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Effects of cytosolic ATP on spontaneous and triggered Ca 2+ ‐induced Ca 2+ release in permeabilised rat ventricular myocytes
Author(s) -
Yang Zhaokang,
Steele Derek S.
Publication year - 2000
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.2000.00029.x
Subject(s) - cyclopiazonic acid , egta , biophysics , chemistry , cytosol , endoplasmic reticulum , ryanodine receptor , calcium , tetracaine , biochemistry , biology , enzyme , organic chemistry , lidocaine , neuroscience
The effects of cytosolic ATP on sarcoplasmic reticulum (SR) Ca 2+ regulation were investigated in saponin‐permeabilised rat ventricular myocytes. [Ca 2+ ] within the cells was monitored using Fura‐2 or Fluo‐3 fluorescence. Spontaneous cyclic Ca 2+ release from the SR was induced by increasing the bathing [Ca 2+ ] to 200–300 nM, in solutions weakly Ca 2+ buffered with 0.05 m m EGTA. Alternatively, Ca 2+ ‐induced Ca 2+ release (CICR) was triggered by a rapid increase in [Ca 2+ ] induced by flash photolysis of Nitr‐5 (0.08 m m ), replacing EGTA in the solution. Stepwise reductions in [ATP] were associated with corresponding decreases in the frequency and increases in the amplitude of spontaneous Ca 2+ transients. A decrease from 5 m m to 0.1 m m ATP, reduced the release frequency by 48.6 ± 7 % ( n = 7 ) and almost doubled the amplitude of the Ca 2+ transient. Marked prolongation of the spontaneous Ca 2+ transient occurred when [ATP] was further reduced to 10 μM, consistent with inhibition of the SR Ca 2+ pump. These effects of ATP were compared with other interventions that inhibit Ca 2+ uptake or reduce the sensitivity of the SR Ca 2+ release mechanism. Inhibition of the SR Ca 2+ pump with cyclopiazonic acid (CPA) markedly reduced the spontaneous Ca 2+ release frequency, without changing the amplitude. The descending phase of the Ca 2+ transient was prolonged in the presence of CPA, while the rising phase was unaffected. In contrast, desensitisation of the SR Ca 2+ release mechanism with tetracaine decreased the frequency of spontaneous release, but markedly increased the amplitude. CICR triggered by flash photolysis of Nitr‐5 appeared to be more sensitive to cytosolic [ATP] than spontaneous release and was generally delayed by a decrease to 2.5 m m ATP. In the presence of 0.1‐0.2 m m ATP, release often failed completely or was not consistently triggered. Some preparations exhibited Ca 2+ release ‘alternans’, whereby every alternate trigger induced a response. These results suggest that the increase in spontaneous Ca 2+ release amplitude and the decrease in frequency that occurs as [ATP] is reduced from 1 m m to 100 μM, is mainly due to desensitisation of the SR Ca 2+ release mechanism, which allows the SR Ca 2+ content to reach a higher level before release occurs. At very low [ATP], a reduction in the SR Ca 2+ uptake rate may also contribute to the decrease in release frequency. CICR triggered by photolysis of Nitr‐5 appeared to be more sensitive to cytosolic [ATP]. The possible underlying mechanisms and the relevance of these results to myocardial ischaemia or hypoxia is considered.