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Biomedical subjects

M F Shuba

Publications and source records attributed to M F Shuba.

At least 37 records · Page 2Linked to original sources
PubMed1992

[Effects of the membrane potential level on serotonin-induced contraction of the pulmonary artery smooth muscle in rabbits].

The effects of changes in membrane potential level on the electrical and contractile responses induced by serotonin (10(-6) mol/l) were investigated in muscle strips from rabbit main pulmonary artery using sucrose-gap technique. In spite of the fact that serotonin-induced depolarization did not exceed the threshold level for development of contraction, it was followed by a strong tonic contraction. Nearly a half of this contraction could be relaxed by an electrotonic hyperpolarization of the membrane. A week preliminary depolarization of the muscle cells resulted in an increase while a strong depolarization--in dramatic decrease of serotonin-induced contraction. Nifedipine effectively blocked potassium-induced, but not serotonin induced contraction. We suggest that in addition to voltage-operated and receptor operated Ca channels in vascular smooth muscle cell membrane there is a separate class of nifedipine-insensitive Ca channels operated by both serotonin receptor and membrane potential.

Animals↗
PubMed1991

The inhibitory action of caffeine on calcium currents in isolated intestinal smooth muscle cells.

The patch-clamp method has been used to investigate the action of caffeine on the calcium current (ICa) in single isolated smooth muscle cells of the guinea-pig ileum. Caffeine (10 mM) substantially inhibited ICa. This effect occurred in a biphasic manner and it was not due either to activation of additional ionic currents of opposite direction nor to inhibition of phosphodiesterase activity. It strongly depended upon the ethylenebis-(oxonitrilo)tetraacetate (EGTA) concentration in the pipette solution. When there was K+ in the pipette solution, application of caffeine evoked a transient Ca-dependent K+ current and an abrupt and transient increase in the frequency of channel openings. Such well-known blockers of Ca release as procaine and ruthenium red strongly decreased ICa. Ryanodine had only little effect on ICa, but application of caffeine in the presence of ryanodine led to a complete and irreversible inhibition of ICa. The results of experiments involving different EGTA concentrations and comparison of the time courses of all caffeine-induced phenomena clearly indicated that only the initial, transient component of the ICa inhibition by caffeine was related to a Ca-dependent inactivation of Ca channels, evoked as a result of Ca release from intracellular stores. The tonic component of ICa inhibition was probably due to a direct blocking action of caffeine on Ca channels.

1-Methyl-3-isobutylxanthine↗
PubMed1991

Properties of the late transient outward current in isolated intestinal smooth muscle cells of the guinea-pig.

1. Whole-cell membrane currents in voltage-clamped single isolated cells of longitudinal smooth muscle of guinea-pig ileum were studied at room temperature using patch pipettes filled with either high-K+ solution or high-Cs+ solution, to suppress K+ outward current, and containing 0.3 mM-EGTA. 2. In the presence of high-K+ solution in the pipette, membrane depolarization from the holding potential of -50 mV evoked an initial inward calcium current (ICa) followed by a large initial transient outward current and a sustained outward current with spontaneous oscillations superimposed. Prolonged depolarization above -20 mV produced a late transient outward current which reached a maximum (up to several nanoamps at +10 mV) within approximately 1 s and lasted several seconds. 3. The late outward current (ILTO) was voltage dependent and reversed at the EK (potassium equilibrium potential) in cells exposed to high-K+ external solution. It was blocked by TEA+ (tetraethylammonium) or Ba2+ applied externally (calculated Kd (dissociation constant) values were 0.67 and 4.43 mM, respectively) or by high-Cs+ solution perfusing the cell. The removal of extracellular Ca2+, application of Ca2+ channel blockers (3 mM-Co2+, 0.2 mM-Cd2+ or 1 microM-nifedipine) or perfusion of 5 mM-EGTA inside the cell also abolished the current. Thus, the current seems to be a Ca(2+)-activated K+ current. 4. There is a great discrepancy between the time course of the ICa and that of the late ILTO, which suggests that Ca2+ release from intracellular storage sites may contribute to the generation of the ILTO. 5. Bath application of caffeine (10 mM) during the development of ILTO enhanced the current. However, in the presence of caffeine ILTO was inhibited. Moderate inhibition of ICa by caffeine was also observed. 6. Ryanodine (5 microM) applied to the bathing solution completely inhibited ILTO within 3.5 min; however, it had no or little effect on the ICa. 7. Ruthenium Red (10 microM) completely blocked the ILTO and slightly and more slowly inhibited the ICa. 8. Increasing Mg2+ concentration in the pipette solution from 1 to 6 mM abolished the ILTO. 9. It was concluded that the ILTO was activated mainly by Ca2+ released from the intracellular storage sites following Ca2+ entry, presumably by a Ca(2+)-induced Ca2+ release mechanism.

