Dihydropyridine receptor isoform expression in adult rat skeletal muscle.

The expression of isoform-specific dihydropyridine receptor Ca2+ channel (DHPR) alpha1-subunit genes in rat diaphragm, soleus and extensor digitorum longus muscles was investigated using RNase protection assays. As expected, mRNA expression levels for the DHPR skeletal muscle isoform were highest in extensor digitorum longus. Unexpectedly, both diaphragm and soleus expressed mRNA for the cardiac isoform at a significant level. Moreover, immunohistochemical experiments provided evidence of the cardiac DHPR isoform at the protein level in muscle fibres. The presence of the cardiac DHPR in the soleus and diaphragm is consistent with a degree of reported cardiac-like excitation-contraction coupling in these muscles, and may be an explanation for some of the therapeutic effects of theophylline in asthmatics, but is likely to serve some other role(s) as well.