The effect of luminal calcium on the stability of coupled gating between cardiac ryanodine receptors

Gaburjakova M.* and Gaburjakova J.

Institute of Molecular Physiology and Genetics, Slovak Academy of Sciences, Bratislava, Slovak Republic

*marta.gaburjakova@savba.sk

In cardiac muscle, the Ca²⁺ required for a contractile activation is released from the Ca²⁺ stores in response to entry of a small amount of Ca²⁺ from the extracellular space to the cytosol. The key role in this process, termed Ca²⁺-induced Ca²⁺ release (CICR), is played by the cardiac ryanodine receptor (RyR2) channel (Fabiato, 1985). Local CICR is intrinsically regenerative, and thus a mechanism leading to robust termination of the Ca²⁺ release is required to ensure the periodic contraction and relaxation of cardiac muscle. The phenomenon termed “coupled gating of RyR2 channels” has been suggested as an attractive candidate for such a termination mechanism. It is manifested by simultaneous openings and closings of multiple RyR2 channels (Marx et al., 2001).

Using the method of reconstitution of a channel into bilayer lipid membrane, we investigated the potential effect of luminal Ca²⁺ on the stability of interaction between coupled RyR2 channels. We introduced a new parameter - the coupling stability for each detected simultaneous opening and closing. We found that the coupling stability during simultaneous opening of RyR2 channels was significantly lower in comparison to the simultaneous closing under the same experimental conditions. Furthermore, high concentration of luminal Ca²⁺ (53 mM) as well as the absence of luminal Ca²⁺ noticeably destabilized functional coupling between coupled RyR2 channels during opening, in contrast to lower tested concentrations (8-20 mM).

Our study led to new observations that may have important implications for understanding the principles of the mechanism terminating CICR in cardiac muscle. We provided experimental evidence that the strength of interaction between coupled RyR2 channels depends on the channel functional state. Furthermore, we showed, for the first time, the new role of luminal Ca²⁺ in a recently suggested “dynamic inter-RyR2 coupling mechanism,” a process that could be involved in the acceleration of termination of CICR in cardiac muscle (Liang et al., 2007).

References

Fabiato A. Simulated calcium current can cause both calcium loading in and trigger calcium release from the sarcoplasmic reticulum of a skinned cardiac Purkinje fiber. J. Gen. Physiol. 245: 291-320, 1985

Liang X., Hu X.-F., Hu J. Dynamic interreceptor coupling: a novel working mechanism of two-dimensional ryanodine receptor array. Biophys. J. 92: 1215-1223, 2007

Marx S.O., Gaburjakova J., Gaburjakova M., Henrikson Ch., Ondrias K., Marks A.R. Coupled gating between cardiac calcium release channels (ryanodine receptors). Circ. Res. 88: 1151-1158, 2001