Electrophysiology of a myoid epithelium inChelophyes (Coelenterata: Siphonophora)

Summary

The passive electrical properties, and the ionic basis of the action potential have been examined in the subumbrella myoid epithelium of the siphonophoreChelophyes. The myoepithelial cells are electrically coupled, and are 20 μm wide, some 1 mm long, and only 5 μm thick. Membrane constants determined by a 2-electrode study were: λ = 280 μm; R_m = 0.11 kOhm/cm²; R_i = 24 Ohm/cm. Mean resting potential was − 85 mV. The first action potential of a series (whether evoked by repetitive stimulation, or occurring in a natural unstimulated swimming burst) shows a rapid rise and fall with no afterpotential. The overshoot is small, but successive action potentials show a remarkable facilitation, overshooting by as much as 70 mV. They also show a plateau phase after the initial rapid rise, which is terminated by a rapid fall. Conduction velocity was 27 cm/s.

Changes in the external milieu, and the effects of Ca²⁺ blocking agents indicated that the action potentials are complex events. Although insensitive to TTX, the action potential is dependent on external sodium concentration, and is not abolished by Ca²⁺ blocking agents: in this respect it resembles the sodium-dependent action potentials of other siphonophore tissues.

The ionic basis of the facilitated action potentials is not yet clear, but it seems probable that a fast potassium conductance terminating the unfacilitated action potential is progressively inactivated during repetitive activity, and that the plateau phase of the facilitated action potential is maintained by a sodium conductance mechanism, to be terminated by a calcium-activated potassium conductance.