Inactivation of a Ca2+-induced Ca2+ release channel from skeletal muscle sarcoplasmic reticulum during active Ca2+ transport.
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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Morii
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Takisawa
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Yamamoto
D000227:Adenine Nucleotides / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D000252:Calcium-Transporting ATPases / Q000032:analysis; D066298:In Vitro Techniques; D007473:Ion Channels / Q000378:metabolism*; D011817:Rabbits; D012430:Ruthenium Red / Q000494:pharmacology; D012519:Sarcoplasmic Reticulum / Q000378:metabolism*
D000227:Adenine Nucleotides; D007473:Ion Channels; D012430:Ruthenium Red; D000252:Calcium-Transporting ATPases; D002118:Calcium