EFFECTS OF BIOGENIC AMINES ON THE ISOLATED VENTRICLE OF THE ATLANTIC RIBBED MUSSEL, Geukensia demissa.

Recent reports suggest that octopamine, a biogenic amine that is ubiquitous in arthropods, is a cardioexcitatory agent in the blue mussel, Mytilus edulis.   To investigate the effects of octopamine on bivalve hearts, we isolated ventricles from Guekensia demissa and suspended the preparations in organ baths.   The mechanical activity of the isolated ventricles was monitored with force transducers (Grass FT 03) and an ink-writing oscillograph (Grass 7).   The ventricles were challenged with doses of octopamine, dopamine, histidine, epinephrine, and serotonin ranging from 1 x 10^-8 to 1x10^-4 M.  Octopamine had mixed effects.  Some preparations were excited by bath concentrations of 1 x10^-5 M; 10^-4 M octopamine excited some preparations and inhibited others.  Excitation consisted of slight increases in the amplitude of contraction.  Inhibitory responses consisted of negative ionotropy and chronotropy.  Dopamine was cardioexcitatory at concentrations of 10^-5 M and above.  Bath concentrations of 10^-6 M of either histidine or epinephrine increased the amplitude of contraction.  Serotonin was cardioinhibitory, causing arrest at bath concentrations of 10^-6 M.  

The results of these experiments with epinephrine and dopamine are largely consistent with published data on the isolated ventricles of a number of other bivalves.   In most species, the heart is excited by both of these agents, albeit at high concentrations.  Serotonin is cardioexcitatory in most species, but can be inhibitory (as in the ventricles from Louisiana populations of G. demissa) in some species.  The effects of octopamine and histidine on the hearts of bivalves have not been extensively studied.  With respect to G. demissa hearts, histidine mimics the excitatory effects of epinephrine while octopamine results in both excitatory and inhibitory effects.  The generality of these effects to other species is unknown.  The key question is:  what is the nature of the biogenic amine receptors in bivalve ventricles?  The effects of epinephrine and histidine at relatively low bath concentrations suggests that authentic receptors for these neurohumors are present.  The higher effective concentrations of octopamine and dopamine may indicate that these agents activate receptors for other amines.  On the other hand, there may be octopamine and histidine receptors with low binding affinity or at low density in the hearts of some bivalves.