Depletion of Ca2+ from the sarcoplasmic reticulum of cardiac muscle prompts phosphorylation of phospholamban to stimulate store refilling

Nonmuscle cells have almost ubiquitously evolved a mechanism to detect and prevent Ca2+ store depletion-store operated calcium entry, No such mechanism has, as yet, been reported in cardiac myocytes, However, it is conceivable that such a mechanism may play an important role in cardiac Ca2+ homeostasis to ensure the availability of sufficient stored Ca2+ to maintain normal excitation contraction coupling, We present data that confirms the presence of a mechanism that is able to monitor the Ca2+ load of the SR and initiate a signaling process to accelerate Ca2+ uptake by the SR when store depletion is detected. Depletion of SR Ca2+ activates a protein kinase, the principal SR substrate of which is phospholamban, Phosphorylation of this SR protein promotes Ca2+ pump activity and therefore store refilling, Furthermore, a protein kinase activity associated with the SR that is inhibited by Ca2+ ions has been identified. We have measured lumenal [Ca2+] by using a fluorescent Ca2+ indicator and found that by initiating Ca2+ uptake and increasing Ca2+ load, we can inhibit the protein kinase activity associated with the SR, This confirms that a protein kinase, that is regulated by lumenal [Ca2+], has been identified and represents part of a previously unidentified signalling cascade, This local feedback mechanism would allow the myocyte to detect and prevent SR Ca2+ load depletion.