Animals↗
PubMed1991

Inactivation of calcium channels in single vascular and visceral smooth muscle cells of the guinea-pig.

Inactivation of currents carried through calcium channels by calcium (ICa), barium (IBa) and monovalent cations (In.s.) was studied in single smooth muscle cell (SMC) of the guinea-pig coronary artery and taenia caeci by the whole-cell patch-clamp method. The rate of ICa inactivation in the coronary artery SMC was correlated with ICa amplitude, and acceleration was observed with the increasing ICa peak amplitude. The availability curve of ICa in double-pulse experiments was found to be U-shaped, however, no complete restoration of ICa availability was observed. Inactivation of IBa was considerably slower than that of ICa. These findings may indicate that inactivation of calcium channels in the membrane of coronary artery SMC is, at least partially, a Ca-dependent process. However, some facts observed contradict the validity of this hypothesis for coronary artery SMC in contrast to taenia caeci: 1) elevation of external Ca2+ concentration did not affect the time course of ICa inactivation; 2) inactivation of In.s., i.e. without calcium entry into the cell, was faster than that of ICa. It was concluded that the characteristics of Ca channel inactivation were changed by the removal of divalent cations from extracellular solution. Differences and similarities in Ca channel inactivation between coronary artery and taenia caeci SMC are discussed.

Animals↗
PubMed1990

[The generation of nonadrenergic inhibiting synaptic potentials in the smooth muscles of the gastrointestinal tract by substituting cesium ions for potassium ions].

Nonadrenergic inhibitory junction potentials (IJPs) evoked by intramural stimulation were investigated in smooth muscle of guinea pig stomach, caecum and colon by means of sucrose-gap method. IJPs disappeared in the smooth muscle preexposed to K-free Krebs solution for 4-9 h and restored by addition 6mM Cs+. The amplitude of IJPs was half as much as one in normal conditions but the latency and duration were significantly prolonged. In most cases apamin blocks IJPs in these muscles. The results presented suggest that IJPs generation is due to Cs+ ions permeated through Ca(2+)-activated apamin-sensitive potassium channels of small conductance in these conditions. As the responses to ATP were affected in parallel with IJPs, these results are consistent with the purinergic hypothesis.

Animals↗
PubMed1989

Studies of the inhibitory non-adrenergic neuromuscular transmission in the smooth muscle of the normal human intestine and from a case of Hirschsprung's disease.

A modified sucrose-gap method was used to study both non-adrenergic inhibitory neuromuscular transmission and effects of adenosine 5'-triphosphate (ATP) on isolated smooth muscle preparations from the human intestine. It was found that non-adrenergic inhibition in the circular smooth muscle layer was of larger amplitude than in the longitudinal layer. Study of the ionic mechanisms underlying non-adrenergic inhibition indicated that an increase in K+ conductance was responsible for the generation of non-adrenergic inhibitory junction potentials (IJPs). The results suggest that the inhibitory actions of the endogenous neurotransmitter and exogenous ATP are due to increases in Ca2+-dependent K+ conductance. The K+-channel blockers tetraethylammonium and 4-aminopyridine had no effect on IJPs or ATP, while apamin slightly decreased both the amplitude of the IJP and the hyperpolarization of the circular smooth muscle caused by ATP. These results are consistent with the purinergic hypothesis of non-adrenergic inhibition. In addition to inhibitory purinoceptors, the existence of excitatory purinoceptors was identified in the longitudinal muscle, activation of which probably caused an increase in Na+-conductance. The excitatory purinoceptor-mediated contraction in the longitudinal muscle from the constricted region of large intestine from patients with Hirschsprung's disease was greater than that found in control specimens. It is possible that excitatory purinoceptors play a role in the pathophysiology of Hirschsprung's disease.

Action Potentials↗
PubMed1989

[Adrenergic regulation of contractile activity of smooth muscles in umbilical vessels].

