Critical quantum content for shortening of endplate currents in the frog skeletal muscle.

The decay time of endplate currents was followed during progressive lowering of quantum content of endplate responses by reduced Ca2+. A certain critical value of about 100 quanta was found, when the decay of endplate currents remained constant even though the quantal content was reduced further.     

Effects of hyaluronidase on miniature endplate potentials and currents at the frog neuromuscular junction

The effects of 0.1% testicular hyaluronidase on miniature endplate potentials and currents (MEPP and MEPC) were investigated in frog pectorocutaneous muscle. The action of hyaluronidase on preparations with armine-induced blockade of acetylcholinesterase was associated with decreased amplitude and duration of MEPP and MEPC half-decay time and rising phase. The correlation between amplitude and half-decay time of MEPP and MEPC declined at the same time, while MEPC decay remained exponential. Treating preparations having intact acetylcholinesterase with hyaluronidase increased the length of MEPC halfdecay, with duration of the rising phase and amplitude remaining constant. It is suggested that enzymatic breakdown of a proportion of the glycocalix of cells forming the neuromuscular junction and a portion of the extracellular matrix at the synaptic cleft leads to attenuation of nonspectific acetylcholine binding, thus facilitating acetylcholine diffusion into the synaptic cleft.A. A. Zhdanov State University, Leningrad. Translated from Neirofiziologiya, Vol. 20, No. 1, pp. 113–119, January–February, 1988.     

Quantitative estimation of synaptic acetylcholinesterase inhibition with galanthamine using parameters of miniature endplate currents

Miniature end-plate currents (MEPC) were recorded in voltage clamped muscle fibers of the rat diaphragm at different degrees of acetylcholinesterase (AChE) inhibition with galanthamine. A model has been suggested connecting the increase in MEPC amplitude with the concentration of a competitive reversible AChE inhibitor. Using the model suggested, the changes in the junctional AChE activity inhibited with different concentrations of galanthamine were estimated. The calculated value of the inhibitory galanthamine constant is 2.8 X 10(-7) M.     

The permeability of the endplate channel to organic cations in frog muscle

The relative permeability of endplate channels to many organic cations was determined by reversal-potential criteria. Endplate currents induced by iontophoretic "puffs" of acetylcholine were studied by a Vaseline gap, voltage clamp method in cut muscle fibers. Reversal potential changes were measured as the NaCl of the bathing medium was replaced by salts of organic cations, and permeability ratios relative to Na+ ions were calculated from the Goldman-Hodgkin-Katz equation. 40 small monovalent organic cations had permeability ratios larger than 0.1. The most permeant including NH4+, hydroxylamine, hydrazine, methylamine, guanidine, and several relatives of guanidine had permeability ratios in the range 1.3--2.0. However, even cations such as imidazole, choline, tris(hydroxymethyl)aminomethane, triethylamine, and glycine methylester were appreciably permeant with permeability ratios of 0.13--0.95. Four compounds with two charged nitrogen groups were also permeant. Molecular models of the permeant ions suggest that the smallest cross-section of the open pore must be at least as large as a square, 6.5 A x 6.5 A. Specific chemical factors seem to be less important than access or friction in determining the ionic selectivity of the endplate channel.     

Potent inhibition of human leukocyte elastase by 1,2,5-thiadiazolidin-3-one 1,1 dioxide-based sulfonamide derivatives

The design, synthesis, and in vitro biochemical evaluation of a class of mechanism-based inhibitors of human leukocyte elastase (HLE) that incorporate in their structure a 1,2,5-thiadiazolidin-3-one 1,1 dioxide scaffold with appropriate recognition and reactivity elements appended to it is described. The synthesized compounds were found to be efficient, time-dependent inhibitors of HLE. The interaction of the inhibitors with HLE is postulated to lead to the formation of a highly reactive N-sulfonyl imine (a Michael acceptor) that arises from an enzyme-induced sulfonamide fragmentation cascade. Subsequent reaction ultimately leads to the formation of a relatively stable acyl enzyme. The results cited herein demonstrate convincingly the superiority of the 1,2,5-thiadiazolidin-3-one 1,1 dioxide scaffold over other scaffolds (e.g., saccharin) in the design of inhibitors of (chymo)trypsin-like serine proteases.     

