Transient elevations of intracellular Ca2+ play a signalling role in such complex cellular functions as contraction, secretion, fertilization, proliferation, metabolism, heartbeat and memory. However, prolonged elevation of Ca2+ above about 10 microM is deleterious to a cell and can activate apoptosis. In muscle, there is a narrow window of Ca2+ dysregulation in which abnormalities in Ca2+ regulatory
proteins can lead to disease, rather than apoptosis. Key
proteins in the regulation of muscle Ca2+ are the voltage-dependent,
dihydropyridine-sensitive, L-type Ca2+ channels located in the transverse tubule and Ca2+ release channels in the junctional terminal cisternae of the sarcoplasmic reticulum. Abnormalities in these
proteins play a key role in
malignant hyperthermia (MH), a toxic response to
anesthetics, and in
central core disease (CCD), a muscle
myopathy. Sarco(endo)plasmic reticulum Ca2+
ATPases (SERCAs) return sarcoplasmic Ca2+ to the lumen of the sarcoplasmic reticulum. Loss of SERCA1a Ca2+ pump function is one cause of exercise-induced impairment of the relaxation of skeletal muscle, in
Brody disease.
Phospholamban expressed in cardiac muscle and
sarcolipin expressed in skeletal muscle regulate SERCA activity. Studies with knockout and transgenic mice show that gain of inhibitory function of
phospholamban alters cardiac contractility and could be a causal feature in some
cardiomyopathies.
Calsequestrin,
calreticulin, and a series of other acidic, lumenal, Ca2+
binding proteins provide a
buffer for Ca2+ stored in the sarcoplasmic reticulum. Overexpression of cardiac
calsequestrin leads to
cardiomyopathy and ablation of
calreticulin alters cardiac development.