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K Boev

Publications and source records attributed to K Boev.

At least 19 recordsLinked to original sources
PubMed1999

Corticotropin-releasing hormone acts on guinea pig ileal smooth muscle via protein kinase A.

In contraction studies corticotropin-releasing hormone (CRH) was found to relax ileal but not gastric and jejunal smooth muscles of the guinea-pig, precontracted with BaCl2. Under whole-cell patch-clamp conditions, CRH concentration-dependently activated Ca2+-sensitive K+ currents (IK) with ED50=20 pM at 100 nM and ED50=0. 13 pM at 500 nM intracellular Ca2+ respectively. This increase was accompanied by significant hyperpolarization of the cell membranes. CRH 9-41 peptide fragment did not affect IK amplitude, membrane potential or contraction. The CRH-induced increase of IK densities was accelerated in the presence of high intracellular Ca2+ concentrations (500 nM) and was abolished by pretreatment of cells with either ryanodine or thapsigargin, which cause depletion of intracellular Ca2+ stores, as well as in cells treated under conditions prohibiting intracellular Ca2+ store refilling. The effect of CRH on IK was not affected by bath application of various selective inhibitors of membrane-bound phospholipases, protein kinase C, cGMP-dependent protein kinase or Ca2+/calmodulin-dependent protein kinase II, but was effectively antagonized by blockers of protein kinase A (PKA) or adenylyl cyclase. Neither forskolin nor the catalytic subunit of PKA could mimic the effect of CRH on IK. Thus, it was suggested that CRH exerts its relaxing activity on ileal smooth muscle cells via PKA-dependent phosphorylation of some intracellular target coupled to sarcoplasmic reticulum Ca2+ storage machinery.

Animals↗
PubMed1992

Effect of cromakalim on the smooth muscle of the cat gastric antrum.

The effect of the K(+)-channel opener cromakalim (BRL 34915) on the electrical and contractile activity of the smooth muscle of the cat gastric antrum has been studied. Cromakalim induced a concentration-dependent inhibition of the contractions and shortening of the sustained partial repolarization phase of the plateau action potential. High concentrations of cromakalim produced hyperpolarization and shortening of the repolarization and depolarization phases of the plateau action potential. The K(+)-channel blockers 4-aminopyridine (10(-2) M) and tetraethylammonium (10(-2) M) decreased the effect of cromakalim on the phasic contractions, while glibenclamide (5 x 10(-5) M) completely abolished it. We suggested that the inhibitory effect of cromakalim on the electrical and contractile activity of the gastric antrum smooth muscle is due to the cromakalim-induced increase of the outward K(+)-current through glibenclamide-dependent K(+)-channels.

Action Potentials↗
PubMed1992

Effects of isosorbide-5-mononitrate and isosorbide-2-mononitrate on the contractile and electrical activity and on the content of cyclic nucleotides in isolated heart muscles of the guinea-pig and dog.

Isosorbide-5-mononitrate and isosorbide-2-mononitrate, the metabolites of isosorbide dinitrate, were studied for their effects on the contractile and electrical activity and on the content of cyclic nucleotides in isolated heart muscles of guinea-pig and dog. Isosorbide-5-mononitrate at all concentrations tested and isosorbide-2-mononitrate at low concentrations did not produce significant changes in the mechanical activity of guinea-pig heart preparations. Isosorbide-2-mononitrate in concentrations above 10(-4) M inhibited the contractility and shortened the action potential of guinea-pig and dog heart muscle. Neither mononitrate had an effect on cAMP concentration but both induced a significant elevation of cGMP content in guinea-pig heart tissue, the effect of isosorbide-2-mononitrate being greater.

Animals↗
PubMed1991

The dual effect of BAY K 8644 on excitation-contraction coupling in gastric smooth muscle.

1. BAY K 8644 at concentrations of 10(-10)-10(-6) M had a stimulant effect on the spontaneous electrical and contractile activity of smooth muscle preparations from cat and guinea pig stomach. 2. Nifedipine (10(-6) M) antagonized the BAY K 8644-induced spike potentials and the related phasic contractions. 3. Neither the excitatory nor the inhibitory effect of BAY K 8644 was significantly influenced by atropine (10(-7) M), phentolamine (10(-7) M), propranolol (10(-7) M) or TTX (10(-6) M). 4. TEA (10(-3) M) abolished the inhibitory effect of BAY K 8644 on the spike generation and increased the amplitude of the phasic contractions.

