Electrically Triggered All-or-None Ca²+-Liberation during Action Potential in the Giant Alga Chara

Electrically triggered action potentials in the giant alga Chara corallina are associated with a transient rise in the concentration of free Ca²+ in the cytoplasm (Ca²+_cyt). The present measurements of Ca²+_cyt during membrane excitation show that stimulating pulses of low magnitude (subthreshold pulse) had no perceivable effect on Ca²+_cyt. When the strength of a pulse exceeded a narrow threshold (suprathreshold pulse) it evoked the full extent of the Ca²+_cyt elevation. This suggests an all-or-none mechanism for Ca²+ mobilization. A transient calcium rise could also be induced by one subthreshold pulse if it was after another subthreshold pulse of the same kind after a suitable interval, i.e., not closer than a few 100 ms and not longer than a few seconds. This dependency of Ca²+ mobilization on single and double pulses can be simulated by a model in which a second messenger is produced in a voltage-dependent manner. This second messenger liberates Ca²+ from internal stores in an all-or-none manner once a critical concentration (threshold) of the second messenger is exceeded in the cytoplasm. The positive effect of a single suprathreshold pulse and two optimally spaced subthreshold pulses on Ca²+ mobilization can be explained on the basis of relative velocity for second messenger production and decomposition as well as the availability of the precursor for the second messenger production. Assuming that inositol-1,4,5,-trisphosphate (IP₃) is the second messenger in question, the present data provide the major rate constants for IP₃ metabolism.