Release and sequestration of calcium by ryanodine‐sensitive stores in rat hippocampal neurones

Release and sequestration of calcium by ryanodine‐sensitive stores in rat hippocampal neurones 1 The properties of ryanodine‐sensitive Ca2+ stores in CA1 pyramidal cells were investigated in rat hippocampal slices by using whole‐cell patch‐clamp recordings combined with fura‐2‐based fluorometric digital imaging of cytoplasmic Ca2+ concentration ((Ca2+)i). 2 Brief pressure applications of caffeine onto the somata of pyramidal cells caused large transient increases in (Ca2+)i (Ca2+ transients) of 50–600 nm above baseline. 3 The Ca2+ transients evoked by caffeine at −60 mV were not associated with an inward current, persisted after blocking voltage‐activated Ca2+ currents and were completely blocked by bath‐applied ryanodine. Similar transients were also evoked at +60 mV. Thus, these transients reflect Ca2+ release from intracellular ryanodine‐sensitive Ca2+ stores. 4 The Ca2+ transients evoked by closely spaced caffeine pulses rapidly decreased in amplitude, indicating progressive depletion of the Ca2+ stores. The amplitude of the Ca2+ transients recovered spontaneously with an exponential time constant of 59 s. Recovery was accelerated by depolarization‐induced elevations in (Ca2+)i and blocked by cyclopiazonic acid (CPA) and thapsigargin, indicating that store refilling is mediated by endoplasmic reticulum Ca2+‐ATPases. 5 Even without prior store depletion the caffeine‐induced Ca2+ transients disappeared after 6 min exposure to CPA, suggesting that ryanodine‐sensitive Ca2+ stores are maintained at rest by continuous Ca2+ sequestration. 6 Caffeine‐depleted Ca2+ stores did not refill in Ca2+‐free saline, suggesting that the refilling of the stores depends upon Ca2+ influx through a ‘capacitative‐like’ transmembrane influx pathway operating at resting membrane potential. The refilling of the stores was also blocked by Ni2+ and gallopamil (D600). 7 Elevations of basal (Ca2+)i produced by bath‐applied KCl markedly potentiated (up to 6‐fold) the caffeine‐induced Ca2+ transients. The degree of potentiation was positively related to the increase in basal (Ca2+)i. The Ca2+ transients remained potentiated up to 9 min after reversing the KCl‐induced (Ca2+)i increase. Thus, the ryanodine‐sensitive Ca2+ stores can ‘overcharge’ when challenged with an increase in (Ca2+)i and slowly discharge excess Ca2+ after basal (Ca2+)i returns to its resting level. 8 Pressure applications of caffeine onto pyramidal cell dendrites evoked local Ca2+ transients similar to those separately evoked in the respective somata. Thus, dendritic ryanodine‐sensitive Ca2+ stores are also loaded at rest and can function as independent compartments. 9 In conclusion, the ryanodine‐sensitive Ca2+ stores in hippocampal pyramidal neurones contain a releasable pool of Ca2+ that is maintained by a Ca2+ entry pathway active at subthreshold membrane potentials. Ca2+ entry through voltage‐gated Ca2+ channels transiently overcharges the stores. Thus, by acting as powerful buffers at rest and as regulated sources during activity, Ca2+ stores may control the waveform of physiological Ca2+ signals in CA1 hippocampal pyramidal neurones. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png The Journal of Physiology Wiley

Release and sequestration of calcium by ryanodine‐sensitive stores in rat hippocampal neurones

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Publisher
Wiley
Copyright
Copyright © 1997 Wiley Subscription Services, Inc., A Wiley Company
ISSN
0022-3751
eISSN
1469-7793
D.O.I.
10.1111/j.1469-7793.1997.013bl.x
Publisher site
See Article on Publisher Site

