereas with a lower dose of milrinone, 30 micrograms kg-1 plus 3 micrograms kg-1 min-1 (n = 10), only 30% of rabbits fibrillated and ST-segment changes were attenuated. 3. Acute administration of both IBMX and milrinone reduced arterial blood pressure. With the higher dose of milrinone a significant effect was still present after 10 min of drug infusion. A greater hypotensive response to the higher dose of milrinone was observed in the rabbits which subsequently fibrillated during ischaemia. A marked tachycardia was also observed after administration of the higher dose of milrinone. 4. At the end of the experiment platelet aggregation was studied ex vivo. ADP-induced aggregation was reduced by pretreatment of the rabbits with milrinone but not IBMX. Both PDE inhibitors enhanced the ability of isoprenaline to inhibit ADP-induced platelet aggregation but milrinone was more effective, particularly at the higher dose.(ABSTRACT TRUNCATED AT 250 WORDS)     

血小板激活因子和银杏内酯B对洗涤兔血小板中cAMP含量的影响

研究了血小板激活因子(PAF)和PAF拮抗剂银杏内酯B对洗涤兔血小板中cAMP含量的作用. 结果表明PAF(0.1-1.0 μmol·L^-1)对血小板的基础cAMP水平无影响, 但对前列腺素E₁(PGE₁) 2 μmol·L^-1及4,5-二氢-6-［4-(1H-咪唑-1)苯基］-5-甲基-3-(2H)-哒嗪酮(CI-930) 20 μmol·L^-1引起的cAMP升高有显著的抑制作用. 银杏内酯B能完全拮抗PAF抑制PGE₁和CI-930升高cAMP的作用, IC₅₀分别为4.7和12.5 μmol·L^-1. 合用磷酸肌酸/磷酸肌酸激酶和阿司匹林对PAF和银杏内酯B的作用均无影响. 提示PAF对磷酸二酯酶的激活作用及腺苷酸环化酶的抑制作用是PAF的直接作用，与其同PAF受体结合有关.     

Effects of beraprost and cilostazol and renal function on serum thrombomodulin levels in diabetic patients

Serum thrombomodulin (TM) levels were determined in diabetic patients, and the effects of diabetic complications and renal function on TM were studied. Serum TM levels increased in diabetics, and patients with diabetic nephropathy tended to manifest higher levels of TM. There was a significant correlation between TM and serum creatinine levels. In addition, there was a significant elevation in serum TM levels in diabetics over time (1 year to 1 year 8 months), and the changes were particularly evident in patients who had a higher TM level from before the observation period. Furthermore, when patients were treated with an antiplatelet agent--beraprost (CAS 88475-69-8) or cilostazol (CAS 73963-72-1)--a significant reduction in TM levels was observed after 3 months. It is suggested that TM could be used as index to assess the development of clinical complications in diabetics and that anti-platelet agents have potential usefulness in delaying the aggravation of diabetic complications.     

米力农治疗小儿重症肺炎合并心衰的临床疗效及对心功能的影响

目的探讨米力农治疗小儿重症肺炎合并心衰的临床疗效及对心功能的影响。方法将108例小儿重症肺炎合并心衰者随机分入对照组与观察组,对照组患儿接受常规治疗及多巴胺静点,观察组患儿同时接受米力农治疗。比较两组临床疗效、心功能指标的差别。结果观察组治疗有效率显著高于对照组,差异具有统计学意义(P<0.05);观察组患者症状、体征改善时间显著早于对照组(P<0.05);观察组治疗后左室射血分数(LVEF)、左室短轴缩短率(LVSF)、左室收缩末径(LVDD)及房室瓣E、A峰比值(E/A)显著优于对照组(P<0.05)。结论米力农可显著提高小儿重症肺炎合并心衰的临床疗效,改善心功能。     

