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

Abstract

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.

Thrombin downregulates muscle acetylcholine receptors via an IP3 signaling pathway by activating its G‐protein‐coupled protease‐activated receptor‐1

“…Similar observations have been recorded in endothelial cells stimulated with thrombin (Ikeda et al, 1996). The cytoplasmic calcium signals induced by thrombin probably result from calcium release from the sarcoplasmic reticulum via IP3R, which is localized in a cross-striated pattern in differentiated myotubes but is not present in young myotubes (Jaimovich et al, 2000). These results are in agreement with the fact that we visualized IP3R in a cross-striated pattern in sarcoplasmic areas where striations were forming.…”

“…in the sarcoplasm, in accordance with a previous study showing more radiolabeled IP3 binding in the nuclear than in the microsomal fraction in cultured rat myotubes (Jaimovich et al, 2000). Calcium signals in nuclei can then result from a cytoplasmic wave by passive diffusion through the nuclear pore complex and/or from calcium released from the nuclear envelope pool via IP3R located on the inner nuclear membrane (Rogue and Malviya, 1999).…”

About the T-system in the myofibril-free sarcoplasm of the frog muscle fibre

“…Since the T-system serves as source of inositol-trisphosphate (Carrasco et al, 1988;Volpe et al, 1986), which can trigger Ca 2+ -waves, the T-system could be shared at the regulation of the perinuclear Ca 2+ -release (Jaimovich et al, 2000;Powell et al, 2001) in those muscle fiber regions where the nuclei are far a way from myofibrils.…”

Amplitude modulation of nuclear Ca2+ signals in human skeletal myotubes: A possible role for nuclear Ca2+ buffering

“…On the other hand, IP 3 receptors were demonstrated to be preferentially present in the nuclear fraction of cultured rat myotubes [13]. Elevations in extracellular K + concentration were shown to cause a biphasic increase in intracellular Ca 2+ concentration ([Ca 2+ ]).…”

“…Elevations in extracellular K + concentration were shown to cause a biphasic increase in intracellular Ca 2+ concentration ([Ca 2+ ]). Detailed analysis revealed that the first increase was fast and associated with contraction, whereas the second increase, which was most prominent in the nuclei, was slow, mediated by IP 3 and responsible for the activation of the transcription factor CREB (cAMP response element-binding protein; [13][14][15]). Similarly, a Ca 2+ ionophore, known to cause a sustained increase in intracellular [Ca 2+ ], was found to increase the activity of the transcription factor NFAT (nuclear factor of activated T cells), in a primary culture of rabbit skeletal muscle [16,17].…”