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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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Maximal concentrations of Q2 as a function of pulse interval. The maximal concentration of Q2 (solid line) achieved during electrical stimulation with pulses of constant strength an duration was calculated from  for kQ1 = 2.5 s−1, kQ3 = 0.05 s−1, and c2 = 1.8. They were derived from the following data set: pl = 3 μA; 200 ms, ph = 5 μA; 50 ms, I0 = 2.8 μA, q = 109 nC, Δt2,min = 300 ms, and Δt2,max = 3,000 ms. The thin line indicates the threshold, which needs to be exceeded for effective Ca2+ mobilization.
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Figure 9: Maximal concentrations of Q2 as a function of pulse interval. The maximal concentration of Q2 (solid line) achieved during electrical stimulation with pulses of constant strength an duration was calculated from for kQ1 = 2.5 s−1, kQ3 = 0.05 s−1, and c2 = 1.8. They were derived from the following data set: pl = 3 μA; 200 ms, ph = 5 μA; 50 ms, I0 = 2.8 μA, q = 109 nC, Δt2,min = 300 ms, and Δt2,max = 3,000 ms. The thin line indicates the threshold, which needs to be exceeded for effective Ca2+ mobilization.

Mentions: To examine the dependency of the concentration of Q2 on pulse intervals, we calculated with the appropriate rate constants the maximal concentration of Q2 achieved at the second pulse as a function of the pulse intervals. The plot in Fig. 9 shows that only pulse intervals between 0.3 and 3 s cause elevation of Q2 over the threshold and, thus, are able to trigger a transient calcium rise. Shorter and longer intervals are predicted to not stimulate a transient calcium rise. This features are in good agreement with the experimental data.


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

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

Maximal concentrations of Q2 as a function of pulse interval. The maximal concentration of Q2 (solid line) achieved during electrical stimulation with pulses of constant strength an duration was calculated from  for kQ1 = 2.5 s−1, kQ3 = 0.05 s−1, and c2 = 1.8. They were derived from the following data set: pl = 3 μA; 200 ms, ph = 5 μA; 50 ms, I0 = 2.8 μA, q = 109 nC, Δt2,min = 300 ms, and Δt2,max = 3,000 ms. The thin line indicates the threshold, which needs to be exceeded for effective Ca2+ mobilization.
© Copyright Policy
Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC2233747&req=5

Figure 9: Maximal concentrations of Q2 as a function of pulse interval. The maximal concentration of Q2 (solid line) achieved during electrical stimulation with pulses of constant strength an duration was calculated from for kQ1 = 2.5 s−1, kQ3 = 0.05 s−1, and c2 = 1.8. They were derived from the following data set: pl = 3 μA; 200 ms, ph = 5 μA; 50 ms, I0 = 2.8 μA, q = 109 nC, Δt2,min = 300 ms, and Δt2,max = 3,000 ms. The thin line indicates the threshold, which needs to be exceeded for effective Ca2+ mobilization.
Mentions: To examine the dependency of the concentration of Q2 on pulse intervals, we calculated with the appropriate rate constants the maximal concentration of Q2 achieved at the second pulse as a function of the pulse intervals. The plot in Fig. 9 shows that only pulse intervals between 0.3 and 3 s cause elevation of Q2 over the threshold and, thus, are able to trigger a transient calcium rise. Shorter and longer intervals are predicted to not stimulate a transient calcium rise. This features are in good agreement with the experimental data.

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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