Biomedical subjects

I Efimov

Publications and source records attributed to I Efimov.

10 recordsLinked to original sources

PubMed2005

Film resonance on acoustic wave devices: the roles of frequency and contacting fluid.

The dynamics of composite films of polypyrrole and sodium poly(styrenesulfonate) were studied by means of the electrochemical quartz crystal microbalance. Admittance spectra recorded after successive cycles of electrodeposition showed dramatic changes, which were interpreted in terms of acoustic resonance of the film. Reports of this phenomenon are rare and unquantified, presenting a unique opportunity for the first test of a recently reported theoretical model. The model, valid at frequencies in the vicinity of film resonance, is represented in terms of an equivalent electrical circuit with parallel LCR elements in the motional arm of the resonator. Since it was developed for viscoelastic films exposed to a vacuum, this provides an opportunity to test the importance of the fluid necessarily present in in situ electrochemical experiments. Measurements at the fundamental frequency and at higher harmonics reveal the sensitivity of film resonance effects to frequency and provide insights into film dynamics through the variation of shear moduli with time scale (frequency).

Journal Article↗

PubMed2004

A physical approach to remove anatomical reentries: a bidomain study.

Controlling cardiac chaos is often achieved by applying a large damaging electric shock-defibrillation. It removes all waves, without differentiating reentries and normal waves, anatomical and functional reentries. Anatomical reentries can be removed by anti-tachycardia pacing (ATP) as well. But ATP requires the knowledge of the position of the reentry, and an access to it with an invasive stimulating electrode. We show that the physics of electric field distribution between cardiac cells permits one to deliver an electric pulse exactly to the core of an anatomical reentry, without knowing its position and even to locations where access with a stimulating electrode is not possible. The energy needed is two orders of magnitude less than defibrillation energy. The results are insensitive to both a detailed ionic model and to the geometry of the fibers.

Arrhythmias, Cardiac↗

PubMed2004

Pinning force in active media.

Pinning of vortices by defects plays an important role in various physical (superconductivity, superfluidity, etc.) or biological (propagation in cardiac muscle) situations. Which defects act as pinning centers? We propose a way to study this general problem by using an advection field to quantify the attraction between an obstacle and a vortex. A full solution is obtained for the real Ginzburg-Landau equation (RGLE). Two pinning mechanisms are found in excitable media. Our results suggest strong analogies with the RGLE when the heterogeneity is excitable. Unpinning from an unexcitable obstacle is qualitatively harder, resulting in a stronger pinning force. We discuss the implications of our results to control vortices and propose experiments in a chemical active medium and in cardiac tissue.

Heart↗

PubMed2004

Unpinning and removal of a rotating wave in cardiac muscle.

Rotating waves in cardiac muscle may be pinned to a heterogeneity, as it happens in superconductors or in superfluids. We show that the physics of electric field distribution between cardiac cells permits one to deliver an electric pulse exactly to the core of a pinned wave, without knowing its position, and even to locations where a direct access is not possible. Thus, unpinning or removal of rotating waves can be achieved. The energy needed is 2 orders of magnitude less than defibrillation energy. This opens a way to new manipulations with pinned vortices both in experiments and in cardiac clinics.

Animals↗

PubMed2001

Effects of noncovalent and covalent FAD binding on the redox and catalytic properties of p-cresol methylhydroxylase.

Each flavoprotein subunit (alpha or PchF) of the alpha(2)beta(2) flavocytochrome p-cresol methylhydroxylase (PCMH) from Pseudomonas putida contains FAD covalently attached to Tyr384. PCMH oxidizes p-cresol to 4-hydroxybenzyl alcohol, which is oxidized subsequently by PCMH to 4-hydroxybenzaldehyde. The Y384F mutant form of PchF (apo-PchF[Y384F]) displayed stoichiometric noncovalent FAD binding. PchF[Y384F]FAD associated with the cytochrome subunit (beta or PchC) (producing PCMH[Y384F]), although not as avidly as with wild-type PchF containing covalently bound FAD (PchF(C)). Dramatic increases in the two-electron E(m,7) (NHE) values for FAD were observed when it bound noncovalently to either apo-PchF or apo-PchF[Y384F], and the two-electron E(m,7) value for FAD was increased further by about 75 mV upon covalent binding to PchF, i.e., PchF(C). The E(m,7) values increased by approximately 20 and 45 mV, respectively, when PchF(C) and PchF[Y384F]FAD associated with PchC. The two-electron E(m,7) for covalently bound FAD in PCMH is 84 mV, the highest measured for a flavoprotein. The values for the one-electron redox potentials (E(m,7), NHE) for FAD were measured also for various forms of PchF. Under anaerobiosis, the reduction of PchF[Y384F]FAD by substrates was similar to that observed previously for PchF containing noncovalently bound FAD. Stopped-flow kinetic studies indicated a rapid substrate reduction of the FAD and heme in PCMH[Y384F] which produced PchF[Y384F]FAD(rad) x PchC, the mutant enzyme containing the flavin radical and reduced heme. These experiments also revealed a slow reduction of unassociated PchC(ox) by PchF[Y384F]FAD(rad) x PchC. Steady-state kinetic studies of the reaction of PCMH[Y384F] with p-cresol indicated that the K(m) for this substrate was unchanged relative to that of PCMH, but that the k(cat) was diminished by an order of magnitude. The data indicate that the covalent attachment of FAD to PchF assists catalysis by raising the E(m,7) of the flavin. Contributions to this effect likely result from conformational changes.

