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Biomedical subjects

N Toda

Publications and source records attributed to N Toda.

At least 415 records · Page 23Linked to original sources
PubMed1969

Interactions of ouabain and noradrenaline in isolated rabbit's atria.

1. The present study was made using sympathetic nerve-atria preparations, conventional atrial preparations and left atria isolated from rabbits in order that interactions of ouabain and noradrenaline on the transmembrane potential of S-A nodal pacemaker fibres, the pacemaker rate and the contractile force of the left atrium could be investigated.2. In pacemaker action potentials, the maximal diastolic potential was decreased, the early repolarization was accelerated and the late repolarization was slowed by toxic concentrations of ouabain. Oscillating potentials of the diastolic pacemaker membrane were produced.3. Stimulation of the postganglionic sympathetic nerve caused an increase in the slope of diastolic depolarization, resulting in an acceleration of the pacemaker rate. In the presence of ouabain, the pacemaker action potential configuration was significantly altered by nerve stimulation in preparations which showed ouabain-induced bradycardia, but the potential configuration was not changed by nerve stimulation in preparations which showed ouabain-induced tachycardia. Cardiac noradrenaline altered the ion permeability of the cardiac pacemaker membrane during diastole, but it had no effect during systole. Sympathetic nerve stimulation transiently corrected the overshoot and the repolarization of atrial action potentials altered by toxic concentrations of ouabain.4. The positive chronotropic effect of sympathetic nerve stimulation was not affected by either therapeutic or toxic concentrations of ouabain. Ouabain shifted the concentration-chronotropic response curve for exogenous noradrenaline to the left, but did not shift the curve for tyramine.5. Relationship between the developed tension and the rate of contraction was altered differently by ouabain and by noradrenaline. The positive inotropic effect of noradrenaline was not significantly altered by therapeutic concentrations of ouabain but it was inhibited by toxic concentrations.

Action Potentials↗
PubMed1969

Effects of calcium, sodium and potassium ions on contractility of isolated atria and their responses to noradrenaline.

1. Rabbit left atrial preparations driven electrically at different rates were used for studies on inotropic effects of cations, drugs and coupled pacing. Sino-atrial node-atrial preparations were used for investigating the chronotropic effect of noradrenaline.2. The contractile tension-driving rate relationship was moved upwards by an elevation of [Ca(++)]o, coupled pacing and noradrenaline. In preparations exposed to Na(+)-poor and K(+)-free solutions the contractile strength at low driving rates (6 to 30 c/min) was markedly enhanced, but at high rates (120 to 240 c/min) it was not influenced. The contractile strength was reduced at low [Ca(++)]o and at high [K(+)]o.3. The positive inotropic effect of noradrenaline was markedly inhibited by a reduction of [Ca(++)]o and to some extent by a reduction of [K(+)]o. The noradrenaline-inotropy was not appreciably affected by an elevation of [Ca(++)]o and a reduction of [Na(+)]o.4. Cardiac excitability studied in preparations driven at high rates was enhanced by noradrenaline, a reduction of [K(+)]o and an elevation of [Ca(++)]o, but was reduced at low [Ca(++)]o, low [Na(+)]o and high [K(+)]o.5. The positive chronotropic response to noradrenaline was enhanced at high [Ca(++)]o, low [Na(+)]o and low [K(+)]o, but was reduced in solutions deficient in Ca(++) or rich in K(+).6. Inotropic effects of the ions and of coupled pacing were compared with those of ouabain. It is suggested that characteristic changes in the tension-rate curve seen in Na(+)-poor, K(+)-free and ouabain-containing solutions are correlated with an inhibition of active processes in the cardiac cell membrane, which affect ionic movements across it. It seems likely that mechanisms mediating adrenergic responses of the contractile tissue and the S-A node are associated with [Ca(++)]o.

Animals↗
PubMed1968

Influence of sodium ions on the membrane potential of the sino-atrial node in response to sympathetic nerve stimulation.

1. Sympathetic nerve-atrial preparations isolated from rabbits were used in the present study. Transmembrane potentials were recorded from sino-atrial (S-A) nodal pace-maker fibres.2. A reduction of [Na(+)](o) (extracellular concentration of sodium ions) decreased the maximal diastolic potential, reduced the size of the overshoot, prolonged the depolarization time, increased the rate of early repolarization and decreased the rate of late repolarization.3. A Na(+)-deficiency markedly augmented the positive chronotropic effect of stimulation of post-ganglionic sympathetic nerves at a frequency of 5/sec but slightly augmented the effect of stimulation at 1 and 20/sec.4. The marked membrane effect of sympathetic nerve stimulation at low [Na(+)](o) was an increase in the slope of slow depolarization during diastole. Corresponding to an induced acceleration of S-A nodal rate, the 90% duration of the action potential was decreased but the other action potential parameters measured were not influenced.5. A reduction of [Na(+)](o) to 51.7 mM produced in the pace-maker membrane, subthreshold oscillations the rate and the amplitude of which were increased by sympathetic nerve stimulation until conducted potentials were generated.6. It is presumed that cardiac noradrenaline elicits an increase in the slope of slow depolarization during late diastole which is due to an increase in Na(+) permeability, a decrease in K(+) permeability or a non-selective increase in ion permeability, but that it does not influence the pace-maker membrane during systole.

Animals↗