An adrenergic neuron blocking action of propranolol in isolated tissues

Propranolol was tested for adrenergic neuron blocking activity in three isolated sympathetically-innervated smooth muscle preparations; the rat vas deferens, rabbit ileum and rabbit ear artery. In each preparation propranolol impaired the responses to sympathetic stimulation without reducing the responses to added noradrenaline. This blocking action of propranolol resembled that of guanethidine in time of onset and persistence of blocking activity but, unlike blocking by guanethidine, was not reversed by (+)-amphetamine. Desipramine and noradrenaline also failed to reverse the blocking action of propranolol. In the rat vas deferens preparation lignocaine had a weaker and more transient sympathetic blocking action than propranolol. It is suggested that the sympathetic blocking action of propranolol may contribute to its antihypertensive effect in man.     

Local anesthetics (benzyl alcohol, lidocaine, procainamide) inhibit aminopyrine accumulation in isolated rat parietal cells

The present studies were designed to examine the effect of local anesthetics (benzyl alcohol, lidocaine, and procainamide) on the secretory response of parietal cells to histamine, dbcAMP, and carbachol. Studies were performed in vitro using isolated cells from rat stomachs, and acid production was determined by 14C-aminopyrine accumulation. In addition, the (H(+)-K+)-ATPase activity of microsomal vesicles isolated from parietal cells was determined. Lower concentrations of the drugs studied increased the basal aminopyrine accumulation and potentiated the secretory response of parietal cells to histamine and dbcAMP. At higher concentrations local anesthetics progressively inhibited both the basal 14C-aminopyrine accumulation and that stimulated by histamine, dbcAMP or carbachol. While a low concentration of local anesthetics increased gastric microsomal (H(+)-K+)-ATPase activity, higher concentrations inhibited enzyme activity to about 80% of those activities found in resting parietal cells. We conclude that increased aminopyrine accumulation may reflect the activation of membrane-bound enzyme(s) involved in the cAMP-dependent signal transduction pathway mediating acid secretion by parietal cells. In turn, it is possible that the inhibition of aminopyrine accumulation by local anesthetics at higher concentrations can relate to two different mechanisms: (1) the nonspecific effect of local anesthetics that causes simple proton neutralization (as weak bases), and (2) to a minor extent their inhibitory effect on proton pump activity.     

Mercury compounds: lipophilicity and toxic effects on isolated myocardial tissue

Lipophilicity is suggested to modulate the diffusion and the cytotoxic effects of mercury compounds. To investigate this, the positive inotropic effect of four Hg compounds (HgCl₂, CH₃HgCl, chlormerodrin, bromomercurihydroxypropane) was studied in catecholamine-depleted isolated heart muscle preparations. The rate of development of the positive effect was inversely correlated to the concentration in the case of HgCl₂ and chlormerodrin, i.e. the product of concentration (c) and time to halfmaximal effect (t₅₀) remained constant. This was in accordance with the assumption of a permeation-controlled rate of action, as was shown earlier forp-chloromercuriphenylsulfonic acid. In addition, the c×t₅₀ values of the individual mercurials decreased hyperbolically with the increase in lipophilicity as measured by the octanol/water partition. The results support the view that the toxicity of mercurials increases with their lipid solubility. In conjunction with the previously reported negative inotropic effect of Hg compounds, a model is proposed allocating thiol groups responsible for the negative inotropic action to lipid compartments within the cell membrane, while SH groups conveying the increase in contraction force are thought to be situated at the internal surface of the sarcolemma.     

Effects of charge and lipophilicity on mercurial-induced reduction of 45Ca2+ uptake in isolated nerve terminals of the rat.

The goal of this study was to compare the ability of neurotoxic mercurials which differ in ionic charge and/or lipophilicity to block nerve-terminal calcium channels. To do so, we examined the acute effects of methyl mercury (MeHg+), ethyl mercury (EtHg+), inorganic mercury (Hg2+), dimethyl mercury (Me2Hg), p-chloromercuribenzoate (PCMB), and p-chloromercuriphenyl-sulfonate (PCMBS-) (10-1000 microM) on 45Ca2+ flux into rat forebrain synaptosomes at rest and during depolarization. Basal (depolarization-independent) entry of 45Ca2+ was measured during 10-sec exposure to mercurials in solutions containing 5 mM KCl. Concentrations of 50, 100, 250, 500, and 1000 microM of Hg2+, MeHg+, and EtHg+ reduced basal influx of 45Ca2+. PCMB reduced basal influx at concentrations of 10, 50, and 100 microM, but increased influx at 1000 microM. PCMBS- and (Me)2Hg had no effect on basal flux at any concentration tested. Uptake of 45Ca2+ was measured after 1 sec of K(+)-induced depolarization (41.25 mM) to determine influx through voltage-dependent Ca2+ channels ("fast" phase) or during the last 10 sec of a 20-sec period of depolarization for uptake associated with a reversed Na+/Ca2+ exchanger and a residual noninactivating Ca2+ channel component ("slow" phase). Fast and slow components of 45Ca2+ uptake into synaptosomes were blocked in a concentration-dependent manner by MeHg+, EtHg+, and Hg2+. For block of the fast component, the calculated IC50's and confidence intervals were (microM) EtHg+, 92 (82, 102); Hg2+, 155 (149, 161); and MeHg+, 196 (120, 272). IC50's and the confidence intervals for the slow component of uptake were (microM) Hg2+, 49 (43, 55); MeHg+, 72 (67, 77); and EtHg+, 147 (142, 152). In contrast, Me2Hg, PCMB, and PCMBS- (10-1000 microM) caused no appreciable reduction in either phase of 45Ca2+ uptake. Increasing [Ca2+]e was unable to overcome the block induced by MeHg+ and EtHg+ (100 microM) on either phase of 45Ca2+ uptake into synaptosomes. Likewise, increasing [Ca2+]e failed to overcome block of the slow component by Hg2+ (100 microM). Increasing [Ca2+]e was able to overcome, in part, block of the fast phase induced by Hg2+ (100 microM) although the percentage of reversal was not statistically significant. The magnitude of block of 45Ca2+ uptake increased as a function of increasing [K+]e for MeHg+ and EtHg+, suggesting the block to be voltage-dependent. Thus, mercurials of dissimilar charge and lipophilicity affect synaptosomal Ca2+ uptake differentially.(ABSTRACT TRUNCATED AT 400 WORDS)     

