Aims: 

Ryanodine receptor (RyR) is a calcium-activated, calcium-permeable channel of the sarcoplasmic reticulum (SR) that mediates excitation-contraction coupling in cardiac muscle cells. There is growing evidence that Ca²⁺in the lumen of the SR can be effectively involved in the different aspects of RYR channel regulation. In this study, we tested whether luminal Ca²⁺exerts any effect on the sensitivity of the RYR channel to adenosine triphosphate (ATP) – a well known activator of the RYR channel with potential physiological importance. Methods:

RyR channels isolated from the rat heart were reconstituted into a planar lipid membrane. Single-channel currents were recorded under asymmetric conditions with either luminal Ca²⁺, luminal Ba²⁺or a mixture of Ca²⁺/Ba²⁺. Luminal Ba²⁺mimicked the situation when no Ca²⁺is present on the luminal face of the channel.

Results: 

In the absence of luminal Ca²⁺, the channels were only marginally activated by ATP (P_max = 0.01 at [ATP] = 10 mM). In the presence of luminal Ca²⁺, ATP induced dose-dependent activation of the channel. The maximum open probability increased, while the EC₅₀ of ATP activation decreased with elevating luminal Ca²⁺concentration from 1 mM to 53 mM (Pmax ~ 0.025, EC50 ~ 7.5 mM at 1 mM luminal Ca²⁺; P0 ~ 0.019, P_max ~ 0.89, EC₅₀ ~ 0.5 mM at 53 mM luminal Ca²⁺).

Conclusion: 

The potency and efficacy of ATP as activator of the cardiac RYR channel at diastolic Ca²⁺concentrations is dramatically modulated by the luminal Ca²⁺concentration.

Support: APVT-51-031104, VEGA-2/6011/26, EU Contract No. LSHM- CT- 2005- 018802/ CONTICA, APVV- 0139- 06