IP3 receptors, IP3 transients, and nucleus-associated Ca2+ signals in cultured skeletal muscle

Inositol 1,4,5-trisphosphate (IP3) receptors (IP3R) and ryanodine receptors (RyR) were localized in cultured rodent muscle fractions by binding of radiolabeled ligands (IP3 and ryanodine), and IP3R were visualized in situ by fluorescence immunocytological techniques. Also explored was the effect of K+ depolarization on IP3 mass and Ca2+ transients studied using a radio-receptor displacement assay and fluorescence imaging of intracellular flue 3. RyR were located in a microsomal fraction; IP3R were preferentially found in the nuclear fraction. Fluorescence associated with anti-IP3R antibody was found in the region of the nuclear envelope and in a striated pattern in the sarcoplasmic areas. An increase in external K+ affected membrane potential and produced an IP3 transient. Rat myotubes displayed a fast-propagating Ca2+ signal, corresponding to the excitation-contraction coupling transient and a much slower Ca2+ wave. Both signals were triggered by high external K+ and were independent of external Ca2+. Slow waves were associated with cell nuclei and were propagated leaving "glowing'' nuclei behind. Different roles are proposed for at, least two types of Ca2+ release channels, each mediating an intracellular signal in cultured skeletal muscle.