Cryptophycin 1 cellular levels and effects in vitro using L1210 cells

Cryptophycin 1 is a natural product that was initially isolated from blue-green algae which has shown potent broad spectrum antitumor activity in preclinical in vitro and in vivo models. The drug strongly binds to tubulin and disrupts microtubule assembly for more than 24 hours after its removal. We evaluated cell survival, intracellular levels and inhibition of macromolecular synthesis in L1210 cells following exposure to cryptophycin 1 in vitro. Cell survival was strongly inhibited following drug exposure for either 1 or 4 hours. Intracellular drug levels were minimally affected by temperature (4°C versus 37°C) or exposure times up to 1 hour. However, extracellular drug concentration in culture media and increasing cell numbers did affect the concentration of intracellular drug levels in a nearly proportional manner. The synthesis of DNA and RNA was inhibited less than 5%, while protein synthesis inhibition was near 30%. Thus, none of the macromolecules were inhibited enough to explain the inhibition of tumor cell growth.     

Cryptophycin 1 cellular levels and effects in vitro using L1210 cells

Cryptophycin 1 is a natural product that was initially isolated from blue-green algae which has shown potent broad spectrum antitumor activity in preclinical in vitro and in vivo models. The drug strongly binds to tubulin and disrupts microtubule assembly for more than 24 hours after its removal. We evaluated cell survival, intracellular levels and inhibition of macromolecular synthesis in L1210 cells following exposure to cryptophycin 1 in vitro. Cell survival was strongly inhibited following drug exposure for either 1 or 4 hours. Intracellular drug levels were minimally affected by temperature (4 degrees C versus 37 degrees C) or exposure times up to 1 hour. However, extracellular drug concentration in culture media and increasing cell numbers did affect the concentration of intracellular drug levels in a nearly proportional manner. The synthesis of DNA and RNA was inhibited less than 5%, while protein synthesis inhibition was near 30%. Thus, none of the macromolecules were inhibited enough to explain the inhibition of tumor cell growth.     

Inactivation of multidrug resistance proteins disrupts both cellular extrusion and intracellular degradation of cAMP

In addition to xenobiotics and several other endogenous metabolites, multidrug-resistance proteins (MRPs) extrude the second-messenger cAMP from various cells. Pharmacological and/or genetic inactivation of MRPs has been shown to augment intracellular cAMP signaling, an effect assumed to be a direct consequence of the blockade of cAMP extrusion. Here we provide evidence that the augmented intracellular cAMP levels are not due exclusively to the prevention of cAMP efflux because MRP inactivation is also associated with reduced cAMP degradation by phosphodiesterases (PDEs). Several prototypical MRP inhibitors block PDE activity at concentrations widely used to inhibit MRPs. Their dose-dependent effects in several paradigms of cAMP signaling are more consistent with their potency in inhibiting PDEs than MRPs. Moreover, genetic manipulation of MRP expression results in concomitant changes in PDE activity and protein levels, thus affecting cAMP degradation in parallel with cAMP efflux. These findings suggest that the effects of MRP inactivation on intracellular cAMP levels reported previously may be due in part to reduced degradation by PDEs and identify MRP-dependent transport mechanisms as novel regulators of cellular PDE expression levels. Mathematical simulations of cAMP signaling predict that selective ablation of MRP-dependent cAMP efflux per se does not affect bulk cytosolic cAMP levels, but may control cAMP levels in restricted submembrane compartments that are defined by small volume, high MRP activity, limited PDE activity, and limited exchange of cAMP with the bulk-cytosolic cAMP pool. Whether this regulation occurs in cells remains to be confirmed experimentally under conditions that do not affect PDE activity.     

Comparative effects of fluoxetine and amitriptyline on cardiac function.

1. The effects of fluoxetine and amitriptyline on the electrocardiogram (ECG) and systolic time intervals (STIs) were measured during a double-blind parallel-group study in depressed patients. 2. ECGs and STIs were measured after a 1 week placebo run-in, following 1 week's treatment with fluoxetine 40 mg daily or amitriptyline 100 mg daily, and then after 3 weeks' treatment with fluoxetine 60-80 mg daily or amitriptyline 150-200 mg daily. 3. Fluoxetine had no effect on the ECG or STIs at any dose. Amitriptyline 150-200 mg daily shortened the sinus cycle length by a mean of 12%, prolonged the PR interval by 8% and the QRS duration by 10%. Amitriptyline did not significantly alter the STIs.     

