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Biomedical subjects

H Reuter

Publications and source records attributed to H Reuter.

At least 199 records · Page 11Linked to original sources
PubMed1969

Sodium current in ventricular myocardial fibers.

Membrane currents were measured in thin bundles of dog ventricular myocardium under voltage-clamp conditions. A rather large initial inward current which had an equilibrium potential at about + 55 millivolts could be recorded. When the external sodium concentration was reduced, the equilibrium potential for this current was shifted by the amount predicted theoretically for a current carried solely by sodium ions. The size of the sodium inward current (I(Na)) was largely dependent on the preceding membrane potential. The I(Na) was completely inactivated if the membrane potential was as low as -45 millivolts. Sodium ions are the main carrier of charge during the rapid depolarization phase of the action potential.

Action Potentials↗
PubMed1969

Calcium current and activation of contraction in ventricular myocardial fibers.

In thin bundles of dog ventricular myocardium, a slow inward current (distinct from the sodium inward current) could be recorded under voltageclamp conditions. This inward current was influenced by changes in external calcium concentration, but it was not dependent on external sodium concentration. Therefore, this current which contributes an appreciable amount of charge transfer during the plateau of the action potential, is carried by calcium ions. In sodium-free solution, the flow of calcium ions into the fiber is directly related to activation of contraction. In sodium-containing solution, however, calcium inward current serves primarily to fill up some intracellular stores from which calcium can be released by moderate depolarization.

Action Potentials↗
PubMed1968

The dependence of calcium efflux from cardiac muscle on temperature and external ion composition.

1. Exchangeable Ca in guinea-pig auricles and ventricular trabeculae of sheep and calf hearts was labelled with (45)Ca and the loss of radioactivity into inactive rinsing solutions of different ion composition was measured for periods up to 6 hr. At no time did the decrease of radioactivity in the muscle follow a single exponential course, while the rate coefficient k (= fraction of (45)Ca lost per minute from muscle into rinsing solution) decreased slightly with time.2. On the basis of the temperature-sensitivity of Ca efflux from auricles the activation energy has been calculated to have a value of 5900 cal/mole, corresponding to a Q(10) of 1.35. 2,4-Dinitrophenol (5.5 x 10(-5) - 5.5 x 10(-4)M) had either no effect on Ca efflux or increased it slightly.3. Compared to control efflux in 1.8 mM [Ca](o) Ca efflux decreased to 70% in Ca-free solution, to 20% in Ca-free, Na-free solution and to 65% in Ca-containing, Na-free solution, NaCl being replaced by either sucrose or LiCl. Quantitatively, Ca efflux from auricles has been shown to depend to a large extent on the ratio [Ca(2+)](o)/[Na(+)](o) (2). The affinity for Na of the activation site for Ca efflux (carrier) is much less than for Ca.4. The efflux from muscles soaked for about 2 hr in Ca-free solution was not linearly related to Ca-concentration in the tissue but followed a square law.5. While Ca content in auricles increased in Ca-containing, Na-poor solution it decreased again when Tyrode solution was readmitted indicating a Na-sensitive Ca net transport in cardiac muscle.6. The results are interpreted in terms of a modified exchange diffusion mechanism (Ussing, 1947) which is responsible for Ca extrusion from mammalian cardiac muscle.

Animals↗
PubMed1968

Slow inactivation of currents in cardiac Purkinje fibres.

1. When membrane currents, associated with repolarization to the holding potential, were measured in cardiac Purkinje fibres after depolarizing voltage clamp steps to the plateau level of the action potential (-20 to -10 mV), tails of inward current were observed which turned into outward current. Amplitude and duration of the tails of inward current increased when the preceding depolarization was progressively shortened. This current could only be recorded in Na-containing solution which strongly supports the view that the inactivation of sodium conductance (g(Na)) has a slow and incomplete component.2. Initial outward current (most probably chloride current (I(Cl))) during strong depolarization (beyond -20 mV) could be inactivated either by increasing the frequency of depolarizations or by applying a conditioning depolarizing prepulse.3. In Na-free solution a small, slowly decreasing inward current was recorded. This is most probably carried by calcium ions and is at least partly responsible for the negative conductance of the current-voltage relation in the voltage range -60 to -30 mV.4. If a strong conditioning depolarization (V(1)) to the inside positive potential range preceded repolarizing clamp steps to different membrane potentials (V(2)) inward current (probably calcium current (I(Ca)) could be activated in Na-free and Na-containing solutions. This current depended on the duration of V(1) as well as on the potential level of V(2). After the maximum inward current, increasing outward current occurred when the duration of V(1) was increased. This current could be explained either by an inactivation of calcium conductance (g(Ca)) with time or by a very slow activation of an additional outward current.5. The main conclusions from these experiments are that (i) slow and incomplete inactivation of g(Na) are important features for the plateau of the action potential and (ii) calcium ions carry additional charge during this phase of the action potential. Chloride conductance (g(Cl)) should be largely inactivated at a heart rate of 90/min.

Action Potentials↗