3-Pyridinecarboxylic acid, 1,4-dihydro-2,6-dimethy↗
PubMed1989

Electromechanical coupling in cat stomach smooth muscle in Ca2+-free EGTA-containing solutions.

The electromechanical coupling in the smooth muscles of cat stomach was studied in Ca2+-free solutions containing EGTA. The experiments were carried out on circular muscle strips from the corpus of cat stomach using the single sucrose gap technique. Complete disappearance of the spontaneous electrical and contractile activity of the muscle strips was observed in Ca2+-free solution. Addition of EGTA (0.1 to 1 mM) to the Ca2+-free solution caused a considerable depolarization of the membrane and appearance of fast fluctuations of the membrane potential, grouped in periodically appearing spindles. After 10-15 min the fluctuations of the membrane potential obtained the shape of flat plateau-like waves. This electrical and contractile activity was completely eliminated by Mn2+ (2.5 mM), D600 (10(-6) M) or nifedipine (10(-8) M). Light inactivation of nifedipine-induced restoration of the chelator-evoked potentials and their reduction to the initial values. The chelator-induced electrical activity depended on the concentration of extracellular Na+, and it was not recorded when the Na+ concentration was decreased below 60 mM. These results support the assumption that the chelator-induced flat plateau-like waves are determined by Na ions penetrating through the Ca-channels. These potentials, appearing in Ca2+-free EGTA-containing solution, probably cause the release of Ca2+ from the intracellular stores in amounts sufficient for the realization of the electromechanical coupling.

Animals↗
PubMed1988

Photoinduced removal of nifedipine blockade of Ca2+ entry in different phases of gastric plateau action potential.

Spontaneous electrical and contractile activities of gastric antral smooth muscle preparations were recorded using a modified sucrose gap technique, which allowed for the exposition of the muscle strips to a short-light pulse from a xenon flash lamp. Nifedipine (3 x 10(-8)-10(-7) mol/l) inhibited spontaneous spike potentials, modified the shape of plateau action potentials and suppressed or abolished spontaneous contractile activity of muscle preparations. Short UV-illumination rapidly inactivated nifedipine's action and restored both the shape of action potentials and the amplitude of contractions. The data obtained showed that nifedipine-dependent Ca2+ entry participated in excitation-contraction coupling both during initial depolarization and during the plateau of action potentials. After nifedipine inactivation spike potentials superimposed on plateau action potentials and related high-amplitude phasic contraction were recovered.

Action Potentials↗
PubMed1988

Differences in the kinetics of ionic currents in the somatic membrane of identified T- and N-mechanosensory neurons of medical leech.

The ionic currents in the membrane of the body of single T- and N-mechanosensory neurons were studied. Transient inward NA+-current followed by outward K+-current was recorded in the T-cells. The current was activated at membrane potential of -40 mV, reaching its maximum at -10 mV for 2 ms, whereby the reversal potential was about 25 mV. This current was inhibited by tetrodotoxin (TTX, 3 X 10(-6) M) and was eliminated in a Na+-free medium. In an Na+-free medium no inward current was registered upon increasing the Ca2+-concentration and after substituting Ca2+ with 10 mM Ba2+. In the N-cells the inward current was activated at membrane potential of -20 mV, reaching its maximum at -5 mV for 3-4 ms, while the reversal potential was about 45 mV. A time- and potential-dependent residual inward current was observed after application of TTX (3 X 10(-6) M) or in a Na+-free medium. This current increases upon rising the concentration of Ca2+ or when Ca2+ was replaced by 10 mM Ba2+. This current was blocked by methoxyverapamil (D-600, 5 X 10(-5) M). The results obtained manifest differences in the kinetics of the Na+-channels in the T- and N-neurons. In the membranes of the T-cells the Ca2+-channels are either poorly represented or totally lacking. Ca2+-conductivity was established in the N-cells and probably in their membranes exist Ca2+-channels which participate in their excitatory processes.