Antagonism towards endogenous adenosine and inhibition of cGI-PDE in the cardiac effects of amrinone, milrinone and related analogues.

1. The effect of amrinone, milrinone and of three milrinone analogues was tested on spontaneous chronotropic and inotropic activity of guinea-pig isolated atria, on the activity of cGMP-inhibited phosphodiesterase (cGI-PDE) from guinea-pig heart and on specific binding of N6-cyclohexyl[3H]adenosine ([3H]CHA) to Ri adenosine receptors in guinea-pig atria. 2. The Ki-values towards [3H]CHA binding to Ri receptors were linearly related to the EC50S for the increase in force of contraction but not to the EC50S for the increase in frequency of the atria. The Ki values towards cGI-PDE were linearly related to the EC50S for the positive chronotropic effect.     

Effects of sodium arsenite on the cytoskeleton and cellular glutathione levels in cultured cells.

The effects of As3+ (NaAsO2) on the microtubule and microfilament organization, cytoskeletal protein synthesis, cytoskeletal and cytosolic (soluble) protein sulfhydryls, and cellular glutathione (GSH) levels were examined in Swiss 3T3 mouse cells. Exposure of cells to 2.5 microM As3+ for 16 hr resulted in apparent cell retraction and loss of thick cables of actin filaments. However, the cells still retained numerous thinner microfilaments distributed in a disorganized manner. Microtubule organization was relatively undisturbed. At higher doses (greater than or equal to 20 microM), As3+ treatment caused a severe loss of microtubules and the remaining dense finer actin filaments formed smearing clusters in perinuclear areas. Treatment of cells with As3+ also induced a dose-dependent inhibition of cytoskeletal protein synthesis. Furthermore, As3+ exposure enhanced cellular GSH synthesis since the elevated cellular GSH content in As(3+)-treated cells could be abolished by treatment with buthionine sulfoximine, an inhibitor of gamma-glutamylcysteine synthetase required for GSH biosynthesis. As determined by the N-[3H]-ethylmaleimide binding assay, As3+ exposure also increased the amount of protein sulfhydryls in both the cytoskeletal and the cytosolic protein fractions. Moreover, a greater increase in protein sulfhydryls occurred in the cytoskeletal fraction than in the soluble fraction. These results indicate that the cytoskeleton could be a cellular target for injury by As3+ exposure. The elevated cellular GSH content induced by As3+ could provide a protective mechanism against further injury from this metal insult.     

Differential effects of 20 non-dioxin-like PCBs on basal and depolarization-evoked intracellular calcium levels in PC12 cells

Non-dioxin-like polychlorinated biphenyls (NDL-PCBs) are environmental pollutants that are well known for their neurotoxic effects. Numerous in vitro studies reported PCB-induced increases in the basal intracellular calcium concentration ([Ca(2+)](i)), and in vivo NDL-PCB neurotoxicity appears at least partly mediated by these disturbances. However, effects of NDL-PCBs on depolarization-evoked calcium influx are poorly investigated, and effects of several congeners, including PCB53, on calcium homeostasis are still unknown. We therefore studied the effects of 20 selected NDL-PCBs on basal and depolarization-evoked [Ca(2+)](i) in fura-2-loaded PC12 cells using single-cell fluorescence microscopy. The results demonstrate that hexa- and heptachlorobiphenyls (with the exception of PCB136) were unable to affect basal and depolarization-evoked [Ca(2+)](i). However, most tri- and tetrachlorinated as well as some pentachlorinated NDL-PCBs (at 1 and 10μM) increased basal [Ca(2+)](i) during a 15-min exposure. The increase in basal [Ca(2+)](i), which differed in kinetics for the different congeners, depended partly on influx of extracellular calcium and calcium release from the endoplasmic reticulum. Importantly, all tested tri- and tetrachlorinated biphenyls and some pentachlorinated NDL-PCBs (PCB95, PCB100, and PCB104) reduced depolarization-evoked [Ca(2+)](i), with PCB51 and PCB53 being most potent (near complete inhibition at 1μM). The reduction in depolarization-evoked calcium influx depended on the exposure duration but not on the foregoing PCB-induced increase in basal [Ca(2+)](i). The inhibition of voltage-gated calcium channels is a novel and sensitive mode of action for NDL-PCBs that contributes to the disturbances in calcium homeostasis and likely is related to NDL-PCB-induced (developmental) neurotoxicity.     

