Junction potentials and action potentials of the crayfish myocardium.

Introduction: The purpose of this investigation was to study the properties of junction and action potentials elicited by nerve stimulation in the absence of the cardiac ganglion in crayfish myocardium.
Methods and Results: The cardiac ganglion was surgically removed in isolated crayfish hearts. Electrical stimulation of one of the peripheral anterolateral nerves provided isolated junction potentials and action potentials free of the usual postspike bursts of junction potentials. The single junction potentials displayed amplitudes of up to 25 mV and slow exponential decay; the mean time constant was 170 +/- 13 msec (approximately x +/- SD). In fully recovered tissue, the junction potentials triggered action potentials free of repetitive subthreshold discharges. Tetrodotoxin did not alter the amplitude or shape of action potentials initiated by direct electrical stimulation of the muscle cells. Calcium channel blocking agents such as Cd2+ and Ni2+ eliminated the action potentials but not the junction potentials. Tetraethylammonium markedly prolonged the action potential duration.
Conclusions: Our data suggest that: (1) A slow decay of the junction potentials may result from the disappearance of the neurotransmitter; this process also accounts for the late slow repolarization of the final part of the action potentials; (2) The equilibrium potential of the junction potentials is close to 0 mV; (3) The upstroke of the action potentials is carried by calcium currents; (4) The fast repolarization phase of the action potentials is likely caused by the delayed rectifier; and (5) The refractory phase outlasts the duration of the action potential.