Action Potentials↗
PubMed1988

Bombesin-induced changes in membrane potential-dependent phasic contractions of cat gastric muscle.

Electrical and contractile activities of smooth muscle strips isolated from the circular muscle layer of cat gastric antrum were studied using the sucrose gap technique. Bombesin (10(-8) mol/l) depolarized the gastric muscle; this was accompanied by an increase in the strip tone, in the plateau action potential frequency and in both the frequency and the amplitude of the spike potentials as well as by a shortening of the plateau action potential duration. Both the frequency and the amplitude of the phasic contractions increased thereafter. The changes in the frequency of the plateau action potentials and contractions were not influenced either by antagonists of cholinergic and adrenergic receptors or by TTX. In the presence of the Ca antagonists D600 (10(-6) mol/l) and nifedipine (10(-7) mol/l) or in Ca-free medium containing EGTA the effect of bombesin on the frequency of the plateau action potentials and phasic contractions remained unchanged; however, spike potentials were not observed and no increase in the amplitude of phasic contractions occurred. UV-light inactivation of nifedipine restored the typical bombesin effect on the electrical and contractile activities of the gastric smooth muscle. The present data suggest that the effect of bombesin on the frequency of both plateau action potentials and phasic contractions is not linked with Ca2+ influx.

Action Potentials↗
PubMed1985

4-Aminopyridine-induced changes in the electrical and contractile activities of the gastric smooth muscle.

Effects of 4-aminopyridine (4-AP) on the electrical and contractile activities of the fundus and antrum of the cat stomach were studied using the sucrose-gap technique. In the fundus, low concentrations of 4-AP (up to 1 mmol/l) induced membrane depolarization and appearance of spike potentials and phasic contractions. After preliminary administration of atropine, 4-AP produced an opposite effect: hyperpolarization and relaxation. On the background of tetrodotoxin (TTX) plus antagonists of cholinergic and adrenergic receptors, high concentrations of 4-AP (greater than 5 mmol/l) caused membrane depolarization and appearance of spike potentials and phasic contractions. In the antrum, 4-AP in low concentrations (up to 1 mmol/l) decreased both the amplitude and the duration of the second component of the plateau-action potential, as well as those of the phasic contractions. This effect decreased in the presence of adrenergic receptor antagonists and was abolished by TTX. On this background, high concentrations of 4-AP (greater than 5 mmol/l) led to the appearance of spike potentials superimposed on the second component of the plateau-action potentials, and to a further increase in the phasic contraction amplitudes. The present data suggest that 4-AP exerts its effects via an increase in neurotransmitter release (low concentrations) and/or directly on the smooth muscle cell membrane (high concentrations).

4-Aminopyridine↗
PubMed1983

Participation of Na+ in the excitation-contraction coupling of the phasic and tonic contractions in stomach smooth muscles.

The sucrose gap method was used to study the effect of Na+ on the electrical and contractile activity of smooth-muscle strips from the antrum and fundus of cat stomach. Na+ reduction or elimination from the nutrient medium leads to reduction of the depolarization phase of the plateau-action potentials and to a transient increase in the amplitude of the phasic contractions of the antrum, followed by their complete disappearance. In Na-free medium direct electrical stimulation leads not to the appearance of plateau-action potentials, but of a group of Ca2+-dependent spike-potentials and high-amplitude phasic contractions. In fundic preparations the decrease in the Na+-concentration of the nutrient solution results in a gradual rise of the tonic tension, without appearance of action potentials. This effect of the reduced Na+ is inhibited by Ca-antagonists La3+ (10(-2) mol/l) D 600 (10(-6) - 10(-5) mol/l) and by sodium nitroprusside (10(-6) mol/l). On the other hand, an increase in the Ca-conductivity of the membrane of the fundic smooth muscle cells was observed in Na+-free medium. It is assumed that at the level of the fundic smooth muscle cell membrane there is competition between Na+ and Ca2+ with respect to slow Ca-channels.

Action Potentials↗
PubMed1983

Ionic interactions in the smooth muscles of cat pulmonary artery and portal vein.

