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Biomedical subjects

J Zachar

Publications and source records attributed to J Zachar.

At least 55 records · Page 3Linked to original sources
PubMed1978

Membrane currents in a calcium type muscle membrane under voltage clamp.

Four ionic current components were identified in the total membrane current recorded under voltage clamp conditions from the muscle membrane of the crayfish (Astacus fluviatilis). The early inward current component is dependent on the presence of Ca2+ ions, disappears in Ca2+ free solutions and is insensitive to variaton of external Na+ ions and to tetrodotoxin. The outward current consists of at least three components, an early, a late and a slow outward current. The outward currents are sensitive to TEA and their reversal potentials differ. The early potassium current may be separated in a proportion of fibres by a hump from the later potassium current. An insufficient space clamp as a cause of the hump was excluded by comparing the size of the clamped membrane area with the distribution of large membrane clefts in the fibre. The early outward current is critically dependent on the presence of Ca2+ ions and is relatively more sensitive to TEA ions and to conditioning depolarisation than the late outward current.

Action Potentials↗
PubMed1978

Changes in electrical properties of muscle membrane systems during decoupling and recoupling induced by glycerol.

In isolated muscle fibres of the frog and of the crayfish the following electrical parameters were determined during the glycerol procedure from the voltage transients at 20 degrees C: the sarcoplasmic resistivity, Ri; the membrane resistance, Rm; the series tubular resistance, Rs; the surface membrane capacity, Cm; the tubular membrane capacity CT. No significant changes were found in fibres equilibrated with glycerol (G) saline. During the washout of glycerol only Ri and Cm remained unchanged. In reversibly decoupled crayfish fibres (300 mM-G) CT decreased to 70%, Rs increased to 175% and Rm increased to 200% of the control values. The changed parameters returned to control values upon reapplication of glycerol. In irreversibly decoupled fibres (500 and 600 mM-G) the changes in CT and Rs were more pronounced; and Rm was decreased. The resting potential remained constant with few mV. In frog fibres the changes in electrical parameters were in the same direction except the decrease of Rm during reversible decoupling (150 mM-G). The corresponding changes in reversible and irreversibly (300 mM-G) detubulated fibres were as follows: CT--60 (80) %; Rs--10 (14) times; Rm--50 (35) %.

Animals↗
PubMed1977

Calcium currents and conductances in the msucle membrane of the crayfish.

1. Membrane currents in calcium type muscle membrane of the cray-fish Astacus fluviatilis were analysed by a method in which a membrane microarea was isolated by circulating sucrose rings contacting the fibre perpendicular to the fibre surface.2. The early calcium inward currents were separated from the total membrane currents by subtraction of the early and delayed potassium currents from the total membrane current.3. The isolated calcium currents show a time course characteristic for a transient change of calcium conductance. The presence of inactivation was further checked by the time course of the tail currents at the end of voltage clamp pulses of variable duration.4. The reversal potential of the early calcium currents determined from the current-voltage relations was +85 +/- 4.2 mV. The calcium potentials were used to express the calcium currents in the form of chord conductances.5. Calcium conductances (g(Ca)) as functions of time and voltage were found to be described quantitatively on the assumption that g(Ca) is determined by two variables (m and h), according to the equation g(Ca) = m(6)hg(Ca), where g(Ca) is a constant and m and h obey first order differential equations of the Hodgkin-Huxley type.6. The activation parameters of the g(Ca) were determined by fitting the solutions of the above equations to the experimental values of the g(Ca). This method was also used to check the inactivation parameters.7. The inactivation parameters of the g(Ca) were obtained from the inactivation curves, which were determined for several membrane potentials by variation of the duration of the conditioning step.8. The average calcium conductance constants were tabulated and compared with sodium conductance constants in excitable membranes.

Animals↗
PubMed1966

Potassium contractures in single muscle fibres of the crayfish.

1. Contractures were evoked in isolated muscle fibres of the extensor carpopoditi muscle of the crayfish (Astacus fluviatilis) by increasing [K](o) at constant [Cl](o) or at constant [K](o)[Cl](o) product.2. The relation between tension and log [K](o) is S-shaped with a less steep slope if [K](o) was increased at constant [Cl](o). This is due to a smaller drop in membrane potential for a given change in [K](o) in the latter case.3. The curves relating the tension to the membrane potential overlap in either case. In the linear part of the curve, the slope is around 0.3 kg cm(-2)m V(-1).4. The mechanical threshold of contracture is about -55 mV and mechanical saturation is at -20 mV.5. Fibres exert the greatest tension when stretched to 1.25 l(o) (8 kg/cm(2)), where l(o) is the length at which the fibres are just taut in the solution. Tension falls on either side of this optimal length. Tension vanishes when the fibre is stretched to 1.95 l(o).6. Sarcomere length at optimal fibre length is 10.5 +/- 0.3 mu. The A band is 3.95 +/- 0.8 mu long and does not alter during stretch.7. The crayfish muscle fibres were of the phasic type, since they relaxed spontaneously at maintained high [K](o).8. At [K](o) near saturation point, contractures attain the maximum tension in 5 +/- 1.3 sec and the time to half decay is 8.1 +/- 0.5 sec.9. If the contracture is allowed to relax spontaneously, it is not possible to obtain initial tension until after 20-30 min. When the contracture is terminated by a return to low [K](o) after reaching its maximum, but before spontaneous relaxation appears, the fibre is capable of repeatedly exerting the initial tension.10. The rate of recovery after a spontaneously relaxed contracture depends on [K](o) in the solution, in which the fibre lies before evoking the test contracture. The relation of recovery upon log [K](o) is S-shaped and the tension is the greater, the lower the [K](o) in the solution in which recovery is taking place.

Animals↗