The effect of agonists and antagonists of alpha- and beta-adrenoreceptors on mechanical properties of the smooth muscle was studied in the isolated muscle strips of the human umbilical artery and vein. Noradrenaline was shown to cause the contraction of smooth muscles of the umbilical vein. Tonic and phase components in the contraction process were observed. Isoprenaline caused tonic contraction of the muscle strip. Phentolamine, and alpha-adrenoblocker did not inhibit the stimulating activity of noradrenaline. In presence of obsidane (beta-adrenoblocker) noradrenaline triggered the contraction with lesser tonic component as compared to the initial one. So the selective stimulation of beta-adrenoreceptors of smooth muscles of the umbilical vein by isoprenaline and blocking of the same receptors by obsidane have proved that activation of alpha-adrenoreceptors is responsible for the phasic component of contraction, while that of beta-adrenoreceptors--for the tonic component of contraction. As both alpha- and beta-adrenoblockers in smooth muscles of the umbilical artery inhibit the noradrenaline-induced contraction, it was concluded that the differentiation of adrenoreceptors of the vessel is rather low.

Humans↗
PubMed1988

Saturation of calcium channels in single isolated smooth muscle cells of guinea-pig taenia caeci.

1. Calcium channel currents were recorded in Cs+-dialysed voltage-clamped single smooth muscle cells isolated from the guinea-pig taenia caeci to evaluate the current-carrying ability of Ca2+, Ba2+, Sr2+ and Mg2+ ions. 2. Ba2+ and Sr2+ ions, as well as Ca2+ ions, were able to carry an inward current through calcium channels. Calcium channel current was not observed when Mg2+ was the only divalent cation in the external solution. 3. Concentration dependences of calcium (ICa), barium (IBa) and strontium (ISr) currents were studied. It was found that currents through calcium channels saturated with increasing the extracellular concentration of a current carrier. Saturation of each current can be fitted with a Langmuir curve with apparent dissociation constants of 1.2 mM for Ca2+, 1.8 mM for Sr2+ and 9.6 mM for Ba2+ ions. 4. External Mg2+ ions reduced both ICa and IBa.IBa was depressed to a greater extent than ICa by Mg2+ ions. Reduction of ICa by Mg2+ ions seems to agree with competitive antagonism between Ca2+ and Mg2+ ions (Hagiwara & Takahashi, 1967). 5. When the external divalent cation concentration [( C2+]o) was changed, the current-voltage relationship of currents through calcium channels was shifted along the potential axis suggesting that activation gating of calcium channels was affected by [C2+]o. These voltage shifts can be fitted with the Gouy-Chapman theory supposing the density of surface charges near calcium channels to be 0.5 e nm-2 and including more potent binding of Ca2+ ions to surface charges than of Ba2+, Sr2+ and Mg2+ ions. 6. The changes in the Ca2+, Ba2+ and Sr2+ concentrations at the surface of the membrane were calculated. It was found that saturation of IBa can be explained by saturation of Ba2+ surface concentration while saturation of ICa and ISr cannot. 7. It was suggested that barium ions were able to carry the larger current through calcium channels in smooth muscle cells due to their much weaker binding within the calcium channel.

Animals↗
PubMed1988

[Membrane mechanisms of the excitatory action of serotonin on the smooth muscles of the rabbit pulmonary artery].

Serotonin induced dose-dependent tonic contractions of the rabbit pulmonary artery smooth muscles with KED50, of 2.7 X 10(-7) mol/l. More than 80% of these contractions were found to be dependent on extracellular calcium. Hyperpolarization of cell membrane by inwardly applied electrical current caused nearly 50% reduction in serotonin-induced contractions. The same portion of contractions was inhibited by verapamil and Ca2+. Serotonin-, but not potassium-induced contractions were completely inhibited by sodium nitroprusside which is thought to be selective inhibitor of receptor-operated calcium channels. These findings could indicate that Ca2+ ions, responsible for serotonin-induced contractions enter the cell from the outer surface of the cellular membrane via receptor-operated calcium channels. Nearly half of serotonin-operated Ca2+ channels appear to be also potential-operated.

Animals↗
PubMed1988

[The role of intra- and extracellular Ca in the activation of contraction of pulmonary artery smooth muscles induced by serotonin].

In Ca-free EGTA-containing solution serotonin induced a transient contraction of rabbit pulmonary artery smooth muscle which decayed to nearly steady-state level accounted for 17.7 +/- 1.6% of original contraction in Krebs solution. Both phasic and tonic components of this contraction were effectively inhibited by verapamil and Cd2+. Caffeine induced no contraction of muscle strips if it was applied after withdrawal of serotonin. But when the sequence of these drugs application was reversed, serotonin still evoked contraction with reduced phasic component. The results obtained in these experiments suggest, that serotonin-induced contraction of pulmonary artery smooth muscle is partly (less than 20%) due to mobilization of bound calcium from at least two stores located on the opposite sides of the cell membrane. Calcium released from external store site enters the cell via receptor-operated calcium channels.

