Calcium regulates L-type Ca2+ channel expression in rat skeletal muscle cells

Primary skeletal muscle cells were cultured in a normal- (1.8 mM) or high- (4.8 mM) Ca2+ culture medium to determine whether Ca2+ modulates the number of L-type Ca2+ channels. Skeletal myoballs cultured in a normal medium showed, when exposed to a high extracellular [Ca2+], ([Ca2+]e) a transient increase in intracellular [Ca2+] ([Ca2+]i) from a resting concentration of 60 to 160 nM. By day 3, however, when the experiments were made, [Ca2+]i no longer differed from control (pre-exposure to high Ca2+). The maximum charge movements in myoballs incubated in 1.8 and 4.8 mM were 16.4+/-1.05 (n=56) and 24.1+/-1.18 nC/microF (n=58; P<0.01), respectively, and peak Ca2+ currents at 20 mV were -10.8+/-1.09 (n=46) and -12.8+/-0.75 nA/microF (n=82), respectively (P>0.05). The tail current amplitudes in 1.8 and 4.8 mM Ca2+-treated cells were -9.3+/-1.23 and -14.2+/-1.37 nA/microF (P<0.05), respectively, at 10 mV and -15.3+/-1.76 and -23.6+/-2.02 nA/microF (P<0.05), respectively at 60 mV. The maximum binding of [3H]PN200-110 (a radioligand specific for L-type Ca2+ channel alpha1 subunits) in myoballs cultured in 1.8 and 4.8 mM [Ca2+]e was 1.34+/-0.23 and 3.2+/-0.63 pmol/mg protein (n=8; P<0.02), respectively. The increase in [Ca2+]i associated with the increases in charge movements, tail currents and the number of L-type Ca2+ channel alpha1 subunits in skeletal muscle cells cultured in high [Ca2+]e support the concept that extracellular Ca2+ influx modulates the expression of L-type Ca2+ channels in skeletal muscle cells.