sufficient for 5HT-like activity. Tryptamine analogs differ in mode of 

 action and potency. Congeners without a 5-hydroxyl group tended to act 

 more slowly and irreversibly, and less strongly than 5HT. Methyl sub- 

 stitution also increased the time of action and difficulty of reversal. 

 Potency of such compounds may be increased or decreased depending upon 

 position of substitution and presence of the 5-hydroxyl group. - J.L.M. 



701 



Greenberg, M. J. 1963. 



The effect of changes in rate on the amplitude of beat of bivalve hearts. 

 Proc. 16th. Internatl. Congr. Zool. 2: 109 (abstract). 



Quiescent, isolated ventricles of Mercenaria mercenaria can be induced to 

 beat by electrical stimulation. Changes in amplitude of an isotonic 

 preparation induced by alterations in rate of stimulation can then be 

 determined. Height of contraction decreases as general interval between 

 beats is decreased from 20 sec to 0.1 sec at 25°C. Tone rises markedly as 

 interval becomes smaller than 1 sec. The amplitude-interval relationship 

 helps in understanding phenomena observable in performance of isotonic 

 bivalve heart preparations. Acetylcholine (ACh) usually decreases amplitude 

 and frequency of normally beating bivalve hearts. Near threshold, only 

 frequency is depressed, while amplitude of beat increases. Drugs which 

 primarily increase frequency and tone, like epinephrine, produce diminished 

 contraction at near threshold doses. 5-Hydroxytryptamine (5HT) usually 

 increases amplitude, but this effect can be blocked selectively, and then 

 5HT also decreases amplitude while increasing rate of beat. Effects of 

 agents on interval-dependent processes appear to be important components of 

 bivalve heart drug responses. As bath temp of Meraenaria heart preparations 

 is increased from 4° to 35°C, frequency increases linearly while amplitude 

 peaks at about 10° to 15°C. The amplitude-interval relationship suggests 

 that this nonlinearity might happen, at least in part, from superposition of 

 a frequency-dependent change on a set of temperature-dependent changes. 

 Occasional preparations beat in sustained bouts separated by periods of 

 quiescence. The first beat of the bout is large; successive contractions 

 decrease until a lower equilibrium value is reached. Sometimes bouts are 

 very short (4-5 contractions) and equilibrium is never achieved. This is 

 "rest contraction". Another phenomenon is appearance of frequent 

 extrasystoles in irregularly beating ventricles. Similar phenomena have 

 been observed in mammalian hearts. Intropic and chronotropic control 

 mechanisms probably are similar in mammalian and molluscan cardiac muscle. 

 - J.L.M. 



702 



Greenberg, Michael J. 1965. 



A compendium of responses of bivalve hearts to acetylcholine. Comp. 

 Biochem. Physiol. 14: 513-539. 



About 400 ventricles from some 40 bivalve species were isolated and 

 challenged with a wide range of acetylcholine (ACh) doses. Results varied 

 with species. Even exceptionally high concentrations (10~3 m) of ACh 

 produced no increase in tone or frequency of beat of the heart of Mercenaria 

 meraenaria. Frequency, tone and amplitude are not independent. Amplitude 

 decreases as frequency is increased to 12/min (10-15°C) . At higher 

 frequencies systolic tone begins to rise, but diastolic tone also rises. An 

 inverse relation between force and frequency has been demonstrated only in 

 M. mercenaria among bivalves. Actions of ACh and 5HT on quiescent heart of 

 hard clam are similar to those demonstrated by Greenberg and Jegla (1963) 

 q.v. In M. mercenaria , depressor action of ACh mimics release of this 

 neurohumor from inhibitor neurons running to the heart from the visceral 

 ganglion. There is no evidence that the tone increase produced by high 

 concentrations of ACh is physiological. ACh contracture of the anterior 

 byssus retractor muscle of blue mussel is an analog of the action of 

 cholinergic excitor nerve fibers. But no data show that excitatory 

 cholinergic neurons exist in bivalve hearts. In over 30 experiments it was 



l% 