Animals↗
PubMed1987

Calcium-dependent inactivation of potential-dependent calcium inward current in an isolated guinea-pig smooth muscle cell.

1. Calcium current (ICa) was studied in single isolated smooth muscle cells of a guinea-pig taenia caeci dialysed with Cs+-containing solution to suppress K+ outward current. 2. With increasing step depolarizations up to +10 mV, acceleration of ICa inactivation was observed. With further increase of step depolarization, ICa inactivation was slowed down. The largest ICa (observed at +10 mV) was characterized by the maximal speed of inactivation. 3. Comparison of ICa in different external concentrations of Ca2+ ions ([Ca2+]o) revealed that at the same membrane potential the time course of ICa inactivation was slower, the smaller the amplitude of ICa. Slowing down of ICa inactivation was observed also during its partial block by Co2+ ions. 4. Elevation of temperature increased ICa peak amplitude and accelerated its decay. The amplitude of ICa was increased by a factor of 1.7 +/- 0.14 (n = 6) when the temperature was raised by 10 degrees C. 5. Calculations of Ca2+ entry during ICa as a time integral of Co2+-sensitive current, and comparison with the degree of ICa inactivation, showed that inactivation was tightly related to Ca2+ entry in the membrane potential range -20 to +40 mV. 6. Ba2+ current through Ca2+ channels was larger than ICa and its inactivation was considerably slower. 7. Recovery of ICa from inactivation was found to be potential dependent. When the cell membrane was hyperpolarized, ICa recovery was accelerated. 8. It was concluded that inactivation and recovery of ICa in smooth muscle cells were influenced by both Ca2+ entry and membrane potential. It was also pointed out that the observed events are difficult to explain by the hypothesis that inactivation was produced simply by accumulation of Ca2+ ions near the inner side of the membrane, and that recovery was due to lowering of internal free Ca2+ ion concentration ([Ca2+]i).

Action Potentials↗
PubMed1987

[Electrophysiological analysis of the action of kavinton on the smooth muscles].

Cavinton at a concentration of 10(-7)-10(-5) M was found to have a dose-dependent relaxing effect on bovine cerebral artery smooth muscles, without changing the resting potential and membrane resistance. Smooth muscles of the rabbit portal vein and guinea-pig taenia coli were insensitive to low cavinton concentrations. The results are consistent with the hypothesis that relaxing action of cavinton is due to the blocking of Ca2+ ions influx into the cells of cerebral artery through receptor-operated calcium channels. At higher concentrations (exceeding 10(-5) M) cavinton exerts nonspecific influence on the smooth muscles under study, inhibiting their excitability and decreasing membrane resistance resulting in the attenuation of tetanic contractions in the smooth muscles of the portal vein and taenia coli.

Animals↗
PubMed1986

Potential-dependent calcium inward current in a single isolated smooth muscle cell of the guinea-pig taenia caeci.

A single glass micropipette voltage-clamp technique was used to study a potential-dependent calcium inward current in isolated smooth muscle cells of the guinea-pig taenia caeci. Experiments were performed at 22-24 degrees C. With potassium as the main cation in the pipette solution, a transient inward current appeared in response to a depolarizing pulse, followed by an outward current. The replacement of potassium ions by caesium ions and TEA (tetraethyl-ammonium) in the pipette solution resulted in an effective suppression of potassium outward current permitting a study of the calcium current solely. The calcium inward current was blocked by 5 mM-cobalt and 5 X 10(-6) M-verapamil. Activation of the calcium current occurred at a membrane potential of between -35 and -25 mV. The calcium current was maximal in the potential range +10 to +20 mV and did not reverse even at +60 mV. Inactivation of the calcium current had a complex nature. It did not inactivate completely even during depolarizations lasting many seconds. During the first 400 ms the decay of the calcium current followed a time course described by two exponentials. The fast time constant of decay was in the range of 40 to 53 ms (n = 3) and the slow time constant was approximately 10-fold greater (at 0 mV). The fast time constant did not depend on the membrane potential while the slow time constant decreased with depolarization. Availability of the calcium current was estimated in double-pulse experiments. It had a U-shaped dependence on the conditioning potential; maximal inactivation was observed at potentials corresponding to the maximal calcium current. It was suggested that a component of inactivation was dependent on the calcium current which flowed. Calculations of calcium entry at various depolarizations showed that large amounts of calcium ions enter the cell. Also, it was suggested that calcium ions are effectively bound within the smooth muscle cell.

Action Potentials↗