REVERSAL OF THE RELATIVE EXPRESSION OF CARDIAC AND SKELETAL ALPHA1 SUBUNIT ISOFORMS OF L-TYPE CALCIUM-CHANNEL DURING IN-VITRO MYOGENESIS

Cardiac and skeletal type of excitation-contraction coupling (ECC) are quite different. Those differences could be explained by structural o nes in the molecular entities involved in ECC, ie dihydropyridines (DH P) receptors (alpha 1 subunit of L-type calcium channels) of the sarco lemma or ryanodine receptors of the sarcoplasmic reticulum membrane. A s previously demonstrated by means of electrophysiology, the two types of ECC coexist during the first stages of in vitro development of ske letal muscle, whereas the skeletal type predominates at the later ones . In order to see whether evolution of ECC could be correlated with th e one of alpha 1 subunit expression, we determined by Northern Blottin g which isoforms of alpha 1 subunit are expressed during the in vitro myogenesis. mRNA corresponding to the cardiac isoform are present in m yoblasts (before fusion), but patch-clamp experiments showed that they are not functional. After fusion, skeletal and cardiac mRNA are coexp ressed in myotubes, with different intensities: whereas expression of skeletal mRNA (which are the more intensive) stabilized at the later s tages tested, cardiac mRNA decreased. We conclude that evolution in mR NA alpha 1 subunit isoforms expression could partly explained evolutio n of ECC features during in vitro myogenesis.