The ionic requirements for the generation of action potentials in the ventral longitudinal muscle fibres of the stick insect, Carausius morosus, were investigated. Ca-free Ringer rapidly and reversibly abolished the action potential. In the presence of tetraethylammonium (TEA) ions (to suppress outward currents) the overshoot of the action potential changed 26 mV for a 10-fold change in [Ca]o. The maximum rate of rise of the action potential (measured in TEA Ringer) showed saturation at high [Ca]o. Cobaltous ions (20 mM) and the organic Ca antagonist D 600 (5×10−4g/ml) reversibly inhibited the action potential; the inhibitory effect of 1 mM-La3+ was irreversible. Barium and strontium, but not magnesium, were able to substitute for calcium as charge carriers. These results suggest that an inward movement of Ca2+ underlies the action potential of Carausius fibres.

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