Expression of the hexokinase (HK) II gene in skeletal muscle is upregulated
by electrically stimulated muscle contraction and moderate-intensity exerc
ise. However, the molecular mechanism by which this occurs is unknown. Alte
rations in intracellular Ca2+ homeostasis accompany contraction and regulat
e gene expression in contracting skeletal muscle. Therefore, as a first ste
p in understanding the exercise-induced increase in HK II, the ability of C
a2+ to increase HK II mRNA was investigated in cultured skeletal muscle cel
ls, namely L6 myotubes. Exposure of cells to the ionophore A-23187 resulted
in an approximately threefold increase in HK II mRNA. Treatment of cells w
ith the extracellular Ca2+ chelator EGTA did not alter HK II mRNA, nor was
it able to prevent the A-23187-induced increase. Treatment of cells with th
e intracellular Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetra
acetic acid tetra(acetoxymethyl) ester (BAPTA-AM) also resulted in an appro
ximately threefold increase in HK II mRNA in the absence of ionophore, whic
h was similar to the increase in HK II mRNA induced by the combination of B
APTA-AM and A-23187. In summary, a rise in intracellular Ca2+ is not necess
ary for the A-23187-induced increase in HK II mRNA, and increases in HK II
mRNA occur in response to treatments that decrease intracellular Ca2+ store
s. Depletion of intracellular Ca2+ stores may be one mechanism by which mus
cle contraction increases HK II mRNA.