Electrophysiology of bioluminescent excitable epithelial cells in a polynoid polychaete worm

Abstract

1. An electrophysiological study was made of the bioluminescent epithelial cells (photocytes) of the wormHesperonoe complanata (Polychaeta: Polynoidae.) 2. Resting photocyte membrane potential is −72±2 (s.d.) mV and decreases by 55 mV as external K concentration is increased from 10 to 100 mM. 3. Depolarization activates two separate regenerative inward currents, both resulting in all-or-none action potentials. The action potential with the lower threshold has an amplitude of 43±4 (s.d.) mV, is Na-dependent but tetrodotoxin-insensitive and is not associated with luminescence. The action potential with the higher threshold overshoots the reference potential by 13±6 (s.d.) mV, increases by 29 mV for a tenfold increase in Ca concentration and persists in Na-free solution. Under some conditions this spike is followed by a low conductance, Ca-dependent depolarized plateau which abruptly terminates after 274±17 (s.d.) s. 4. Luminescence accompanies each Ca spike. Reducing Ca influx reduces light emission. Depolarization in Ca-free medium does not produce light. It is therefore concluded that the intracellular light producing mechanism is Ca-activated and that Ca ions mediate excitation-luminescence coupling. 5. Photocytes receive direct excitatory innervation. Large depolarizing postsynaptic potentials occur upon nerve stimulation. 6. These electrical properties adequately explain in vivo bioluminescence.