IP(3)-induced tension and IP(3)-receptor expression in rat soleus muscle during postnatal development

No evidence for inositol 1,4,5-trisphosphate-dependent Ca2+ release in isolated fibers of adult mouse skeletal muscle

The presence and role of functional inositol 1,4,5-trisphosphate (IP₃) receptors (IP₃Rs) in adult skeletal muscle are controversial. The current consensus is that, in adult striated muscle, the relative amount of IP₃Rs is too low and the kinetics of Ca²⁺ release from IP₃R is too slow compared with ryanodine receptors to contribute to the Ca²⁺ transient during excitation–contraction coupling. However, it has been suggested that IP₃-dependent Ca²⁺ release may be involved in signaling cascades leading to regulation of muscle gene expression. We have reinvestigated IP₃-dependent Ca²⁺ release in isolated flexor digitorum brevis (FDB) muscle fibers from adult mice. Although Ca²⁺ transients were readily induced in cultured C2C12 muscle cells by (a) UTP stimulation, (b) direct injection of IP₃, or (c) photolysis of membrane-permeant caged IP₃, no statistically significant change in calcium signal was detected in adult FDB fibers. We conclude that the IP₃–IP₃R system does not appear to affect global calcium levels in adult mouse skeletal muscle.

Inositol trisphosphate receptor Ca2+ release channels

The inositol 1,4,5-trisphosphate (InsP3) receptors (InsP3Rs) are a family of Ca2+ release channels localized predominately in the endoplasmic reticulum of all cell types. They function to release Ca2+ into the cytoplasm in response to InsP3 produced by diverse stimuli, generating complex local and global Ca2+ signals that regulate numerous cell physiological processes ranging from gene transcription to secretion to learning and memory. The InsP3R is a calcium-selective cation channel whose gating is regulated not only by InsP3, but by other ligands as well, in particular cytoplasmic Ca2+. Over the last decade, detailed quantitative studies of InsP3R channel function and its regulation by ligands and interacting proteins have provided new insights into a remarkable richness of channel regulation and of the structural aspects that underlie signal transduction and permeation. Here, we focus on these developments and review and synthesize the literature regarding the structure and single-channel properties of the InsP3R.