Amino Acid Substitution↗

PubMed2001

Fluorescent imaging of a dual-pathway atrioventricular-nodal conduction system.

A dual-pathway theory to explain atrioventricular-nodal (AVN) reentry has been proposed previously. However, the exact anatomical and functional correlates of the fast pathway (FP) and slow pathway (SP) have not yet been elucidated. We used optical mapping to reconstruct patterns of activation during retrograde conduction through the AVN and during AVN reentry in the triangles of Koch of 12 rabbits. Reentry was inducible by a premature stimulation of the bundle of His in 6 preparations (50%). A functional FP and SP appear to be anatomically correlated with posterior and posterolateral extensions of the AVN, which were recently described. Retrograde breakthrough points in 6 noninducible preparations were clustered near the apex of the triangle of Koch (FP), whereas 6 inducible preparations had either cycle length-dependent FP and SP exits (n=3) or only SP exits located near the coronary sinus orifice. The shift of breakthrough points from FP to SP during progressive shortening of the coupling interval was accompanied by a discontinuity in the conduction curve. We observed a transmural reentrant circuit involving the AVN, FP, SP, and the superficial endocardial layer of atrial and transitional cells. The presence of a functional SP during retrograde conduction was associated with inducibility of AVN reentry. The full text of this article is available at http://www.circresaha.org.

Animals↗

PubMed1999

Properties of p-cresol methylhydroxylase flavoprotein overproduced by Escherichia coli.

The alpha(2)beta(2) flavocytochrome p-cresol methylhydroxylase (PCMH) from Pseudomonas putida is composed of a flavoprotein homodimer (alpha(2) or PchF(2); M(r) = 119 kDa) with a cytochrome monomer (beta, PchC; M(r) = 9.3 kDa) bound to each PchF subunit. Escherichia coli BL21(DE3) has been transformed with a vector for expression of the pchF gene, and PchF is overproduced by this strain as the homodimer. During purification, it was recognized that some PchF had FAD bound, while the remainder was FAD-free. However, unlike PchF obtained from PCMH purified from P. putida, FAD was bound noncovalently. The FAD was conveniently removed from purified E. coli-expressed PchF by hydroxyapatite chromatography. Fluorescence quenching titration indicated that the affinity of apo-PchF for FAD was sufficiently high to prevent the determination of the dissociation constant. It was found that p-cresol was virtually incapable of reducing PchF with noncovalently bound FAD (PchF(NC)), whereas 4-hydroxybenzyl alcohol, the intermediate product of p-cresol oxidation by PCMH, reduced PchF(NC) fairly quickly. In contrast, p-cresol rapidly reduced PchF with covalently bound FAD (PchF(C)), but, unlike intact PCMH, which consumed 4 electron equiv/mol when titrated with p-cresol (2 electrons from p-cresol and 2 from 4-hydroxybenzyl alcohol), PchF(C) accepted only 2 electron equiv/mol. This is explained by extremely slow release of 4-hydroxybenzyl alcohol from reduced PchF(C). 4-Hydroxybenzyl alcohol rapidly reduced PchF(C), producing 4-hydroxybenzaldehyde. It was demonstrated that p-cresol has a charge-transfer interaction with FAD when bound to oxidized PchF(NC), whereas 4-bromophenol (a substrate analogue) and 4-hydroxybenzaldehyde have charge-transfer interactions with FAD when bound to either PchF(C) or PchF(NC). This is the first example of a "wild-type" flavoprotein, which normally has covalently bound flavin, to bind flavin noncovalently in a stable, redox-active manner.

