Aims: 

The basis of Ca signalling in cardiac muscle is the sarcoplasmic reticulum (SR). This organelle releases Ca from its lumen into the cytoplasm of the cell via the Ca release channel it has within its membrane. This channel is also known as the ryanodine receptor (RyR). The activity of the RyR (its open probability: Po) is determined by the local concentration of Ca. In addition there are several accessory proteins that associate with the RyR to alter its Po.

Methods: 

We have examined one of these: Calmodulin. Recently it has been estimated that the free [CaM] is somewhere less than 100 nM¹. We have used 50 nM to determine if this protein increases the Po of the RyR thereby increasing the frequency of spontaneous Ca releases that take place in the cell.

Results: 

Spontaneous sparks taking place in a single skinned ventricular myocyte isolated from a rat. Note that in Calmodulin (50 nM) the frequency of sparks is increased (7.2±0.5 to 9.4±1.1, n=3, p<0.05). Similar behaviour has been seen under Ca overloaded conditions when waves of Ca-induced Sa release (CICR) are increased in frequency from 8.7±1.3 min^-1 to 12.7±2.0 (n=14; p<0.001).

Conclusion: 

As would be expected this increase of SR Ca release activity is associated with a decrease of the SR Ca content of about 30% and a reduction of wave amplitude.

¹Wu X, Bers DM. Free and bound intracellular calmodulin measurements in cardiac myocytes. Cell Calcium 2007 April;41(4):353–64.