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S I Zakharov

Publications and source records attributed to S I Zakharov.

At least 19 recordsLinked to original sources
PubMed1999

Monovalent cation and L-type Ca2+ channels participate in calcium paradox-like phenomenon in rabbit aortic smooth muscle cells.

1. The effects of removal of extracellular divalent cations (experimental calcium paradox conditions) were studied on the whole-cell current in freshly isolated smooth muscle cells (SMCs), and on contraction in rabbit aortic rings. 2. Aortic rings treated for 30-60 min with extracellular Ca2+- and Mg2+-free solution contracted following readmission of extracellular Ca2+, even in the presence of nifedipine. 3. In isolated SMCs, the removal of extracellular Ca2+ and Mg2+ induced a non-inactivating whole-cell inward current and membrane depolarization. This current was a monovalent cation (MC) current which reversed at around 0 mV and conducted K+ >= Cs+ > Na+ > Li+. Extracellular divalent cations (Ca2+, Mg2+, Ba2+, Mn2+ and Ni2+) inhibited MC current. 4. Using noise analysis of the whole-cell MC current, the single MC channel conductance was estimated to be < 450 fS. 5. MC current was insensitive to nifedipine, TEA, 4-aminopyridine, SK&F 96365 and S-nitroso-N-acetyl-penicillamine (SNAP), but was decreased by amiloride and low pH. 6. When EGTA was present in Ca2+- and Mg2+-free solution, a significant nifedipine-sensitive Na+ current through L-type Ca2+ channels developed in addition to MC current. 7. It is concluded that upon the removal of extracellular Ca2+ and Mg2+ from resting SMCs, an inward MC current develops allowing Na+ influx and causing SMC depolarization which could be the important steps leading to vessel contraction upon Ca2+ readmission. Addition of EGTA to Ca2+- and Mg2+-free solution greatly potentiates Na+ influx and vessel contraction by allowing additional Na+ influx through L-type Ca2+ channels which are activated presumably by MC current-induced depolarization.

4-Aminopyridine↗
PubMed1997

Rebound stimulation of the cAMP-regulated Cl- current by acetylcholine in guinea-pig ventricular myocytes.

1. Acetylcholine (ACh)-induced rebound stimulation of the cAMP-regulated Cl- current was studied in isolated guinea-pig ventricular myocytes using dialysing and dialysis-limiting configurations of the whole-cell patch-clamp technique. 2. Exposure to and subsequent washout of ACh produced a transient rebound stimulation of the Cl- current. However, this rebound response was only observed in the presence of submaximally stimulating concentrations of the cAMP-producing agonists isoprenaline (Iso) or histamine. ACh-induced rebound stimulation was not observed in the presence of maximally stimulating concentrations of Iso, nor was it observed in the absence of Iso. 3. To prevent saturation of responses during rebound, the effects of ACh were studied in the presence of a subthreshold concentration of Iso (0.001 microM). Varying the duration of exposure to ACh before washout demonstrated that the stimulatory effect of 1 microM ACh approaches steady state with a time constant of 34 s. Exposing myocytes to varying concentrations of ACh for 90 s demonstrated that the EC50 for the stimulatory effect of ACh was 0.15 microM with a maximum response equal to 67% of that obtained by a maximally stimulating concentration of Iso alone. 4. Rebound stimulation of the Cl- current could also be elicited by washing in 2 microM atropine during exposure to ACh, instead of washing out ACh. Furthermore, ACh-induced rebound was blocked by the M2 muscarinic receptor antagonist methoctramine but not by the M1 receptor antagonist pirenzepine. Rebound was also blocked in pertussis toxin (PTX)-treated myocytes. 5. ACh-induced rebound stimulation was not blocked by: (a) L-NMMA, an inhibitor of nitric oxide synthase activity; (b) Methylene Blue, LY-83583, and ODQ, inhibitors of cGMP production; or (c) milrinone, an inhibitor of cGMP-dependent phosphodiesterase activity. 6. These results indicate that ACh can stimulate cAMP-regulated ion channel activity in cardiac ventricular myocytes by facilitating beta-adrenergic and histaminergic responses. This is opposite to the inhibitory actions more typically associated with muscarinic receptor stimulation in ventricular myocardium. This stimulatory effect of ACh is mediated through M2 muscarinic receptors and a PTX-sensitive G-protein, but it does not appear to involve the production of nitric oxide or cGMP.