Phosphodiesterase isozyme inhibition, activation of the cAMP system, and positive inotropy mediated by milrinone in isolated guinea pig cardiac muscle

The purpose of the present study was to examine the interrelationships among phosphodiesterase (PDE) isozyme inhibition, cAMP formation, activation of cAMP-dependent protein kinase (cAPK), and positive inotropy in isolated guinea pig cardiac muscle mediated by the cardiotonic/vasodilator agent, milrinone. Milrinone was a potent and selective inhibitor of the "low Km" cAMP PDE isozyme (peak III) isolated by diethylaminoethyl ether cellulose chromatography, with IC50 values of 0.7 microM for peak III PDE and 100 microM for peak I PDE. In isolated papillary muscles frozen at peak inotropic responses, positive and significant correlations were evident between isometric force development as a function of cAMP content (r = 0.72, p less than 0.05) or cAPK activity ratio, an index of activation of cAPK (r = 0.79, p less than 0.001), for concentrations of milrinone from 0.1-1000 microM. Similar correlations were evident in muscles frozen at peak inotropic responses for the beta-adrenoreceptor agonist isoproterenol (r = 0.96, p less than 0.001; r = 0.98, p less than 0.001, respectively), but not for ouabain or Bay K-8644. The temporal sequence of these events was also quantitated for concentrations of milrinone (100 microM) and isoproterenol (3 nM) that produced approximately a 100% increase in isometric force. Whereas early time interval of force development (30 s, 1 min, isoproterenol; 30 s milrinone) were not accompanied by significant increases in either cAMP content or cAPK activity ratio, peak increases in force development for both isoproterenol (2 min) and milrinone (1 min) were related to peak increases in cAPK activity ratios. In summary, these results show that significant increases in cAMP content or cAPK activation are correlated with positive inotropy in isolated guinea pig papillary muscles with milrinone. These correlations occur at concentrations of milrinone that inhibit cardiac PDE isozymes and are similar to the known cAMP-dependent cardiostimulant isoproterenol. These data support the hypothesis that selective PDE isozyme inhibition is a mechanism by which milrinone effects positive inotropy.     

Comparison of the cellular and RNA-dependent effects of sangivamycin and toyocamycin in human colon carcinoma cells

The effects of the pyrrolopyrimidine antibiotics sangivamycin and toyocamycin on the synthesis of RNA and protein, ribosomal RNA processing, and cell viability were examined in colon carcinoma cell line HT-29. Exposure for 24 hr to toyocamycin caused an exponential type of cell lethality resulting in a 4-log reduction of cell viability, while sangivamycin produced a gradual and self-limiting type of cell lethality resulting in a 1-log reduction of cell viability. Toyocamycin, at a concentration of 1 microM produced total cessation of precursor rRNA processing, while 10 microM sangivamycin produced little or no effect on processing. On the contrary, sangivamycin caused a significant decrease in protein synthesis after 6 hr, while toyocamycin had less effect. The inhibition of protein synthesis by sangivamycin results from an inhibition of the formation of complexes essential to the initiation of protein synthesis. The results suggest that the mechanisms of action of these closely related agents are quite distinct. The marked loss of cell viability caused by toyocamycin correlates with its effect on rRNA processing, while the slow inhibition of protein synthesis appears to be secondary to the loss of ribosome synthesis. On the other hand, the lesser cytotoxicity produced by sangivamycin results from a more direct effect on protein synthesis. Importantly, cells are much less capable of resuming normal proliferative activity after 24 hr of impaired rRNA processing than after a similar interval of reduced protein synthesis.