RESUMO
1. The effects of rilmakalim, a potassium channel opener, were studied on rabbit cardiac Purkinje, ventricular muscle and atrial fibers, with the use of conventional microelectrode techniques. 2. Rilmakalim (0.24-7.2 microM) shortened, in a concentration-dependent manner, the action potential duration (APD) in Purkinje and ventricular muscle without affecting other parameters of the action potential. Pinacidil (30-300 microM) also decreased the APD of Purkinje fibers. 3. Rilmakalim (2.4 microM) and cromakalim (100 microM) hyperpolarized and abolished abnormal automaticity of cardiac Purkinje fibers pretreated with barium (0.2-0.3 mM). Glibenclamide (5 microM) blocked the hyperpolarizing effect. 4. Stable early afterdepolarizations induced in Purkinje fibers by berberine (100 microM) were reversibly blocked by rilmakalim (2.4 microM), which also suppressed late afterdepolarizations induced in Purkinje fibers treated with ouabain (0.3-0.5 microM). 5. The rate of spontaneous discharges of the rabbit sinoatrial node was not affected by rilmakalim (7.2 microM) or by pinacidil (100 microM). Both agents were also unable to affect the APD of atrial muscle fibers. 6. In cardiac Purkinje fibers, tetraethylammonium (TEA; 20 mM) significantly reduced the effects of rilmakalim (2. 4 microM) on the APD. However, neither TEA nor glibenclamide (100 microM) reduced the shortening of the APD induced by dinitrophenol (30 microM) or by salicylate (1 mM).
Assuntos
Potenciais de Ação/efeitos dos fármacos , Antiarrítmicos/farmacologia , Cromanos/farmacologia , Canais de Potássio/efeitos dos fármacos , Ramos Subendocárdicos/efeitos dos fármacos , Pirrolidinas/farmacologia , Nó Sinoatrial/efeitos dos fármacos , 2,4-Dinitrofenol/farmacologia , Animais , Glibureto/farmacologia , Técnicas In Vitro , Ramos Subendocárdicos/fisiologia , Coelhos , Nó Sinoatrial/fisiologia , Salicilato de Sódio/farmacologia , Tetraetilamônio , Compostos de Tetraetilamônio/farmacologiaRESUMO
TsTX-V, a new neurotoxin from Tityus serrulatus scorpion venom able to induce a prolongation of the inactivation of Na+ channels, has been purified to homogeneity. The venom was chromatographed on CM-cellulose-52 and 13 fractions were first collected. A subsequent stepwise elution chromatography of fraction XI afforded, among other toxins, highly purified TsTX-V, which showed a single band by PAGE, SDS-PAGE or isoelectric focusing, a distinctive amino acid composition, mol. wt. = 7230, pI = 8.0 and i.v. LD50 = 94 +/- 7 micrograms/kg in mice. TsTX-V induced a long lasting hypertension in anesthetized rats and prolonged the action potential of the B fibers of the rabbit vagus nerve at 0.03 microgram/ml. At 0.3 microgram/ml and higher concentrations it caused also a nerve depolarization. These effects on nerve membranes were irreversible and could be suppressed by tetrodotoxin (200-500 nM). Nerve fibers depolarized by high extracellular K+(15-30mM) concentrations still displayed long duration action potentials after TsTX-V treatment. It is suggested that TsTX-V blocks the Na+ channel inactivation system probably as an alpha-toxin.
Assuntos
Neurotoxinas/isolamento & purificação , Venenos de Escorpião/isolamento & purificação , Canais de Sódio/efeitos dos fármacos , Potenciais de Ação , Aminoácidos/análise , Animais , Pressão Sanguínea/efeitos dos fármacos , Dose Letal Mediana , Camundongos , Peso Molecular , Neurotoxinas/química , Neurotoxinas/farmacologia , Coelhos , Ratos , Venenos de Escorpião/química , Venenos de Escorpião/farmacologia , Nervo Vago/efeitos dos fármacosRESUMO
1. Conventional microelectrode techniques were used for intracellular recordings of the transmembrane electrical potentials, the effects of berberine were studied on canine cardiac Purkinje and ventricular muscle fibres and on rabbit atrial fibres. 2. Berberine (3-30 microM) increased in a concentration-dependent manner, the action potential duration (APD) in canine Purkinje and ventricular muscle without affecting other parameters of the action potential. 3. The berberine-induced enlargement of the APD showed reverse use-dependence, so that the effect was greater at lower rates of stimulation. 4. Preparations perfused with berberine (30 microM) and driven at rates below 0.5 Hz exhibited early after depolarizations which persisted 3-4 h after washing. 5. The early afterdepolarizations were reversibly abolished by perfusion with lignocaine (3 microM) or by the increase in the rate of stimulation. 6. The effective refractory period (ERP) of Purkinje fibres was greatly increased by berberine (30 microM); however, the ratio ERP/APD was not significantly affected. 7. Berberine (10-100 microM) decreased in a concentration-dependent manner the spontaneous frequency of rabbit sinoatrial cells. The decrease in frequency was accompanied by a depression of the phase 4 depolarization, without significant changes in other parameters of the nodal action potential. 8. Atropine (2.5 microM) did not affect the bradycardic effect of berberine. On the other hand, berberine (30 microM) did not alter the chronotropic effect of isoprenaline. 9. Berberine (30 microM) also increased the duration of slow responses in K-depolarized rabbit atrial muscle fibres, other parameters being unaffected. 10. It is suggested that berberine exerts Class III antiarrhythmic and proarrhythmic actions in cardiac muscle of the dog in vitro.
