CHOLINOCEPTIVE AND ADRENOCEPTIVE PROPERTIES OF THE TUNICATE HEART PACEMAKER

¹ Department of Zoology, University of Washington, Seattle, Washington 98105

1. Many previous investigators have reported that acetylcholine has little or no effect on the beat frequency of intact but isolated hearts. However, it was found that when hearts were split open, Ach at low concentrations (10^-8 g./ml.) stopped the heart beat for up to a minute. Atropine blocked Ach.

2. D-tubocurarine at a concentration of 3 x 10^-7 g./ml. stopped the beat of opened hearts. Its effect was additive to that of Ach.

3. Adrenaline (10^-5 g./ml.) could stop pacemakers. However, a doubling in beat frequency was frequently observed with higher concentrations. Doubling in frequency resulted from an ectopic center which alternated with the primary center, thus driving the heart at twice its original frequency. In hearts which showed arrhythmia resulting from activity of more than one pacemaker center, adrenaline at a low concentration (10^-5 g./ml.) usually stopped one or the other of the centers, decreasing the beat frequency by half.

4. Hearts recovered in low concentrations of either Ach or adrenaline. However, at higher concentrations (Ach 10^-6, adrenaline 5 x 10^-4g./ml.) arrhythmia was induced and hearts did not recover. The occurrence of arrhythmia is discussed with respect to the decremental nature of conduction in the ends of the heart and to the observation that all cells near the ostia are latent or dormant pacemakers.