Acetylcholine↗
PubMed1996

Nitric oxide synthase activity in guinea pig ventricular myocytes is not involved in muscarinic inhibition of cAMP-regulated ion channels.

It has recently been demonstrated that NO plays an obligatory role in muscarinic inhibition of beta-adrenergically stimulated ion channels in cardiac sinoatrial node cells (J Gen Physiol. 1995;106:45-65). We looked for evidence that NO might play a similar role in ventricular cells by using histochemical staining for NO synthase (NOS) activity and whole-cell patch-clamp recording of cAMP-regulated Cl- currents. Myocytes isolated from guinea pig hearts stained positively for NADPH-diaphorase activity, suggesting that these cells do express NOS. Acetylcholine (ACh) inhibition of the R(-)-isoproterenol bitartrate (Iso)-activated Cl- current was also reversed by the cGMP-lowering agents LY-83583 and methylene blue, consistent with idea that NO activation of guanylate cyclase may contribute to muscarinic responses. However, LY-83583 and methylene blue activated the Cl- current in the presence of subthreshold concentrations of Iso alone, suggesting that their effects may not be due to antagonism of an NO/cGMP-dependent response. Furthermore, ACh inhibition of Iso-activated Cl- currents could not be mimicked by the NO donors sodium nitroprusside,3-morpholinosydnonimine, and spermine-NO. Similarly, ACh inhibition of the Iso-activated Cl- current could not be blocked by the NOS inhibitor NG-monomethyl-L-arginine. These results indicate that even though ventricular myocytes possess NOS activity, NO production does not play an important role in muscarinic inhibition of beta-adrenergically regulated Cl- channels in these cells.

Acetylcholine↗
PubMed1995

Altered beta-adrenergic and muscarinic response of CFTR Cl- current in dialyzed cardiac myocytes.

Autonomic regulation of the cardiac cystic fibrosis transmembrane conductance regulator (CFTR) Cl- current was studied in isolated guinea pig ventricular myocytes using various configurations of the whole cell patch-clamp technique. When currents were recorded using the conventional patch-clamp technique, it was possible to continue to activate the Cl- current on repeated exposure to isoproterenol (Iso) for up to 60 min after initiating dialysis. However, there was significant rundown of the magnitude of the Cl- current response to the maximally stimulating concentrations of Iso. In addition, the concentration of Iso that produced half-maximal activation of the Cl- current (K1/2) increased with time. Conversely, the K1/2 for acetylcholine inhibition of the Iso-activated current decreased with time. When currents were recorded using the perforated patch-clamp technique, the sensitivity to both beta-adrenergic- and muscarinic-receptor stimulation was stable. Immediately after initiation of dialysis with the conventional patch-clamp technique, the sensitivity to Iso was nearly identical to that determined using the perforated patch-clamp technique. However, the initial sensitivity to muscarinic-receptor activation was significantly greater. These results indicate that cell dialysis associated with conventional patch-clamp techniques not only results in a time-dependent rundown of current amplitude, but it also significantly alters the concentration dependence of beta-adrenergic and muscarinic-receptor regulation of ion channel function.

Acetylcholine↗
PubMed1995

Muscarinic regulation of the cardiac CFTR Cl- current by quaternary ammonium compounds.

