微小電極法による心筋細胞活動電位持続時間と刺激頻度との関係について

抄録

The characterisitic configuration of the action potential of the cardiac muscle cell is its long sustaining "plateau" phase. The variations in the cardiac action potential duration produced by the changes in the frequency of stimulation are principally accounted for by the variations in this phase. The relationship between the action potential duration and the stimulation frequency is expected to shed some light on the problem of QT duration in ECG at various heart rates. The influence of the stimulus frequency on the electrical activity of heart muscle cell were studied with the ultramicroelectrode technique at varying conditions of medium extracellular fluid. Material and method. The resting and action potentials of the cardiac ventricular muscle of tortoise (Clemmys japonica) were recorded with the LlNG-GERARD type microelectrode. To overcome the disturbances due to vigorous mechanical movements, flexibly mounted suspending electrodes with copper wire of about 40μ in diameter and 20 cm long were often used. The composition of the reference medium solution was as follows : NaCl 136.8 mM/L, KCl 2.94 mM/L, CaCl₂ 1.84 mM/L, NaHCO₃ 2.5 mM/L, and pH was adjusted to 7.3. Results and comments. 1) There is always stable relationship between the interval of impulses and the action potential durations, which can be described as an exponential function. The results of the experiments are consistent with the expression derived by Carmeliet from the experiments on frog cardiac muscle. At the same time, at least in a certain range, the relation between the impulse frequency and the action potential duration could be described as a straight line. Except in the refractory phase, these relationships are very stable and are exactly reproducible. It is inferred that the variation of action potential duration with the changes in impulse frequency is on a fairly stabler cellular mechanism. 2) The changes in action potential duration depend chiefly on the changes in the duration of the second phase ("plateau" phase) of action potential. In the case of tortoise, there are changes in the third phase of action potential too, which is not found in the case of frog. 3) There is another regularity between the action potential duration and its preceding resting phase duration, which could be observed even when the regularities between action potential duration and interval of impulses and/or impulse frequency are lost in irregular excitation with intensively frequent impulses. 4) In a solution with high concentration of KCl, the variation of action potential duration with the change of impulse frequency is more marked and in the solution with low concentration of KCl milder, Besides NOBLE'S postulation that, at high stimulation frequency, the succeeding action potential would start at a time when the raised K permeability of cellular membrane due to the foregoing one is yet high, it may be supplemented, at high extracellular K concentration, by the possibility that, because of the raising activity in K permeability of high K bathing solution, the late component in the rise of K permeability at the active cellular membrane might be accelerated during repolarization, with consequence of the more intense shortening of action potential duration at frequent impulses. 5) The degree of the. shortening of action potential duration for frequent impulses is marked in a solution with high concentration of NaCl and is mild in a solution with low concentration. It is postulated that the increase of extracellular Na ions influences upon the K permeability of cellular membrane and accelerates the repolarization phase. 6) The degree of the shortening of action potential duration is mild in a solution with high concentration of CaCl₂ and marked with low concentration of CaCl₂. [the rest omitted]