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

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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.