The presence and role of functional inositol 1,4,5-trisphosphate (IP3) receptors (IP3Rs) in adult skeletal muscle is still a debated issue. Calcium release through the IP3R occurs in almost all cell types, including developing muscle, and it was also initially thought to play a role in excitation-contraction coupling in adult skeletal muscle. The current consensus is that IP3Rs don't play a role in excitation-contraction coupling, yet a new role in excitation-transcription coupling has been proposed. However, while numerous studies have examined the role of the IP3R in developing muscle cells, its role in adult muscle remains an open question.

Here we use adult skeletal muscle fibers taken from the flexor digitorum brevis (FDB) and show that these fibers increase intracellular calcium in response to caffeine, yet show no response when using an IP3R agonist UTP, which clearly elicits an increase in intracellular calcium in C2C12 myotubes. Since the specificity of agonists and inhibitors can be questioned we decided to directly micro-inject IP3 into C2C12 myotubes and adult FDB fibers, while simoultaneously measuring the calcium release. While C2C12 myotubes showed a significant increase in intracellular calcium, no change was found in adult fibers. Furthermore, to avoid that the lack of signal is due to a rapid degradation of IP3, or a slow diffusion, we loaded both C2C12 myotubes and FDB fibers with a membrane-permeant caged IP3. Upon release of the caged IP3 by a laser flash again a clear increase in intracellular calcium was found in C2C12 myotubes without any response in adult single fibers.

Taken together, these results clearly show that there is no IP3-dependent calcium release in adult skeletal muscle.