This study demonstrates that isolated tunicate myocardial cells possess intrinsic pacemaker capabilities and can synchronize, with dormant pacemaker centers activated by mechanical stimuli such as increased blood pressure.
Intrinsic pacemaker activity in tunicates leaves open relevance to mammalian hearts; no clinical implications yet.
The primary pacemakers in the tunicate heart are located near the cardiovascular junctions close to the raphe that connects the V-shaped heart to the pericardium. However, small isolated pieces of the ring of myocardial cells at the ends of the heart were found to have nearly equal pacemaker capabilities. Arrhythmia in one primary pacemaker region was found to result from activity of two or more centers. 2. Following isolation of a small piece of tissue in sea water, 20-50% of the cells were observed to pulsate and usually within a few minutes all cells contracted synchronously. Reversals in the direction of conduction in strips of myocardium without any primary pacemaker region were observed. 3. The C center located in the middle of the heart was found to be dormant during normal heart activity but in opened animals it was activated by increasing the blood pressure.
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Mahlon E. Kriebel (1968) studied this question.
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