In guinea pig ventricle, the protein kinase A-regulated Cl- current (ICl) is conducted by an alternatively spliced isoform of the cystic fibrosis transmembrane conductance regulator. We studied muscarinic regulation of this current using the whole-cell configuration of the patch-clamp technique. Acetylcholine (ACh) antagonized activation of ICl activated by 1 microM isoproterenol (ISO) in a concentration-dependent manner. The concentration of ACh that produced a half-maximal effect (K1/2) was 36 nM, the slope factor was 1.1, and the relative magnitude of the Cl- conductance at maximally effective concentrations of ACh (Gmin) was 21% of that observed in the presence of ISO alone. In the presence of 100 nM atropine, a competitive antagonist at the muscarinic receptor, the K1/2 value for ACh inhibition of ICl was increased to 4.3 microM, but the slope factor and Gmin were not affected, which indicated that the dissociation constant (KB) for atropine was < 1 nM. ACh-induced inhibition of the ISO-activated ICl was also blocked by the quaternary ammonium compound tetraethylammonium (TEA). Like atropine, TEA increased the K1/2 value for ACh inhibition of ICl without affecting the slope factor or Gmin. Schild analysis confirmed that TEA is also a competitive antagonist at the muscarinic receptor, with a KB value of 137 microM. However, tetramethylammonium (TMA), a structurally related compound, acted as an agonist at the muscarinic receptor. TMA inhibited ICl activated by 1 microM ISO with a K1/2 value of 342 microM, a slope factor of 0.87 and a Gmin value of 17%. Increasing the concentration of ISO shifted the K1/2 value for both ACh and TMA inhibition of ICl to higher concentrations and increased Gmin, without significantly affecting the slope factor. These results indicate that muscarinic regulation of ICl depends on the level of beta adrenergic stimulation in a functionally uncompetitive manner. They also suggest that TMA acts like ACh, a full agonist at the muscarinic receptor. Furthermore, we conclude that quaternary ammonium compounds, which are often used as ion substitutes and direct ion channel blockers, should be used with caution because of the significant and diverse effects they exert at muscarinic receptors.

Acetylcholine↗
PubMed1993

Tetramethylammonium activation of muscarinic receptors in cardiac ventricular myocytes.

Replacement of extracellular Na+ with tetramethylammonium (TMA) reduces the magnitude of the Cl- current activated by beta-adrenergic receptor stimulation in guinea pig ventricular myocytes. However, the effects of replacing Na+ appear to be associated with the presence of TMA, rather than the absence of Na+. Direct addition of TMA to extracellular solutions, without changing the Na+ concentration, was able to inhibit the Cl- current activated by isoproterenol (Iso) in a concentration-dependent manner. The concentration of TMA that caused half-maximal inhibition was 327 microM when the Cl- current was activated by 1 microM Iso and 29 microM when the Cl- current was activated by 0.03 microM Iso. The effect of TMA was also blocked by atropine, suggesting that TMA exerts its effect through stimulation of the muscarinic receptors. Furthermore, TMA inhibited the Iso-activated Ca2+ current, as would be expected for an effect involving muscarinic receptor stimulation. The response to complete Na+ replacement with TMA could not be overcome by increasing the concentration of Iso 1,000-fold, and direct addition of TMA was able to antagonize the Cl- current activated independently of the beta-adrenergic receptor, using forskolin and histamine. These results are consistent with the hypothesis that TMA does not exert its effects through a competitive mechanism at the beta-adrenergic receptor. It is concluded that TMA is able to antagonize adenosine 3',5'-cyclic monophosphate-dependent activation of ion channels in the heart through activation of muscarinic receptors.

Animals↗
PubMed1991

Effects of ryanodine on ouabain-induced spontaneous mechanical and electrical oscillations in guinea-pig heart.

The effects of ryanodine on ventricular arrhythmias in guinea-pigs in vivo, on delayed after potentials and after contractions, and on spontaneous oscillations of the membrane potential (SOP) and of resting tension (SOT) of guinea-pig papillary muscle under ouabain intoxication were studied. After addition of ouabain (1 microM) the after potentials, after contractions, and SOP and SOT amplitude were significantly increased. The power spectra of SOT and SOP under these conditions had a resonance harmonic with the frequency of about 5 Hz. Three to 5 mins after the addition of ryanodine (0.1-0.5 microM), the after potentials, after contractions, and SOP and SOT were abolished, suggesting a close relationship between these oscillations and the oscillatory activity of sarcoplasmic reticulum. In in vivo experiments, ouabain-induced (75-115 micrograms/kg) ventricular arrhythmias were terminated 4 to 5 min after intravenous injection of ryanodine (15 micrograms/kg); within 8-10 min, sinus rhythm was completely restored. We attribute the antiarrhythmic effect of ryanodine to a cellular effect and alteration of SR function, rather than to effects that are secondary to this.

