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Electrically triggered all-or-none Ca(2)+-liberation during action potential in the giant alga Chara.

Wacke M, Thiel G - J. Gen. Physiol. (2001)

Bottom Line: Electrically triggered action potentials in the giant alga Chara corallina are associated with a transient rise in the concentration of free Ca(2)+ in the cytoplasm (Ca(2)+(cyt)).The present measurements of Ca(2)+(cyt) during membrane excitation show that stimulating pulses of low magnitude (subthreshold pulse) had no perceivable effect on Ca(2)+(cyt).Assuming that inositol-1,4,5,-trisphosphate (IP(3)) is the second messenger in question, the present data provide the major rate constants for IP(3) metabolism.

View Article: PubMed Central - PubMed

Affiliation: Albrecht-von-Haller Institute for Plant Sciences, Plant Biophysics, University of Göttingen, 37073 Göttingen, Germany.

ABSTRACT
Electrically triggered action potentials in the giant alga Chara corallina are associated with a transient rise in the concentration of free Ca(2)+ in the cytoplasm (Ca(2)+(cyt)). The present measurements of Ca(2)+(cyt) during membrane excitation show that stimulating pulses of low magnitude (subthreshold pulse) had no perceivable effect on Ca(2)+(cyt). When the strength of a pulse exceeded a narrow threshold (suprathreshold pulse) it evoked the full extent of the Ca(2)+(cyt) elevation. This suggests an all-or-none mechanism for Ca(2)+ 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 Ca(2)+ 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 Ca(2)+ 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 Ca(2)+ 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 (IP(3)) is the second messenger in question, the present data provide the major rate constants for IP(3) metabolism.

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Summation of subthreshold pulses for induction of a transient calcium rise. (A) One Chara internodal cell was stimulated by two subthreshold pulses to induce a transient calcium rise. (B) After reduction of the pulse interval to 100 ms (B) and 0 ms (C) the stimulation failed to induce a transient calcium rise. All pulses were 200 ms long. (D) Amplitudes of Ca2+cyt changes in response to dual subthreshold pulses as a function of the pulse interval for the cell shown in A–C. (E) Normalized data for experiments with four cells as in D. Normalization as in Fig. 5.
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Figure 7: Summation of subthreshold pulses for induction of a transient calcium rise. (A) One Chara internodal cell was stimulated by two subthreshold pulses to induce a transient calcium rise. (B) After reduction of the pulse interval to 100 ms (B) and 0 ms (C) the stimulation failed to induce a transient calcium rise. All pulses were 200 ms long. (D) Amplitudes of Ca2+cyt changes in response to dual subthreshold pulses as a function of the pulse interval for the cell shown in A–C. (E) Normalized data for experiments with four cells as in D. Normalization as in Fig. 5.

Mentions: Fig. 7 shows another surprising observation with respect to a minimum interval between two effective subthreshold stimuli. In this case, pulses with low strength and long duration were chosen. As a single pulse, these were not able to stimulate a transient calcium rise (not shown). When two pulses of the same kind were applied in series with an interval of 300 ms, a transient calcium rise was stimulated. Subsequently, the interval between the two stimuli was shortened and the effect on Ca2+cyt was monitored. Fig. 7 (B and C) shows that also a reduction in the interval between two subthreshold pulses resulted in a loss of the additive effect of subthreshold pulses as trigger for Ca2+cyt mobilization. Fig. 7 D summarizes the effects of dual pulses on transient calcium rises tested in the same cell. The plot shows the amplitudes of Ca2+cyt changes as a function of the pulse interval. It is apparent that intervals must be longer than ∼200 ms to assure an additive effect of subthreshold pulses. The same pattern for stimulation of transient calcium rises by subthreshold was observed in four other cells tested. The result was independent on whether the experiment was started with a short or a long interval. This renders an endogenous decrease in excitability of the cell unlikely as explanation for the results.