The rising phase of the miniature endplate current at the frog neuromuscular junction

(1) The rising phase of minature endplate currets was recorded at the frog's neuromuscular junction using both the two electrode voltage clamp and a single external electrode, or Strickholm, voltage clamp. (2) The $\text{Q}$(10) of the miniature endplate current rising phase was 2.3 in a variety of solutions selected to alter presynaptic behavior. (3) Increasing the solution's viscosity by an amount sufficient to slow the diffusion coefficient of acetylcholine by a third has no effect on the duration of the rising or the decay phase. This solution does seem to further slow the miniature endplate current decay phase, but not the rising phase, after inhibition of the acetylcholinesterase. (4) As the membrane potential is made more positive, the miniature endplate current rising phase is prolonged, with an $\text{e-}\text{fold}$ slowing per 170 mV change. (5) It is concluded that neither presynaptic nor subsynaptic events determine the rising phase of miniature endplate currents at the frog neuromuscular junction. Rather, the limiting step occurs within the membrane and is most likely a change in the binding constant of the receptor for the acetylcholine molecule.     

Stability constants and multinuclear NMR measurements of phosphonic acid derivatives with aluminum in aqueous solutions

We are reporting the stability constants of the different complexes between phosphonoacetic acid (PAA), phosphonoformic acid (PFA), aminomethylphosphonic acid (AMPA), aminoethylphosphonic acid (AEPA) and methylenediphosphonic acid (MDP) with the aluminum metal ion in aqueous solutions. (In this study the term aluminum is reserved for the 3+ ion.) The affinity of the phosphonic acid derivatives to chelate aluminum has been tested by potentiometric titrations with I = 0.10 M KNO₃ at 25 ± 0.1 °C. The proposed aluminum–ligand complex structures have been confirmed by ³¹P NMR, ¹³C NMR, and ²⁷Al NMR experiments. Both PAA and PFA formed simple one to one complexes. The respective values for PAA are log β₁₁₁ = 13.57, log β₁₁₀ = 10.58, and log β₁₁₋₁ = 5.84. The respective values for PFA are log β₁₁₂ = 15.24, log β₁₁₁ = 13.11, and log β₁₁₀ = 6.88. In contrast to PAA and PFA, the major species formed with AMPA and AEPA consist of a series of dimeric complexes. The ³¹P NMR spectra of these complexes indicate that the amine groups do not co-ordinate to aluminum and remain protonated. In addition to these dimeric complexes, a monoprotonated monomer of Al–AMPA also has been detected. The ²⁷Al NMR experiments indicated that the aluminum is hexacoordinated in all the complexes in this study and the hydroxide ion did not remove aluminium from its complexes. Among the ligands studied, PAA and PFA were able to solubilize aluminum at physiological pH. A comparison between the acidities and the chelating properties of the ligands used is presented.     

The inhibition of deoxyribonucleic acid nucleotidyltransferase by stilboestrol derivatives

The inhibition by diethylstilboestrol of DNA nucleotidyltransferase isolated from calf thymus was studied. The inhibition exercised by diethylstilboestrol appears to obey competitive kinetics with respect to DNA primer. The activities of both replicative and terminal enzymes were affected to the same extent. There was no evidence of binding between DNA and diethylstilboestrol. A comparative study of the inhibitory effects of some stilboestrol derivatives is presented. The alkyl substitution in the alphaalpha'-positions seem to alter the inhibitory effect of these compounds: dimethylstilboestrol was more inhibitory than stilbene, and diethylstilboestrol was more inhibitory than dimethylstilboestrol. Hexoestrol, in which the alphaalpha'-ethylenic linkage is saturated, was the most effective inhibitor.     

