IP(3) receptors, IP(3) transients, and nucleus-associated Ca(2+) signals in cultured skeletal muscle

Inositol 1,4,5-trisphosphate (IP(3)) receptors (IP(3)R) and ryanodine receptors (RyR) were localized in cultured rodent muscle fractions by binding of radiolabeled ligands (IP(3) and ryanodine), and IP(3)R were visualized in situ by fluorescence immunocytological techniques. Also explored was the effect of K(+) depolarization on IP(3) mass and Ca(2+) transients studied using a radio-receptor displacement assay and fluorescence imaging of intracellular fluo 3. RyR were located in a microsomal fraction; IP(3)R were preferentially found in the nuclear fraction. Fluorescence associated with anti-IP(3)R 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 IP(3) transient. Rat myotubes displayed a fast-propagating Ca(2+) signal, corresponding to the excitation-contraction coupling transient and a much slower Ca(2+) wave. Both signals were triggered by high external K(+) and were independent of external Ca(2+). Slow waves were associated with cell nuclei and were propagated leaving "glowing" nuclei behind. Different roles are proposed for at least two types of Ca(2+) release channels, each mediating an intracellular signal in cultured skeletal muscle.