Triadin and junctin are integral sarcoplasmic reticulum membrane proteins that 
form a macromolecular complex with the skeletal muscle ryanodine receptor (RyR1) 
but their roles in skeletal muscle calcium homeostasis remain incompletely 
understood. Here we report that delivery of siRNAs specific for triadin or 
junctin into C2C12 skeletal myoblasts reduced the expression of triadin and 
junctin in 8-day-old myotubes by 80 and 100%, respectively. Knocking down either 
triadin or junctin in these cells reduced Ca2+ release induced by depolarization 
(10mM KCl) by 20-25%. Unlike triadin knockdown myotubes, junctin knockdown and 
junctin/triadin double knockdown myotubes also had reduced Ca2+ release induced 
by 400 microM 4-chloro-m-cresol, 10mM caffeine, 400 microM UTP, or 1 microM 
thapsigargin. Thus, knocking down junctin compromised the Ca2+ stores in the 
sarcoplasmic reticulum of these cells. Our subsequent studies showed that in 
junctin knockdown myotubes at least two sarcoplasmic reticulum proteins (RyR1 
and skeletal muscle calsequestrin) were down-regulated while these proteins' 
mRNA expression was not affected. The results suggest that triadin has a role in 
facilitating KCl depolarization-induced Ca2+ release in contrast to junctin 
which has a role in maintaining sarcoplasmic reticulum Ca2+ store size in C2C12 
myotubes.