异氟烷和七氟烷对缺血神经元c-fos蛋白表达的影响和保护作用

目的　了解吸入麻醉药异氟烷和七氟烷对脑缺血神经元c fos蛋白表达的影响和保护作用。方法　用苏木素 伊红 (HE)染色法检测大鼠脑缺血梗塞体积和海马结构CA1～CA4亚区神经元损伤程度。用免疫组化法检测海马结构c fos蛋白的表达。结果　缺血 1h再灌 72h异氟烷组和七氟烷组脑梗塞体积显著小于缺血组 ;海马CA3和CA4亚区神经损伤程度也显著小于缺血组。缺血 1h再灌 4h ,在海马齿回 ,CA4亚区c fos蛋白样免疫反应率 ,吸入麻醉药组明显低于未吸入组。结论　异氟烷和七氟烷对缺血神经元有一定的保护作用。它们对c fos蛋白表达的衰减作用可能与其拮抗NMDA受体性质有关     

Macrolide antibiotics: binding site,mechanism of action,resistance

Macrolides are among the most clinically important antibiotics. However, many aspects of macrolide action and resistance remain obscure. In this review we summarize the current knowledge, as well as unsolved questions, regarding the principles of macrolide binding to the large ribosomal subunit and the mechanism of drug action. Two mechanisms of macrolide resistance, inducible expression of Erm methyltransferase and peptide-mediated resistance, appear to depend on specific interactions between the ribosome-bound macrolide molecule and the nascent peptide. The similarity between these mechanisms and their relation to the general mode of macrolide action is discussed and the discrepancies between currently available data are highlighted.     

Common modes of drug action on Na+ channels: local anesthetics,antiarrhythmics and anticonvulsants

The development of agents useful in local anesthesia, therapy of cardiac arrhythmias, and prevention of epileptic seizures has proceeded along independent pathways with use of appropriate pharmacological models for drug testing. William Catterall reviews recent studies of the mechanism of action of these agents in peripheral nerve, cardiac muscle cells and central neurons. These have focused on a common array of effects on voltage-sensitive Na⁺ channels which may underly much of their therapeutic utility. Na⁺ channels are found to be inhibited at therapeutic concentrations of these agents in a voltage- and frequency-dependent manner that selectively blocks excitability of abnormally firing cells while leaving normally functioning cells relatively unaffected. An allosteric or ‘modulated receptor’ model of drug action is sufficient to describe the major features of the voltage- and frequency-dependent action of all three classes of drugs.     

Mechanism of action of volatile anesthetics: involvement of intracellular calcium signaling

There have been extensive efforts to characterize the mechanism of action of volatile anesthetics, but their molecular and cellular actions are still a matter of debate. Volatile anesthetics act primarily on synaptic transmission in the central nervous system but proof of this as the predominant mechanism of action remains elusive. Changes in neurotransmitter release may relate to direct interaction of the anesthetic molecule with an ion channel protein or synaptic protein, but can also be a consequence of alterations in intracellular signaling. Calcium is one of the most important messengers in cells and its intracellular concentration may be modified by several agents including volatile anesthetics. Neuronal excitability is in part determined by calcium availability that is controlled by several mechanisms. Because voltage-gated calcium channels (VGCC) play a key role in controlling Ca2+ entry and in initiating cellular responses to stimulation through an elevation of intracellular calcium concentration ([Ca2+](i)), they are thought to be one of the targets for volatile anesthetics. However, [Ca2+](i) can also be altered without the participation of VGCC through receptor-mediated pathways. Indeed, calcium homeostasis is also controlled by plasma membrane Ca2+ -adenosine triphosphatase, sarcoplasmic-endoplasmic reticular Ca2+ -ATPase, the Na+ -Ca2+ exchanger, and mitochondrial Ca2+ sequestration. Alteration of any of those mechanisms that control [Ca2+](i) may lead to a change in presynaptic transmission or postsynaptic excitability. Here we will review some of the recent progress in identifying putative actions of volatile anesthetics, specifically the effect on intracellular calcium homeostasis in neurons.     