Animals↗
PubMed1991

[Arrhythmogenic effect of acoustic cavitation in isolated rat heart perfused with physiological solution].

In experiments on isolated rat hearts the effects of focused continuous and impulse ultrasound (543 Hz, with intensity up to 7.8 W/cm2 at a focal region) on a pressure developed by left ventricle and electrograms were studied. In all experiments ultrasound induced extra-excitations of the heart, which appeared when intensity was 1.35 +/- 0.21 W/cm2 (n = 9). Simultaneously with the extra-excitations the cavitation bursts were recorded at intensity of 1.52 +/- 0.18 W/cm2 (n = 6). Acoustic cavitation (after 30 sec of exposure) resulted in a significant decrease of the developed pressure (from 100.8 +/- 3.8 mm Hg to 95.1 +/- 4.3 mm Hg, p 0.001), measured in 2 min after the end of the exposure. In the absence of cavitation the ultrasound was found to have no effects on cardiac performance. Electrograms recorded during acoustic pacing show that a pattern of the heart excitation changed from stimulus to stimulus.

Acoustic Stimulation↗
PubMed1989

Effects of barium on spontaneous mechanical and electrical oscillations in rat papillary muscle.

In experiments on isolated rat papillary muscles the effects of Ba2+ (0.2-1 mM) on spontaneous resting tension oscillations (RTO) and membrane potential oscillations (MPO) were investigated. In normal Tyrode's solution the muscles were not spontaneously active, however, the addition of barium (1 mM) after a delay of 3-5 min induced the spontaneous excitations of the preparations. During this time period the amplitudes of RTO and MPO of the quiescent muscle increased gradually, and just before the first spontaneous beat their values were many fold above control levels. It was noted that just before the first spontaneous excitation the RTO became very synchronous and resembled the sinusoidal oscillations. By means of a special transducers the displacements of two adjacent points (distance, 1.0 mm) on a muscle surface were continuously recorded at the resting state. The correlation coefficient between these displacement signals was shown to increase from zero (before Ba2+ addition) to 0.5-0.7 (just before the first spontaneous beat). It was suggested that the enhancement of the RTO and MPO is a result of the synchronization of subthreshold electrical and mechanical activities of all muscle cells after the addition of barium.

Animals↗
PubMed1989

The effect of acoustic cavitation on the contraction force and membrane potential of rat papillary muscle.

In experiments on isolated rat papillary muscles an acoustic cavitation induced by continuous wave focused ultrasound (543 kHz with intensity up to 3 W/cm2) was found to result in reversible membrane depolarization by 54.0 +/- 1.4 mV (n = 5), loss of excitability and rise in resting tension up to 53.1 +/- 4.3% (n = 15) of contractile response in the control. It was supposed that the rapid recovery of excitability (69.3 +/- 10.3 s, n = 15) might be a result of Ca2+ pump activation and/or alterations of intercellular coupling when cavitation ends.

Animals↗
PubMed1989

[The action of ultrasound on the contraction strength and action potential of the papillary muscle of the rat heart].

In experiments on isolated rat papillary muscles the effects of therapeutic doses of ultrasound (US) (intensity, less than 2 W/cm2) with frequency of 0.88 MHz on contraction force and action potential (AP) were studied. 12 muscles (from 14) responded to 3-min exposition of the US with a rise both in contraction force and in resting tension. Sensitivity to US and a value of inotropic effect changed significantly between the preparation, and the threshold intensities of US varied from 0.3 to 2 W/cm2. In 3 experiments the inotropic effect of US was more than 100%, but in others it was about 50%. Two preparations were not sensitive to the US. The positive inotropic effect of US was accompanied by membrane depolarization (up to 20 mV) and by prolongation of AP duration measured at 10% of its amplitude (APD10). The correlation between the increase in contraction force and APD10 was demonstrated. Some preparations responded to US with high depolarization (up to 50 mV) and were inexcitable. The US induced an increase in temperature less than 1 degree C, therefore all the effects of US could not be explained as a result of temperature rise.

Action Potentials↗
PubMed1988

Analysis of antiarrhythmic effect of ryanodine in guinea-pigs.