Electrically triggered all-or-none Ca(2)+-liberation during action potential in the giant alga Chara.

Wacke M, Thiel G - J. Gen. Physiol. (2001)

Summation of subthreshold pulses for induction of a transient calcium rise. (A) One Chara internodal cell was stimulated by two subthreshold pulses to induce a transient calcium rise. (B) After reduction of the pulse interval to 100 ms (B) and 0 ms (C) the stimulation failed to induce a transient calcium rise. All pulses were 200 ms long. (D) Amplitudes of Ca2+cyt changes in response to dual subthreshold pulses as a function of the pulse interval for the cell shown in A–C. (E) Normalized data for experiments with four cells as in D. Normalization as in Fig. 5.
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Related In: Results  -  Collection

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Figure 7: Summation of subthreshold pulses for induction of a transient calcium rise. (A) One Chara internodal cell was stimulated by two subthreshold pulses to induce a transient calcium rise. (B) After reduction of the pulse interval to 100 ms (B) and 0 ms (C) the stimulation failed to induce a transient calcium rise. All pulses were 200 ms long. (D) Amplitudes of Ca2+cyt changes in response to dual subthreshold pulses as a function of the pulse interval for the cell shown in A–C. (E) Normalized data for experiments with four cells as in D. Normalization as in Fig. 5.
Mentions: Fig. 7 shows another surprising observation with respect to a minimum interval between two effective subthreshold stimuli. In this case, pulses with low strength and long duration were chosen. As a single pulse, these were not able to stimulate a transient calcium rise (not shown). When two pulses of the same kind were applied in series with an interval of 300 ms, a transient calcium rise was stimulated. Subsequently, the interval between the two stimuli was shortened and the effect on Ca2+cyt was monitored. Fig. 7 (B and C) shows that also a reduction in the interval between two subthreshold pulses resulted in a loss of the additive effect of subthreshold pulses as trigger for Ca2+cyt mobilization. Fig. 7 D summarizes the effects of dual pulses on transient calcium rises tested in the same cell. The plot shows the amplitudes of Ca2+cyt changes as a function of the pulse interval. It is apparent that intervals must be longer than ∼200 ms to assure an additive effect of subthreshold pulses. The same pattern for stimulation of transient calcium rises by subthreshold was observed in four other cells tested. The result was independent on whether the experiment was started with a short or a long interval. This renders an endogenous decrease in excitability of the cell unlikely as explanation for the results.

Bottom Line: Electrically triggered action potentials in the giant alga Chara corallina are associated with a transient rise in the concentration of free Ca(2)+ in the cytoplasm (Ca(2)+(cyt)).The present measurements of Ca(2)+(cyt) during membrane excitation show that stimulating pulses of low magnitude (subthreshold pulse) had no perceivable effect on Ca(2)+(cyt).Assuming that inositol-1,4,5,-trisphosphate (IP(3)) is the second messenger in question, the present data provide the major rate constants for IP(3) metabolism.

View Article: PubMed Central - PubMed

Affiliation: Albrecht-von-Haller Institute for Plant Sciences, Plant Biophysics, University of Göttingen, 37073 Göttingen, Germany.

ABSTRACT
Electrically triggered action potentials in the giant alga Chara corallina are associated with a transient rise in the concentration of free Ca(2)+ in the cytoplasm (Ca(2)+(cyt)). The present measurements of Ca(2)+(cyt) during membrane excitation show that stimulating pulses of low magnitude (subthreshold pulse) had no perceivable effect on Ca(2)+(cyt). When the strength of a pulse exceeded a narrow threshold (suprathreshold pulse) it evoked the full extent of the Ca(2)+(cyt) elevation. This suggests an all-or-none mechanism for Ca(2)+ 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 Ca(2)+ 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 Ca(2)+ 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 Ca(2)+ 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 (IP(3)) is the second messenger in question, the present data provide the major rate constants for IP(3) metabolism.

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