Bacterial Proteins↗

PubMed1998

Proposed steady-state kinetic mechanism for Corynebacterium ammoniagenes FAD synthetase produced by Escherichia coli.

The bifunctional enzyme, FAD synthetase (FS), from Corynebacterium ammoniagenes was overproduced in Escherichia coli and purified, and its steady-state kinetic properties were investigated. Although FMN is an intermediate product in the conversion of riboflavin to FAD, FMN must be released after formation, and then rebind for adenylylation. It was shown that adenylylation of FMN is reversible; FAD and pyrophosphate can be converted to FMN and ATP by the enzyme. In contrast, under the conditions studied, phosphorylation of riboflavin is irreversible. A method is described for analysis of two catalytic cycles, occurring on one enzyme, which have a substrate and/or product in common. The binding order for the phosphorylation cycle of FS was established as riboflavin(in), ATP(in), ADP(out), and FMN(out). The order for the adenylylation cycle was ATP(in), FMN(in), pyrophosphate(out), and FAD(out). A set of steady-state constants was determined, and without additional optimization, these constants were sufficient to describe experimental progress curves for conversion of riboflavin to FAD. In independent studies, it was demonstrated that FMN binds to apo-FS with a dissociation constant of 6-7 microM, which is 2 orders of magnitude higher than the KD value for riboflavin. For the steady-state kinetic analysis, this represents reversible binding of FMN(out) in the phosphorylation cycle (cycle I), which effectively inhibits catalysis in the adenylylation cycle (cycle II).

Adenosine Triphosphate↗

PubMed1997

Mechanism of atrioventricular nodal facilitation in the rabbit heart: role of the distal AV node.

We investigated whether atrioventricular (AV) nodal facilitation is the result of distal AV nodal action potential shortening. Atrial and bundle of His (H) electrograms and microelectrode recordings from proximal and distal AV nodal cells were analyzed in eight superfused rabbit AV node preparations in response to two pacing protocols. In the facilitation protocol, an atrial extrastimulus (A3) was preceded by an atrial impulse (A2) introduced 300, 200, 150, or 125 ms after 30 basic beats (A1). The preexcitation protocol differed from the facilitation protocol by the addition of a premature His depolarization (h2) such that the H1-h2 interval was shorter than the H1-H2 interval. Conduction curves (A3-H3 vs. H2-A3, h2-A3, and A2-A3 intervals) were constructed. Facilitation was demonstrated in all preparations when H2-A3 was used (P = 0.02) but not in the A2-A3 format. Compared with facilitation at the same A1-A2 intervals, preexcitation, despite shortening the distal cellular action potential duration, resulted in longer A3-H3 delays (P = 0.002), shorter A2-A3 intervals, and depression of the proximal nodal cellular response. Thus facilitation does not result from altered distal AV nodal characteristics and instead is a manifestation of an uncontrolled pacing protocol-dependent modulation of proximal AV nodal function.

Action Potentials↗

PubMed1997

Mechanism of atrioventricular nodal facilitation in rabbit heart: role of proximal AV node.

The phenomenon of atrioventricular (AV) nodal "facilitation," described in traditional "black box"-functional studies, implies enhanced AV nodal dromotropic function. We investigated the role of atrial prematurities in the modulation of the nodal cellular responses in the mechanism of AV nodal facilitation. Atrial and His (H) bundle electrograms and microelectrode recordings from proximal AV nodal cells were analyzed in 15 superfused rabbit AV node preparations. The pacing protocol consisted of 30 basic beats (S1; coupling interval S1-S1 = 300 ms) followed by a facilitating prematurity (S2; coupling intervals S1-S2 of 300, 200, 150, and 130 ms) followed by the test beat (S3; coupling interval S2-S3 scanned in 5-ms steps). Conduction curves (S2-H2 vs. S1-S2, S3-H3 vs. S2-S3, and S3-H3 vs. H2-S3) were constructed. Facilitation (i.e., shortening of S3-H3 when S1-S2 was shortened) was demonstrated in all preparations using the H2-S3 (P < 0.001) but not the S2-S3 format. Microelectrode recordings revealed a causal relationship between the improved proximal AV nodal cellular responses in facilitation and the prolonged S2-S3 interval. There was no evidence for enhanced nodal dromotropic function directly resulting from the introduction of the facilitating beats. Thus facilitation is based on inherent cycle-length-dependent properties of the AV node during application of a complex pacing protocol and primarily reflects the uncontrolled modulation of the proximal cellular response.

Animals↗