Zk. Yang et Ds. Steele, Effects of cytosolic ATP on spontaneous and triggered Ca2+-induced Ca2+ release in permeabilised rat ventricular myocytes, J PHYSL LON, 523(1), 2000, pp. 29-44
1. The effects of cytosolic ATP on sarcoplasmic reticulum (SR) Ca2+ regulat
ion were investigated in saponin-permeabilised rat ventricular myocytes. [C
a2+] within the cells was monitored using Fura-2 or Fluo-3 fluorescence. Sp
ontaneous cyclic Ca2+ release from the SR was induced by increasing the bat
hing [Ca2+] to 200-300 nM, in solutions weakly Ca2+ buffered with 0.05 mM E
GTA. Alternatively, Ca2+-induced Ca2+ release (CICR) was triggered by a rap
id increase in [Ca2+] induced by flash photolysis of Nitr-5 (0.08 mM), repl
acing EGTA in the solution.
2. Stepwise reductions in [ATP] were associated with corresponding decrease
s in the frequency and increases in the amplitude of spontaneous Ca2+ trans
ients. A decrease from 5 mM to 0.1 mM ATP, reduced the release frequency by
48.6 +/- 7 % (n = 7) and almost doubled the amplitude of the Ca2+ transien
t. Marked prolongation of the spontaneous Ca2+ transient occurred when [ATP
] was further reduced to 10 mu M, consistent with inhibition of the SR Ca2 pump.
3. These effects of ATP were compared with other interventions that inhibit
Ca2+ uptake or reduce the sensitivity of the SR Ca2+ release mechanism. In
hibition of the SR Ca2+ pump with cyclopiazonic acid (CPA) markedly reduced
the spontaneous Ca2+ release frequency, without changing the amplitude. Th
e descending phase of the Ca2+ transient was prolonged in the presence of C
PA, while the rising phase was unaffected. In contrast, desensitisation of
the BR Ca2+ release mechanism with tetracaine decreased the frequency of sp
ontaneous release, but markedly increased the amplitude.
4. CICR triggered by flash photolysis of Nitr-5 appeared to be more sensiti
ve to cytosolic [ATP] than spontaneous release and was generally delayed by
a decrease to 2.5 mM ATP. In the presence of 0.1-0.2 mM ATP, release often
failed completely or was not consistently triggered. Some preparations exh
ibited Ca2+ release 'alternans', whereby every alternate trigger induced a
response.
5. These results suggest that the increase in spontaneous Ca2+ release ampl
itude and the decrease in frequency that occurs as [ATP] is reduced from 1
mM to 100 mu M, is mainly due to desensitisation of the SR Ca2+ release mec
hanism, which allows the SR Ca2+ content to reach a higher level before rel
ease occurs. At very low [ATP], a reduction in the SR Ca2+ uptake rate may
also contribute to the decrease in release frequency. CICR triggered by pho
tolysis of Nitr-5 appeared to be more sensitive to cytosolic [ATP]. The pos
sible underlying mechanisms and the relevance of these results to myocardia
l ischaemia or hypoxia is considered.