Potent inhibition of serine proteases by heterocyclic sulfide derivatives of 1,2,5-thiadiazolidin-3-one 1,1 dioxide

The existence of subtle differences in the Sn' subsites of closely-related (chymo)trypsin-like serine proteases, and the fact that the 1,2,5-thiadiazolidin-3-one 1,1 dioxide scaffold docks to the active site of (chymo)trypsin-like enzymes in a substrate-like fashion, suggested that the introduction of recognition elements that can potentially interact with the Sn' subsites of these proteases might provide an effective means for optimizing enzyme potency and selectivity. Accordingly, a series of heterocyclic sulfide derivatives based on the 1,2,5-thiadiazolidin-3-one 1,1 dioxide scaffold (I) was synthesized and the inhibitory activity and selectivity of these compounds toward human leukocyte elastase (HLE), proteinase 3 (PR 3) and cathepsin G (Cat G) were then determined. Compounds with P1 = isobutyl were found to be potent, time-dependent inhibitors of HLE and, to a lesser extent PR 3, while those with P1 = benzyl inactivated Cat G rapidly and irreversibly. This study has demonstrated that 1,2,5-thiadiazolidin-3-one 1,1 dioxide-based heterocyclic sulfides are effective inhibitors of (chymo)trypsin-like serine proteases.     

Effect of exogenous acetylcholine on potassium currents in the frog motor nerve ending during acetylcholinesterase inhibition

The effect of exogenous acetylcholine (ACh) on potassium currents in the motor nerve ending (NE) has been studied in neuromuscular preparations of the frog cutaneous-sternal muscle by extracellular recording of evoked electrical potentials from the NE. The investigation was performed during inhibition of acetylcholinesterase (AChE) activity by specific inhibitors and AChE removal from the synaptic cleft by collagenase. After AChE inhibition by either armine or proserine, or after treatment of the preparation with collagenase, no effect of exogenous ACh in concentrations of 1·10^–4–6·^–4 mole/liter was observed, in contrast to results from preparations with intact AChE. However, under the same conditions, as in the case of active AChE, ACh in concentrations of 7·10^–4–2·10^–3 mole/liter inhibited Ca-activated potassium current of the NE membrane. Experiments with dipyroxim, a synaptic AChE reactivator, have shown that the ACh effect on the potential-dependent potassium current is mediated by specific AChE. The role of AChE is discussed in respect to its significance for realization of the ACh action on potential-dependent potassium current in NE.Translated from Neirofiziologiya, Vol. 25, No. 2, pp. 146–149, March–April, 1993.     

Discovery of novel phosphonic acid derivatives as new chemical leads for inhibitors of TNF-alpha production

2-(Acylamino)benzylphosphonic acid 6 derived from an artificial substrate of sphingomyelinase was found to show inhibitory activity of TNF-alpha production. Structural optimization was started with the chemical modification of 6. The discovery of another chemical leads 7, 8, 10 and 16 for the development of structurally new inhibitors of TNF-alpha production is reported.     

Comparative analysis of the effects of synthetic derivatives of batrachotoxin on sodium currents in frog node of Ranvier

