Since the demonstration by Prosser (1940) of the unusual sensitivity to acetylcholine (Ach) of the isolated heart of the bivalve molusc, Venus mercenaria, we have extensively employed this preparation for the bio-assay of Ach in tissue extracts (e.g. Welsh, 1943; Welsh and Hyde, 1944a and b;Prajmovsky and Welsh, 1948).In certain respects it is superior to the classical Ach assay preparations such as the dorsal muscle of the leech, rectus abdominis of the frog, isolated frog heart, and blood pressure of cat. For example, it is more sensitive to Ach, with complete inhibition occurring at about 50 times the threshold inhibitory concentration ; it is relatively unaffected by changes in pH, inorganic ions, and tissue constituents other than Ach ; it recovers quickly, thereby allowing more rapid estimation than the abovementioned preparations. While employing the Venus heart for bio-assay, its responses to a variety of drugs, organic compounds, and inorganic ions have been studied and, in particular, to a series of choline esters and analogs this in the hope of obtaining evidence toward a better understanding of the fundamental mode of action of Ach. An organ with a high specificity for choline esters, exhibiting a response which is easily quantified, and which has so little self-contained cholinesterase that blocking of this enzyme is not necessary when working at great dilutions of the unstable esters, provides a suitable object for studying certain aspects of the mechanism by which Ach acts on cells.

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