Marine Invertebrate Preparations useful in the Bioassay of Acetylcholine and 5-Hydroxytryptamine

Abstract

CERTAIN isolated muscle and heart preparations of marine invertebrates are highly sensitive to acetylcholine, to 5-hydroxytryptamine (enteramine, serotonin), or to both of these substances. Bacq¹ was the first and Ambache and Sawaya² among the more recent to demonstrate the sensitivity to acetylcholine of muscle strips from holothurians (Echinodermata). Inhibition of heart-beat of the bivalve mollusc, Venus mercenaria, was first used by Welsh³ for the quantitative estimation of acetylcholine in tissue extracts. Recently, the same heart has been found to respond by increases in amplitude and frequency to low concentrations of 5-hydroxytryptamine⁴. Earlier, it had been reported^5,6 that certain molluscan hearts had a low threshold to 5-hydroxytryptamine or tissue extracts containing this substance. At the Marine Biological Laboratory, during the summer of 1953, the need arose for the quantitative estimation of acetylcholine and 5-hydroxytryptamine in certain tissues. Several species of echinoderms and molluscs from the Plymouth region were secured, and isolated muscle and heart preparations were screened for their suitability in bioassay of these substances. Available holothurians were Holothuria nigra, Cucumaria normani, C. elongata, and a single specimen of Thyone sp. Pieces or entire lengths of the longitudinal muscle bands were dissected free from the inner body wall and set up in the manner commonly used for muscle strips during recording of contractions. In the case of C. normani the peristomal (pharyngeal) retractor muscles were also tried. While all these preparations contracted in low concentrations of acetylcholine, especially after eserinization, the difficulties that were encountered by earlier workers^1,7 who had used other species of holothurians were obvious. Chief among these were spontaneous contractions, tendency to contract and remain contracted after eserine, and sensitivity to mechanical stimulation after eserine. This last was especially marked in the muscles from the single specimen of Thyone which, otherwise, were most promising since they were not spontaneously active.