Cx39 is expressed in skeletal muscle, low in adult but at higher levels in early postnatal periods.

These work was done by the group of Klaus Willecke. Search Pub Med and Cx39.

Antibodies that work and which we have tested are available from Life Technologies (Invitrogen).

Cx43 and Cx45 are also particularly important in the myoblasts

Start with these references

Cea LA, Riquelme MA, Cisterna BA, Puebla C, Vega JL, Rovegno M, Sáez JC.- and pannexin-based channels in normal skeletal muscles and their possible role in muscle atrophy. J Membr Biol. 2012 Aug;245(8):423-36. \

von Maltzahn J, Wulf V, Matern G, Willecke K. deficient mice display accelerated myogenesis and regeneration of skeletal muscle. Exp Cell Res. 2011 May 1;317(8):1169-78.

Trovato-Salinaro A, Belluardo N, Frinchi M, von Maltzahn J, Willecke K, Condorelli DF, Mudò G.Regulation of connexin gene expression during skeletal muscle regeneration in the adult rat. Am J Physiol Cell Physiol. 2009 Mar;296(3):C593-606.

in skeletal muscle, Cx43, Cx45 and Cx39 are expressed. The level of expression depends on several factors such as health of tissue and state of differentiation (for example from myoblasts to myotubes), tissue regeneration..... You should as indicated by James Nagy & Eric Beyer perform a Pubmed research, because you will find number of articles/reviews that could help you.

OK for the expression of Cx43, Cx45 and Cx39 in vivo in skeletal muscle. Pannexin 1 is also expressed in adult skeletal muscle.

What about immortalized cell lines, such as L6 and C2C12? Would you expect them to express the same as skeletal muscle, or different ones?

I know only C2C12. They express cx43.but not cx40 and in litterature, I saw that they express also cx39