Ifenprodil, a novel NMDA receptor antagonist: site and mechanism of action

Ifenprodil is a novel N-methyl-D-aspartate (NMDA) receptor antagonist that selectively inhibits receptors containing the NR2B subunit. As such, it has become widely used as a tool to study subtypes of NMDA receptors both in vitro and in vivo, and as a tool for molecular studies of the properties and regulation of NMDA receptors. Ifenprodil has an unusual form of activity-dependence and its mechanism of action may involve an increase in proton inhibition of NMDA receptors. These properties are shared by analogs or derivatives of ifenprodil, some of which may be lead compounds for therapeutically useful NMDA antagonists. Such antagonists have potential as neuroprotectants, anticonvulsants, analgesics, and for the treatment of Parkinson's disease and other disorders of the nervous system. The location of the ifenprodil binding site on NMDA receptors and the structural and mechanistic basis of its effects are still unknown. Recent work suggests that at least part of the ifenprodil binding site is located in the R1/R2 domain of the NR1 subunit. This region, like the S1/S2 agonist binding domain, shares homology with bacterial periplasmic binding proteins.     

Dissociation,solubility and lipophilicity of azathioprine

The thermodynamic pK_a values of 7.87 and 7.99 were determined for azathioprine (I) at 25°C by spectrophotometric and solubility methods, respectively. The partition coefficient of undissociated I between l-octanol and 0.01 N acetic acid (pH = 3.6) at 25°C was 1.25, and the partition ratio of I between 0.04 M phosphate buffer (pH = 7.4) and l-octanol at 37°C was 1.04.The solubility of I in water was 0.130 mg/ml and the intrinsic solubility at pH 4.08 in 0.02 N acetate buffer was 0.124 mg/ml at 25°C.Intramolecular hydrogen bonding between the N-7 hydrogen and C-6 sulfur atoms in 6-mercaptopurine (II) accounts for the approximately 1000 times greater K_a of the tautomeric proton at positions 7 and 9 in I compared to the N-7 proton in II.     

Effects of amiloride on active sodium transport by the isolated frog skin: Evidence concerning site of action

1. Amiloride reduces short-circuit current and potential difference across the isolated frog skin.2. Isotopically measured sodium influx and efflux are diminished.3. Total electrical conductance and partial sodium conductance are diminished, the reduction in total conductance being entirely accounted for by the reduction in partial sodium conductance.4. The effect of antidiuretic hormone (ADH), cyclic 3'5'-adenosine monophosphate (cyclic AMP) and theophylline can be antagonized by pretreatment with amiloride but the antagonism can be abolished by increasing the concentration of these compounds.5. Amiloride has no effect on oxygen consumption in concentrations which inhibit sodium transport. However, it prevents the stimulatory effect of ADH on oxygen consumption.6. The results are consistent with an action of amiloride at the passive outside membrane of the transporting cells of isolated frog skin.     

Extravascular transport of drugs in tumor tissue: effect of lipophilicity on diffusion of tirapazamine analogues in multicellular layer cultures

The extravascular diffusion of antitumor agents is a key determinant of their therapeutic activity, but the relationships between physicochemical properties of drugs and their extravascular transport are poorly understood. It is well-known that drug lipophilicity plays an important role in transport across biological membranes, but the net effect of lipophilicity on transport through multiple layers of tumor cells is less clear. This study examines the influence of lipophilicity (measured as the octanol-water partition coefficient P) on the extravascular transport properties of the hypoxic cytotoxin tirapazamine (TPZ, 1) and a series of 13 neutral analogues, using multicellular layers (MCLs) of HT29 human colon carcinoma cells as an in vitro model for the extravascular compartment of tumors. Flux of drugs across MCLs was determined using diffusion chambers, with the concentration-time profile on both sides of the MCL measured by HPLC. Diffusion coefficients in the MCLs (D(MCL)) were inversely proportional to M(r)(0.5) (M(r), relative molecular weight), although this was a minor contributor to differences between compounds over the narrow M(r) range investigated. Differences in lipophilicity had a larger effect, with a sigmoidal dependence of D(MCL) on log P. Correcting for M(r) differences, lipophilic compounds (log P > 1.5) had ca. 15-fold higher D(MCL) than hydrophilic compounds (log P < -1). Using a pharmacokinetic/pharmacodynamic (PK/PD) model in which diffusion in the extravascular compartment of tumors is considered explicitly, we demonstrated that hypoxic cell kill is very sensitive to changes in extravascular diffusion coefficient of TPZ analogues within this range. This study shows that simple monosubstitution of TPZ can alter log P enough to markedly improve extravascular transport and activity against target cells, especially if rates of metabolic activation are also optimized.