Electrophysiology of the Heart of an Isopod Crustacean: Porcellio Dilatatus

Abstract

1. The electrical properties (recorded with intracellular microelectrodes) and the mechanical properties of the myocardium of the wood-louse Porcellio dilatatus (Brandt), a terrestrial isopod crustacean, have been investigated. 2. The diastolic membrane potential varied from - 50 to - 70 mV. Several types of spontaneous electrical responses have been recorded. Their amplitude was usually between 30 and 45 mV and their duration varied from 150 to 400 msec. Except for very rare cases, overshoot did not occur. 3. The phase of depolarization began slowly; it became faster but the maximum rate of rise never exceeded 1-2 V/sec. The rising phase, devoid of steps, was followed by a partial repolarization leading to a more or less sustained smooth plateau. Super-imposed changes of potential occurred very rarely. A jagged appearance of the plateau, as seen in numerous neurogenic hearts and interpreted as junction potentials, could not be observed. 4. During spontaneous activity intracellularly applied currents modified the frequency, the time course and the amplitude of the responses. The junctional nature of the rising phase is suggested. 5. At rest the myocardial membrane displayed a ‘normal’ rectifying property and weak delayed rectification; in addition, some preparations showed an active graded response of the membrane. A complete regenerative activity was never triggered. 6. A tentative explanation of the electrical response is proposed: the rising phase could chiefly correspond to a junctional potential, the plateau might be a response of the membrane to the synaptic depolarization. 7. The value of the membrane space constant (about 1 mm) suggests that the small muscle fibres, as observed under optical microscope, are interconnected electrically. The impulses delivered by the heart ganglion would activate the whole myocardium almost simultaneously, resulting in a high apparent conduction velocity and a good synchronization from one end to the other. 8. The degree of tension or of relaxation of the fibres closely depended on the value of the membrane voltage. The magnitude of the contraction depended on the absolute level of the potential and the length of time during which the depolarization was maintained. The functional importance of the plateau phase is considered. Intracellular depolarizing pulses led to a contraction of the heart; conversely, hyperpolarizing pulses led to its relaxation. 9. GABA (10-6 g/ml) inhibited the functioning of the heart and reduced the membrane resistance. Picrotoxin (10-4 g/ml) acted as an antagonist to GABA. L-Glutamic acid (10-4 g/ml) strongly depolarized the membrane, leading to the cessation of the activity.