STUDIES ON CARDIOVASCULAR PHYSIOLOGY OF TUNICATES

¹ Department of Zoology, University of Washington, Seattle, Washington 98105; the Marine Biological Laboratory, Woods Hole, Massachusetts 02543; and the Friday Harbor Laboratories, San Juan Island, Washington 98250

1. Experiments were conducted on isolated and in situ hearts of 4 species of tunicates in order to investigate beat reversal. The mechanical and electrical properties of in situ and isolated hearts were found to be the same. The intracardiac pressure was found to affect the excitability of pacemakers and it could be adjusted so that reversals did not occur, indicating that pacemakers did not fatigue. Small changes in the intracardiac pressure (or intracardiac blood volume) altered beat frequency and the duration of a pulsation-series. The greatest beat frequency was obtained with 5-mm. intracardiac water pressure. After a change in intracardiac pressure, pacemakers showed the characteristics of adaptation.

2. Localized heating of an active pacemaker or localized cooling of an active pacemaker initiated reversal. Pacemaker regions under localized heating remained active for periods of 8 hours without slowing.

3. Pacemakers which had just stopped after a pulsation-series became active again when either the hearts were bisected, the pacemaker region was locally heated, the surrounding myocardium was stimulated electrically, the arterial vessel was occluded or the venous pressure was increased.

4. Two modes of beat reversal were observed: (1) hearts in relaxed animals decreased in frequency and stopped, after a few seconds the formerly inactive pacemaker increased in frequency, reached a steady frequency, slowed and then stopped; the cycle was repeated; (2) pacemakers in most isolated hearts and in hearts in damaged or contracted animals alternated in dominance after periods in which they were both active. It is suggested that reversals occurring after pauses in pacemaker activity result from a decrease in venous pressure which lowers pacemaker excitability and adaptation of the pacemaker which leads to its arrest; and an increase in arterial pressure which raises the excitability of the previously inactive pacemaker so it begins after the reversal-pause. The second mode of reversal results from a shift in phase of the refractory period of the heart (at each end) and the continuous activity of both pacemakers. Similar reversals were produced when the ends of the heart were electrically stimulated with two stimulators set at different frequencies.

5. Cinematographic analyses of hearts in vivo demonstrate that the average velocity of the wave of contraction was the same in both directions of contraction (6 mm./sec. at 10° C.) and that there was a gradual decrease in diastolic filling and cardiac output during a pulsation-series.

6. The stroke volume in 8 cm. long Ciona (heart 30 mm. long) was 45 mm³. At 20 beats/min., this was 6.6 cc. blood/min./100 g. animal. The blood volume was determined to be 4% of the body weight by exsanguination. During a Ciona heart pulsation-series, 13.5 cc. of blood were ejected, which was equal to the weight of the animal, demonstrating that blood returns to the heart and does not oscillate.

7. The formation of the functional valves in the heart wall (contraction valve and rest-fold) was concluded to be due to differences in arterial and venous blood pressure since there are no specialized areas of the heart wall.

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