CARDIOREGULATION IN LIMULUS. III. INHIBITION BY 5-HYDROXYTRYPTAMINE AND ANTAGONISM BY BROMLYSERGIC ACID DIETHYLAMIDE AND PICROTOXIN

¹ Department of Biological Sciences, Purdue University, Lafayette, Indiana 47907

1. Heart rate in Limulus is slowed by 5-hydroxytryptamine (5-HT). The threshold for this inhibition is 4.9 x 10^-8 M.

2. The strength of beat is also reduced in 5-HT solutions. The calculated threshold for this effect is 5.6 x 10^-8 M.

3. Both of these effects are readily reversible.

4. Neither 5-hydroxytryptophan (10^-4 M) or 5-hydroxyindole acetic acid (10^-4 M) have any detectable effects on rate or strength of beating.

5. Applied to the isolated cardiac ganglion, 5-hydroxytryptamine (10^-6 M) decreases the rate of rhythmic discharge, reduces the number of neurons discharging in each burst, and lessens the duration of each burst. All of these effects are also reversible.

6. Bromlysergic acid diethylamide (BOL), 1.6 x 10^-5 M, decreases the ability of the cardioinhibitory nerves to influence heart rate.

7. BOL prevents the rate and strength changes engendered by exogenous 5-HT applied to the isolated heart.

8. Picrotoxin antagonizes the decrease in heart rate produced by application of 5-HT, but synergizes with 5-HT with respect to its strength-decreasing ability.

9. No interaction between BOL and -aminobutyric acid (GABA) could be demonstrated.