| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 2003: ¥1,000,000 (Direct Cost: ¥1,000,000)
Fiscal Year 2002: ¥1,000,000 (Direct Cost: ¥1,000,000)
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| Research Abstract |
This project was conducted to characterize bi-directional communications between the SR membrane and the T-tubule membrane.
1) Orthograde signal from the DHPR to the RyR : Application of caffeine, a potentiator of Ca^<2+>-induced Ca^<2+> release (CICR), induces continuous release of Ca^<2+> from the SR (caffeine-contracture). When a sustained depolarization is applied during the caffeine-contracture, subsequent repolarization terminates Ca^<2+> release (RISC : PNAS91, 5725, 1994). Because caffeine failed to induce Ca^<2+> release following the depolarizing pulse, it is strongly suggested that CIOR is inhibited at a time ofrepolarization. RISC was absent in dysgenic (DHPR-lacking) mouse myotubes, although expression of the skeletal DHPR in dysgenic myotubes restored RISC, suggesting involvement of the skeletal DH PR in RISC. CICR induced by caffeine was also terminated following the cessation of repetitive field stimulation (20Hz) applied on top of caffeine-contracture. Electron -microscopy revealed that inmyoballs, obtained by colchicines treatment, SR membranes and T-tubule membranes were located in closeproximity with each other.
2) Retrograde signal from the SR to the plasma (T-tubule) membrane : When rat myotubes, pretreated with SR Ca^<2+> pump inhibitors (e.g.TG), were exposed to caffeine (10mM), the cells showed an increase in cytoplasmic Ca^<2+> concentration, due presumably to Ca^<2+> entry from the extracellular space. We also found a novel pathway for Ca^<2+> entry activated at the resting membrane potential in rat skeletal myotubes. This Ca^<2+> entry was present in dysgenic myotubes. We are now investigating whether the novel Ca^<2+> entry is regulated by SR Ca^<2+> content or the state of the RyR.
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