THE ACTION OF ACETYLCHOLINE ANTAGONISTS ON THE HEART OF VENUS MERCENARIA*

Pharmacological Studies on the Heart of Tapes Watlingi: A Mollusc of the Family Veneridae

“…Hexamethonium and decamethonium block cholinergic transmission at vertebrate autonomic ganglia and neuromuscular junctions respectively and might have been expected to provide some information about the nature of the receptor mechanism on the Tapes heart. However, neither compound had any antiacetylcholine action at concentrations of 10-5 M. A similar lack of effect has been reported for pentamethonium and decamethonium on Venus heart (Welsh & Taub, 1953).…”

“…Nicotine had a dual action on the heart with excitation predominating, whereas on the Venus heart it has an inhibitory action at similar concentrations (Welsh & Taub, 1953). Tubocurarine also had mixed effects on the Tapes heart, those illustrated in Fig.…”

“…The action of acetylcholine antagonists In their preliminary report on the pharmacology of the Tapes heart, Ladd & Thorburn (1955) discussed the failure of atropine to modify the inhibitory action of acetylcholine, but did not mention further studies with antagonists of acetylcholine. Extensive investigations into the action of acetylcholine antagonists on the Venus heart (Luduejia & Brown, 1952;Welsh & Taub, 1953) blocked the effects of doses of acetylcholine which were sufficient to inhibit completely cardiac contractions ( Fig. 5 and 7).…”

Depression of Autonomic Ganglia by Barbiturates

“…It would seem unlikely that barbiturates have a similar mode of action, since their chemical structures bear little resemblance either to acetylcholine itself oT to the quaternary ammonium blocking agents. On the other hand, barbitone and phenobarbitone have been shown by Fernando (1952) to block the action of acetylcholine on the heart of Venus mercenaria, and Welsh and Taub (1953) have since suggested that the "carbonyl" The diminished excitability of the ganglion cells to acetylcholine, produced by amylobarbitone, might, however, be due to a less specific " blanketing" effect of the drug on the cell surface, or to a straightforward interference with essential cellular metabolic processes, such as has been described for narcotics on isolated brain tissue by Quastel and Wheatley (1932), and others. For the depression of spinal cord motor-neurone synapses by pentobarbitone, Eccles (1946) has suggested an increased stability of the nerve cell membranes to electrical changes.…”

Actions of Derivatives of Lysergic Acid on the Heart of Venus Mercenaria

“…It is suggested here that the amino-alcohol of lysergic acid derivatives provides a further important combining group, possibly to an adjacent 5-hydroxytryptamine receptor in the heart muscle. There is some indication that, in the Venus heart, acetylcholine receptor sites occur in pairs (Welsh & Taub, 1953). This may account, in part, for the effectiveness of benzoquinonium as an acetylcholine antagonist on this preparation, as first observed by Luduena & Brown (1952).…”