J. Biol. Chem., Vol. 260, Issue 21, 11536-11541, Sep, 1985
Inactivation of a Ca2+-induced Ca2+ release channel from skeletal muscle sarcoplasmic reticulum during active Ca2+ transport
H Morii, H Takisawa and T Yamamoto
ATP-dependent Ca2+ uptake by subfractions of skeletal muscle sarcoplasmic
reticulum (SR) was studied with the Ca2+ indicator dye, antipyrylazo III.
Ca2+ uptake by heavy SR showed two phases, a slow uptake phase and a fast
uptake phase. By contrast, Ca2+ uptake by light SR exhibited a monophasic
time course. In both fractions a steady state of Ca2+ uptake was observed
when the concentration of free Ca2+ outside the vesicles was reduced to
less than 0.1 microM. In the steady state, the addition of 5 microM Ca2+ to
the external medium triggered rapid Ca2+ release from heavy SR but not from
light SR, indicating that the heavy fraction contains a Ca2+-induced Ca2+
release channel. During Ca2+ uptake, heavy SR showed a constant
Ca2+-dependent ATPase activity (1 mumol/mg protein X min) which was about
150 times higher than the rate of Ca2+ uptake in the slow uptake phase.
Ruthenium red, an inhibitor of Ca2+-induced Ca2+ release, enhanced the rate
of Ca2+ uptake during the slow phase without affecting Ca2+-dependent
ATPase activity. Adenine nucleotides, activators of Ca2+ release, reduced
the Ca2+ uptake rate. These results suggest that the rate of Ca2+
accumulation by heavy SR is not proportional to ATPase activity during the
slow uptake phase due to the activation of the channel for Ca2+- induced
Ca2+ release. In addition, they suggest that the release channel is
inactivated during the fast Ca2+ uptake phase.
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