1. In voltage-clamp experiments on frog myelinated nerve fibers, the effects of nine synthetic derivatives of batrachotoxin (BTX) obtained from 7,8-dihydrobatrachotoxinin A (DBTX-A) on Na+ currents (INa) have been investigated. 2. Both of 20 alpha-esters of DBTX-A with 2,4,5-trimethylpyrrol-3-carboxylic acid (DBTX-P) and benzoic acid (DBTX) at a 10(-5) M concentration caused modification of INa qualitatively similar to that induced by BTX. 3. The quaternary derivative of DBTX (QDBTX) produced such changes in INa only at a 5.10(-4) M concentration, apparently due to its much lower lipid solubility. 4. Replacement of a -CH2- by a -C = O. group in the homomorpholine ring near the tertiary nitrogen atom abolished the DBTX activity, strongly suggesting the necessity of tertiary nitrogen protonation for the toxin interaction with the channel receptor. 5. Transfer of an 11-hydroxygroup from the alpha- to the beta-position in the DBTX molecule did not decrease its activity in spite of the fact that in the beta-position this group is sterically very hindered. The activity of 11 beta-DBTX is at variance with the prediction of Codding's (1983) "oxygen triad" hypothesis. 6. DBTX-A and compounds obtained from DBTX by oxidation of the 11 alpha-hydroxygroup (K-DBTX), acetylation (Ac-DBTX), or reduction of the hemiketal moiety (H2DBTX) even at a concentration as high as 10(-3) M were able to modify only a very small fraction of the Na channels. However, a clear-cut reversible blocking action on both normal and modified Na channels was observed. 7. These results led us to conclude that BTX modifies the Na channels only in the charged form and hemiketal and 20 alpha-ester moieties provide adequate disposition of toxin on the receptor surface. The inability of H2DBTX, DBTX-A, and K-DBTX and Ac-DBTX to modify most of the Na channels can be explained by a low "probability of correct disposition" of these ligands on the receptor surface.     

Synthesis and cholinesterase inhibition of cativic acid derivatives

Alzheimer’s disease (AD) is a neurodegenerative disorder associated with memory impairment and cognitive deficit. Most of the drugs currently available for the treatment of AD are acetylcholinesterase (AChE) inhibitors. In a preliminary study, significant AChE inhibition was observed for the ethanolic extract of Grindelia ventanensis (IC₅₀ = 0.79 mg/mL). This result prompted us to isolate the active constituent, a normal labdane diterpenoid identified as 17-hydroxycativic acid (1), through a bioassay guided fractionation. Taking into account that 1 showed moderate inhibition of AChE (IC₅₀ = 21.1 μM), selectivity over butyrylcholinesterase (BChE) (IC₅₀ = 171.1 μM) and that it was easily obtained from the plant extract in a very good yield (0.15% w/w), we decided to prepare semisynthetic derivatives of this natural diterpenoid through simple structural modifications. A set of twenty new cativic acid derivatives (3–6) was prepared from 1 through transformations on the carboxylic group at C-15, introducing a C2–C6 linker and a tertiary amine group. They were tested for their inhibitory activity against AChE and BChE and some structure–activity relationships were outlined. The most active derivative was compound 3c, with an IC₅₀ value of 3.2 μM for AChE. Enzyme kinetic studies and docking modeling revealed that this inhibitor targeted both the catalytic active site and the peripheral anionic site of this enzyme. Furthermore, 3c showed significant inhibition of AChE activity in SH-SY5Y human neuroblastoma cells, and was non-cytotoxic.     

Synthesis and proteasome inhibition of glycyrrhetinic acid derivatives

This study discovered that glycyrrhetinic acid inhibited the human 20S proteasome at 22.3microM. Esterification of the C-3 hydroxyl group on glycyrrhetinic acid with various carboxylic acid reagents yielded a series of analogs with marked improved potency. Among the derivatives, glycyrrhetinic acid 3-O-isophthalate (17) was the most potent compound with IC(50) of 0.22microM, which was approximately 100-fold more potent than glycyrrhetinic acid.     

The spatial distribution of currents in the receptor layer of the frog retina

Steady state patterns of currents flowing extracellularly in the receptor layer of the frog retina are used to work out an electrical model of vertebrate rods. The model can be thought of as a linear network consisting of resitances and current sources. The model predicts the axial spread of currents along the outside of the receptors. In addition, it predicts membrane voltages in light and darkness. If the dark-adapted receptors are assumed to have a uniform membrane with a specific resistance of 500 ·cm², a single rod gives rise to an extracellular dark current of about 200 pa.