ホヤ心臓の電気刺激に対する反応

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The effect of electrical stimulation on the tunicate heart in situ has been studied on Cioma intestinalis and Styela plicata obtained at the Misaki Marine Biological Station. Most experiments were performed on Ciona. The animal, the heart of which was exposed by removing the surrounding tissues and pericardial sac, was placed in a Petridish containing sea water. Rectangular current pulses from an electronic stimulator (5-20 msec duration) were applied to the heart through a pair of stimulating electrodes: one was a fine Ag-AgCl wire electrode insulated except for the tip (diameter, 20-50μ) which made contact with the heart surface, while the other was a Ag-AgCl plate (1×2cm) immersed in the outside medium. A unipolar cardiac electrogram was also recorded by use of a RC-amplifier (time constant, 1.4sec) and an oscilloscope. The results obtained are summarized as follows: 1) The heart of Ciona normally continued to beat at a constant frequency (50-60/min) with periodic reversal of beating direction. Usually the activity of hypobranchial pacemaker lasted longer than that of the visceral one. The cardiac electrogram consisted of a series of simple diphasic waves or complicated ones. No remarkable potential change could be seen during the pause at beat reversal. 2) Response of the heart to single electrical stimuli was not an all-or-none type. A weak stimulus barely produced a localized contraction around the tip of the electrode, whereas a stronger one gave rise to a propagated contraction wave which sometimes collided with a spontaneous contraction wave. Graded nature of response was most marked in the hearts beating irregularly at very low frequencies (1-10/min). 3) A strong stimulus caused simultaneous contraction including a fairly large part of the heart irrespective of the position of stimulating electrode or the phase of spontaneous heart-beat at the moment of stimulation. Such an extrasystole always eliminated the propagating contraction wave and was followed by a pause of a spontaneous heart-beat which may be comparable with the compensatory pause in vertebrate heart. The duration of the pause was somewhat dependent on the stimulus strength; the stronger the stimulus strength the longer the duration of the pause. The cause of the pause may be the refractory period of the contracted part. 4) The threshold for provoking contraction was not uniform along the heart surface, being highest at he middle part and lowest at the pacemaker regions. The threshold at the active pacemaker region was lower than that at the non-active one. Change in direction of the stimulating current had little effect on the threshold. 5) The refractory period examined by two successive stimuli of varying intervals was fairly uniform along the heart surface, but it showed a wide range of individual variation according to the condition of the heart, ranging from 0.1 to 3sec. A long refractory period was always observed in the heart beating irregularly at very low frequencies (1-10/min). The presence of the relative refractory period was clearly demonstrated. 6) A series of heart-beats could be initiated by stimulating the pacemaker region repetitively with an adequate stimulus strength. The resulting beating frequency was only a fraction of the stimulus frequency if the latter was more than 300/min; each heart-beat was initiated by more than two stimuli. The increase in the stimulus strength increased the resulting beating frequency to a certain extent, indicating the relative refractoriness of the stimulated region. Concerning direct current stimulation, on the other hand, no definite result was obtained. 7) The middle part of an injured heart sometimes showed rhythmic vibration of high frequency (200-250/min), which seemed to be comparable with the atrial or ventricular fibrillation in vertebrate heart.
社団法人日本動物学会の論文
1965-02-15

ホヤ心臓の電気刺激に対する反応

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