Abstract

1 The properties of ryanodine‐sensitive Ca2+ stores in CA1 pyramidal cells were investigated in rat hippocampal slices by using whole‐cell patch‐clamp recordings combined with fura‐2‐based fluorometric digital imaging of cytoplasmic Ca2+ concentration ((Ca2+)i). 2 Brief pressure applications of caffeine onto the somata of pyramidal cells caused large transient increases in (Ca2+)i (Ca2+ transients) of 50–600 nm above baseline. 3 The Ca2+ transients evoked by caffeine at −60 mV were not associated with an inward current, persisted after blocking voltage‐activated Ca2+ currents and were completely blocked by bath‐applied ryanodine. Similar transients were also evoked at +60 mV. Thus, these transients reflect Ca2+ release from intracellular ryanodine‐sensitive Ca2+ stores. 4 The Ca2+ transients evoked by closely spaced caffeine pulses rapidly decreased in amplitude, indicating progressive depletion of the Ca2+ stores. The amplitude of the Ca2+ transients recovered spontaneously with an exponential time constant of 59 s. Recovery was accelerated by depolarization‐induced elevations in (Ca2+)i and blocked by cyclopiazonic acid (CPA) and thapsigargin, indicating that store refilling is mediated by endoplasmic reticulum Ca2+‐ATPases. 5 Even without prior store depletion the caffeine‐induced Ca2+ transients disappeared after 6 min exposure to CPA, suggesting that ryanodine‐sensitive Ca2+ stores are maintained at rest by continuous Ca2+ sequestration. 6 Caffeine‐depleted Ca2+ stores did not refill in Ca2+‐free saline, suggesting that the refilling of the stores depends upon Ca2+ influx through a ‘capacitative‐like’ transmembrane influx pathway operating at resting membrane potential. The refilling of the stores was also blocked by Ni2+ and gallopamil (D600). 7 Elevations of basal (Ca2+)i produced by bath‐applied KCl markedly potentiated (up to 6‐fold) the caffeine‐induced Ca2+ transients. The degree of potentiation was positively related to the increase in basal (Ca2+)i. The Ca2+ transients remained potentiated up to 9 min after reversing the KCl‐induced (Ca2+)i increase. Thus, the ryanodine‐sensitive Ca2+ stores can ‘overcharge’ when challenged with an increase in (Ca2+)i and slowly discharge excess Ca2+ after basal (Ca2+)i returns to its resting level. 8 Pressure applications of caffeine onto pyramidal cell dendrites evoked local Ca2+ transients similar to those separately evoked in the respective somata. Thus, dendritic ryanodine‐sensitive Ca2+ stores are also loaded at rest and can function as independent compartments. 9 In conclusion, the ryanodine‐sensitive Ca2+ stores in hippocampal pyramidal neurones contain a releasable pool of Ca2+ that is maintained by a Ca2+ entry pathway active at subthreshold membrane potentials. Ca2+ entry through voltage‐gated Ca2+ channels transiently overcharges the stores. Thus, by acting as powerful buffers at rest and as regulated sources during activity, Ca2+ stores may control the waveform of physiological Ca2+ signals in CA1 hippocampal pyramidal neurones.

Journal

The Journal of PhysiologyWiley

Published: Jul 1, 1997

References

  • Similarity of junctions between plasma membranes and endoplasmic reticulum in muscle and neurons
    Henkart, Henkart; Landis, Landis; Reese, Reese
  • Calcium release‐activated calcium current in rat mast cells
    Hoth, Hoth; Penner, Penner
  • Ryanodine receptor‐mediated intracellular calcium release in rat cerebellar Purkinje neurones
    Kano, Kano; Garaschuk, Garaschuk; Verkhratsky, Verkhratsky; Konnerth, Konnerth
  • Dendritic calcium transients evoked by single back‐propagating action potentials in rat neocortical pyramidal neurons
    Markram, Markram; Helm, Helm; Sakmann, Sakmann
  • Molecular and cellular physiology of intracellular calcium stores
    Pozzan, Pozzan; Rlzzuto, Rlzzuto; Volpe, Volpe; Meldolesi, Meldolesi
  • Calcium‐induced calcium release in rat sensory neurons
    Shmigol, Shmigol; Verkhratsky, Verkhratsky; Isenberg, Isenberg
  • Calcium channels, stores, and oscillations
    Tsien, Tsien; Tsien, Tsien
  • Ryanodine produces a low frequency stimulation‐induced NMDA receptor‐independent long‐term potentiation in the rat dentate gyrus in vitro
    Wang, Wang; Wu, Wu; Rowan, Rowan; Anwyl, Anwyl

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