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

1. Fluo-3 fluorescence measurements were made in isolated beta -escin permeablised rat cardiac myocytes using confocal microscopy. Perfusion of a mock intracellular solution containing 0.22-0.23 muM Ca2+ and 5 mar 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(nu) and amplitude (sigma). Under the control conditions the ratio of velocity to amplitude (nu/sigma) varied little and was 99.8 +/- 2.5 m s(-1) muM(-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 muM 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 mar ATP st-as larger than normal but the nu/sigma value was 32 +/- 6 % of control (n = B). In the similar burst on removal of 100 muM tetracaine nu/sigma 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 muM tetracaine resulted in SR Ca2+ release with no coherent wave pattern. The reason for this discrepancy is discussed.