The Sites of Action of Pericardial Organ Extract and 5-Hydroxytryptamine in the Decapod Crustacean Heart

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Abstract
Responses of isolated, perfused hearts of Homarm americanus to brief, internal application of extracts of pericardial organs (PO's) ofCancer borealis, or 5-hydroxytryptamine (5HT) are very similar over a thousand-told range of concentration: an increase in rate and amplitude of beating. These reach their maxima after washing out has begun, and recover within ten minutes. External application is ineffective and the substances do not interact with effects of stretch stimulation. Intracellular recording from heart muscle fibers reveals facilitation of depolarization to bursts at heart beat frequencies. There may be some effect of 5HT directly on neuromuscular facilitation. Responses recorded from isolated cardiac ganglia show increased burst rate, burst duration, or both. Thresholds and the range of concentrations for which coordinated responses are recorded correspond to those for perfused hearts. It is concluded that the major sites of action of PO extract and 5HT are in the cardiac ganglion. 5HT tachyphlaxis and LSD block effects of 5HT, but not of PO extract or accelerator nerve stimulation. Intracellular recordings from the large ganglion cells show no effects on resting, synaptic, or spike potentials. Changes in membrane potential to current pulses revealed no changes in membrane resistance or in the resistance of the electrotonic pathway between cells. Results of selective application to large or small cells suggested that PO extract may contain a rate-increasing substance and one prolonging the duration of bursts. The former, and 5HT, may influence pacemaker potentials; the latter may increase the number of spikes a unit can produce before becoming refractory.