The contractile activity of isolated smooth-muscle preparations of pulmonary artery and portal vein was recorded under isometric conditions and the changes in the membrane potential were recorded by the sucrose gap method. The replacement of Na+ by sucrose or cholinechloride ([Na+]o = 0) in the presence of phentolamine and atropine causes contraction of the smooth-muscle strips, characterized by initial fast and subsequent slow tonic component. The contraction is accompanied by hyperpolarization of the membrane. D600 (10(-5) M) and sodium nitroprusside (10(-5) M) do not influence the tonic component of the [Na+]o = 0-induced contraction. Low Na-content in the solution ([Na+]o = 15.5 mM) causes brief contraction only of the portal vein strips. In Ca-free solution the tone of the arterial preparations does not change, while the tone of portal vein strips decreases and phasic contractions are inhibited. In Ca-free solution or after pretreatment of the strips with 10 mM LaCl3, substitution of sodium induces a slowly developing contraction without initial fast component. Pretreatment of the preparations with ouabain (10(-4) M) or their preincubation in K-free solution potentiates the [Na+]o = 0-induced contraction. The results obtained exclude membrane-potential-dependent processes in the development of the [Na+] o = 0-induced contraction and support the existence of Na+-Ca2+ exchange mechanism in the smooth muscles of cat pulmonary artery and portal vein.

Animals↗
PubMed1980

On the changes in the membrane potential and the contractile activity of the smooth muscle of the lower esophageal and ileo-caecal sphincters upon increased K in the nutrient solution.

The influence of increased K+ in the nutrient solution on the excitatory-contractile process in smooth-muscle strips from the lower esophageal sphincter (LES) and the ileo-caecal sphincter (ICS) of cats is studied. The changes in the membrane potential (MP) and the tension (T) are recorded by the sucrose gap method. The rise of the K+ concentration in the nutrient solution to 30 mM causes hyperpolarization and relaxation of LES, while in ICS depolarization is observed irrespective of the relaxation. In LES the higher K+ concentrations, as well as the isotonic K+ lead to initial brief hyperpolarization, followed by depolarization. A biphasic response in the contraction-relaxation and subsequent contraction corresponds to them. Irrespective of the fact that the response in ICS is also biphasic with respect to the contraction, the changes in MP are manifested only as depolarization. These responses of LES and ICS are not influenced by cholinergic and adrenergic blockers (atropinum sulfuricum, Propranolol, Phenoxybenzamine--(10(-6) to 10(-5) g/ml). In ouabaine-pretreated (10(-6) to 5 x 10(-6) M) smooth muscle from LES and ICS, in all K+ concentrations the response is depolarization and contraction. The effects of increased K+ in the nutrient solution on LES and ICS are connected with the ratio [K]i : [K]o ([K]i--intracellular, [K]o--extracellular). It is assumed that the Na-K pump in LES and ICS functions at a different level compared with the other smooth muscles of the gastro-intestinal tract.

Animals↗
PubMed1979

Inhibitory action of acetylcholine on the smooth muscle from the lower esophageal sphincter.

The effect of acetylcholine (Ach) on smooth-muscle strips isolated along the transversal axis of cat lower esophageal sphincter (LES) is studied. Ach in low concentrations (10(-11)--10(-9) g/ml) causes contraction of the muscle strips. Increase of the concentration to 10(-8) g/ml leads to biphasic effect: contraction with relaxation. Inhibitory response predominates at Ach 10(-6) and 10(-5) g/ml. Atropine (10(-6) M) eliminates the excitatory phase but it has no effect on the second relaxation phase. Propranolol (10(-6), 2 X 10(-6) M) as well as phentolamine turn the inhibitory response to Ach into contraction. Noradrenaline leads to LES contraction while isoprenaline induces relaxation. In smooth-muscle LES strips from cats pretreated with reserpine (1 mg/kg for 3 days), Ach in the concentrations used (10(-5), 10(-6) g/ml) leads to contraction. The changes observed are membrane-dependent -- the contraction is accompanied by depolarization, relaxation by hyperpolarization. The inhibitory effect of Ach on LES smooth muscle is discussed in the light of the hypothesis of Burn and Rand (1960) about the release of noradrenaline under the effect of Ach.

Acetylcholine↗