Assuntos
Berberina/farmacologia , Átrios do Coração/efeitos dos fármacos , Ventrículos do Coração/efeitos dos fármacos , Ramos Subendocárdicos/efeitos dos fármacos , Potenciais de Ação/efeitos dos fármacos , Animais , Cães , Potenciais da Membrana/efeitos dos fármacos , Microeletrodos , Coelhos , Nó Sinoatrial/efeitos dos fármacosRESUMO
Toxins T1IV (TsTX-III) and T2IV have been purified to homogeneity from Tityus serrulatus scorpion venom and further characterized. Their amino acid composition and SDS-PAGE reveal an approximate mol. wt of 7000. Their intracisternal LD50 (micrograms/kg) in mice were 12.9 +/- 1.6 and 3.0 +/- 0.5, while their N-terminal amino acid sequences were K-E-G-Y-A-M-D-H-E-G-C-K-F-S- and K-E-G-Y-L-M-D-H-E-G-C-K-L-S-C-F-I-R-P-S-G-Y-C-G-R-E-, respectively. This sequence of T2IV, its amino acid composition and its chromatographic and electrophoretic behaviour identify it as toxin gamma (TsTX-I), which is the major toxin from this venom. TsTX-III (13 to 102 micrograms/kg) produced a long lasting enhancement of the hypertensive effect of noradrenaline and a slight decrease of the hypotensive effect of acetylcholine, while T2IV (115 micrograms/kg) induced a prolonged hypotensive effect on the anesthetized rat. On the isolated guinea-pig vas deferens, TsTX-III (2.1 and 3.0 micrograms/ml) produced a horizontal shift of the dose-response curve for noradrenaline to the left with no change of the maximal response. At a concentration of 1.43 microM, it induced a prolongation of the duration of the B component of the compound action potential. This prolongation was strongly reduced after addition of tetrodotoxin.
Assuntos
Neurotoxinas/análise , Venenos de Escorpião/análise , Acetilcolina/farmacologia , Potenciais de Ação/efeitos dos fármacos , Sequência de Aminoácidos , Animais , Pressão Sanguínea/efeitos dos fármacos , Eletroforese em Gel de Poliacrilamida , Cobaias , Técnicas In Vitro , Masculino , Dados de Sequência Molecular , Peso Molecular , Músculo Liso/efeitos dos fármacos , Neurônios/efeitos dos fármacos , Neurotoxinas/farmacologia , Norepinefrina/farmacologia , Coelhos , Ratos , Ratos Endogâmicos , Venenos de Escorpião/farmacologia , Nervo Vago/citologia , Nervo Vago/efeitos dos fármacos , Ducto Deferente/efeitos dos fármacosRESUMO
1. Using conventional microelectrode techniques for the intracellular recordings of the membrane potential, the effects of labetalol were studied on cardiac Purkinje, atrial and ventricular muscle fibres of the dog. 2. Labetalol (1-10 microM) reduced, in a concentration-dependent manner, the action potential amplitude (APA) and the maximum rate of rise of the action potential (Vmax) in Purkinje fibres. 3. The action potential duration (APD) was decreased in Purkinje fibres but significantly increased in ventricular fibres after small concentrations of labetalol (1-3 microM). The atrial fibres were not very sensitive to labetalol. 4. Depolarization of the cardiac Purkinje fibres by increasing the external potassium concentration (8-12 mM), potentiated the labetalol effects on APA and Vmax but blocked its effects on the APD. 5. The effects of labetalol on Vmax of Purkinje fibres were dependent on the frequency of stimulation. 6. The ratio of the effective refractory period to the APD was increased both in normally polarized and depolarized Purkinje fibres after treatment with labetalol (10 microM). 7. Labetalol (10 microM) shifted the membrane responsiveness curve of Purkinje fibres by about 10 mV in the hyperpolarizing direction. 8. The slow response obtained in K-depolarized, Ba-treated Purkinje fibres was not significantly affected by labetalol (10-100 microM). 9. It is suggested that labetalol can exert Class I and Class III antiarrhythmic actions in cardiac muscle of the dog in vitro.
Assuntos
Coração/efeitos dos fármacos , Labetalol/farmacologia , Potenciais de Ação/efeitos dos fármacos , Animais , Cães , Átrios do Coração/efeitos dos fármacos , Ventrículos do Coração/efeitos dos fármacos , Técnicas In Vitro , Microeletrodos , Ramos Subendocárdicos/efeitos dos fármacosRESUMO
1 The effects of the venom of the scorpion (Tityus serrulatus) on nerve fibres of the rabbit cervical vagus were studied by the single sucrose-gap technique. Scorpion venom (1 microgram/ml) increased irreversibly the duration of the B component of the compound action potential of the vagus nerves, leaving the C component with its normal configuration. Tetrodotoxin (200 nM) suppressed the prolongation of the action potential duration in venom-treated B fibres. 2 At the same concentration (1 microgram/ml), scorpion venom reduced the amplitude and the rate constant of decay of the hyperpolarization produced by tetanic stimulation of non-myelinated nerve fibres. 3 A lower concentration (0.2 micrograms/ml) blocked completely the hyperpolarization of the potassium-activated response. After washing, the potassium-activated response partially recovered its amplitude but there was a significant increase in the time constant of the decay of hyperpolarization. 4 It is suggested that scorpion venom may modify the sodium pumping mechanism within fibres as well as affecting the passive and active sodium permeability systems.