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A comparison of the effects of ATP and tetracaine on spontaneous Ca²⁺ release from rat permeabilised cardiac myocytes

G. L. Smith and S. C. O'Neill *

1. Fluo-3 fluorescence measurements were made in isolated -escin permeablised rat cardiac myocytes using confocal microscopy. Perfusion of a mock intracellular solution containing 0.22-0.23 µM Ca²⁺ and 5 mM ATP elicited regular waves of Ca²⁺ (approximately every 5 s) due to spontaneous release of Ca²⁺ from the sarcoplasmic reticulum (SR).
2. An approximately linear relationship was noted between Ca²⁺ wave velocity (v) and amplitude (). Under the control conditions the ratio of velocity to amplitude (v/) varied little and was 99.8 ± 2.5 m s^-1 µM^-1 (n = 78).
3. Reduction of [ATP] in the bathing solution to 0.5 and 0.2 mM ATP progressively decreased Ca²⁺ wave frequency and propagation velocity while increasing the amplitude. The changes in Ca²⁺ wave characteristics in 0.5 mM ATP were similar to those observed during perfusion with 50 µM tetracaine. In 0.2 mM ATP the decline of [Ca²⁺] during a Ca²⁺ wave was slowed suggesting a lowered rate of Ca²⁺ re-uptake by the SR Ca²⁺ pump.
4. Reduction of [ATP] to 0.1 mM abolished Ca²⁺ waves after 15-20 s. Returning the [ATP] to 5 mM caused a burst of high frequency and large amplitude waves. Mean velocity of the first wave on returning to 5 mM ATP was larger than normal but the v/ value was 32 ± 6 % of control (n = 6). In the similar burst on removal of 100 µM tetracaine v/ was higher than control (166 ± 9 %, n = 6).
5. Rapid application of caffeine (10 mM) was used to assess the SR Ca²⁺ content. This showed that SR Ca²⁺ increased as [ATP] was reduced or [tetracaine] was increased. The highest SR Ca²⁺ content was observed after perfusion with 0.1 mM ATP, which was 245 ± 15 % of control values.
6. Returning [ATP] from 0.1 mM to 5 mM caused a burst of high frequency, large amplitude Ca²⁺ waves. But recovery after incubation with 300 µM tetracaine resulted in SR Ca²⁺ release with no coherent wave pattern. The reason for this discrepancy is discussed.

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