The effects of ryanodine on (1) ventricular arrhythmias in guinea-pigs in vivo, (2) delayed afterpotentials and aftercontractions and (3) spontaneous oscillations of the membrane potential (SOP) and of resting tension (SOT) of guinea-pig papillary muscle under ouabain intoxication have been studied. After addition of ouabain (1 microM), the afterpotentials, aftercontractions and the amplitude of SOP and SOT were significantly increased. The power spectra of SOT and SOP under these conditions had a resonance harmonic with a frequency of about 5 Hz. The afterpotentials, aftercontractions, SOP and SOT were abolished 3 to 5 min after ryanodine addition (0.1 to 0.5 microM), suggesting a close relationship between these oscillations and the oscillatory activity of sarcoplasmic reticulum. During in vivo experiments, ouabain-induced (75 to 115 micrograms/kg) ventricular arrhythmias were terminated 4 to 5 min after the intravenous injection of ryanodine (15 micrograms/kg) and within 8 to 10 min, the sinus rhythm was completely restored. We conclude that the antiarrhythmic effect of ryanodine is related to the inhibition of the diastolic fluctuations of the membrane potential.

Action Potentials↗
PubMed1987

[Amplitude fluctuations in the contractile responses of small fragments of rat papillary muscle].

Contractile responses of small rat papillary muscle segments (length = 0.25 mm) superfused in a normal Tyrode's solution (t = 35 degrees C) and stimulated at a frequency of 0.16 Hz have been measured, using an original technique. The amplitude of contractile responses was found to fluctuate with the variation of 3.0 +/- 0.4 mu, though the contraction amplitude of the entire muscle remained constant from beat to beat. After the addition of caffeine (6 mM) the variation of contractile responses decreased up to 0.83--0.03 mu, demonstrating a crucial role of sarcoplasmic reticulum in the phenomenon. It is believed that contractility inhibition during calcium overload may be caused by the same reason.

Animals↗
PubMed1987

[Anti-arrhythmic effect of ryanodine in the guinea pig with glycoside poisoning].

The effect of ryanodine on membrane potential oscillations in vitro (in isolated guinea-pig papillary muscle) and on ventricular arrhythmias in vivo (in glycoside intoxication) were studied. 3-5 minutes after ryanodine (0.5 microM) addition the membrane potential oscillations induced by ouabain (1 microM) were abolished. 4-5 minutes after intravenous ryanodine infusion (15 micrograms/kg) ventricular arrhythmias induced by ouabain intoxication (75-115 micrograms/kg) disappeared and 8-10 minutes later sinus rhythm was restored. It is suggested that antiarrhythmic effect of ryanodine is a result of the inhibition of diastolic membrane potential oscillations.

Action Potentials↗
PubMed1987

[Action of barium and potassium ions on fluctuations of the myocyte membrane potential in the rat papillary muscle].

Using a common microelectrode technique membrane potential fluctuations (MPF) have been studied in experiments on the papillary muscles of rat cardiomyocytes. Parallel measurements of oscillations in resting tension (mechanical noise--MN) from the muscle ends were performed. A correlation between MPF and MN was absent in the control. After the addition of barium (0.2 mM) MPF increased, assumed a sinusoidal form and synchronized with MN. Membrane depolarization evoked by external current resulted in a similar effect. However, the depolarization induced by a high potassium dose (30 mM) was accompanied by MPF and MN decrease. The opposite effects of barium and potassium on MPF might be a result of their different influences on the muscle length constant (which is increased by Ba2+ and decreased by K+).

Animals↗
PubMed1985

[Mechanical noise of the myocardium as an index of sarcoplasmic reticulum activity].

The effects of caffeine (0.5-12 mM) and cooling on the mechanical noise (MN) and relaxation rate of contractile response (RR) were studied in experiments on rat papillary muscles. Mean frequency of MN and RR were found to have similar temperature dependences from 12 to 32 degrees C. MN amplitude was more sensitive to the action of sarcoplasmic reticulum activity blocker, caffeine, than RR one. MN as an index of sarcoplasmic reticulum activity made it possible to demonstrate a competition between caffeine and local anaesthetics described earlier for skeletal muscles.

Animals↗