Fine structure of the heart and the effects on the heartbeat of some transmitter candidates in crustacean cardioregulatory system were examined in the myogenic heart of the branchiopod crustacean Triops longicaudatus. Electron microscopy revealed that, in each myocardial cell, myofibrils are confined in the part facing the epicardium and intercalated disks are present between the myofibrillar regions of adjacent myocardial cells. No neural elements were found in the heart, suggesting lack of extrinsic cardioregulatory nerves from the central nervous system. Gamma aminobutyric acid and acetylcholine produced no detect-able changes in the myogenic activity of the heart at concentrations up to 10−3 M, respectively. Glutamate induced a depolarizing membrane response in the cardiac muscle with a threshold concentration of approximately 1×10−5 M. The amplitude of the depolarizing response was concetration-dependent and saturated at approximately 1×10−4 M. The myogenic activity of the heart increased in frequency with glutamate of less than approximately 3×10−5 M. With higher dose of glutamate, action potential adaptation occurred in the cardiac muscle and the heart exhibited a systolic arrest.
