ABSTRACT
Many hydromedusae exhibit a protective behaviour known as crumpling in response to noxious stimuli. This behaviour habituates with repetitive stimulation, and is mediated by action potentials in the ectodermal epithelium. Repetitive electrical stimulation (above 1 Hz) causes conduction velocity decreases and eventual failure of epithelial action potentials. The rate and degree of such fatigue is frequency-dependent, while recovery is time–dependent. Fatigue does not occur in the absence of extracellular calcium. Increasing the intracellular calcium concentration by raising the extracellular concentration of calcium or by preventing re-uptake of calcium into intracellular stores using thapsigargin enhances fatigue. Epithelial cells may be grown in primary cultures and their ionic currents recorded using the whole-cell, voltage-clamp technique. There appear to be two major inward currents; sodium and calcium. It is suggested that fatigue is due to modulation of sodium current by calcium.
