COMPARISON OF MYOSIN ISOENZYMES PRESENT IN SKELETAL AND CARDIAC MUSCLES OF THE ARCTIC CHARR SALVELINUS-ALPINUS (L) - SEQUENTIAL EXPRESSION OF DIFFERENT MYOSIN HEAVY-CHAINS DURING DEVELOPMENT OF THE FAST WHITE SKELETAL-MUSCLE

The expression of myosin isoforms and their subunit composition in the white skeletal body musculature of Arctic charr (Salvelinus alpinus) of different ages (from 77-day embryos until about 5 years old) was studied at the protein level by means of electrophoretic techniques. Myosin from the white muscle displayed three types of light chain during all the developmental stages examined: two myosin light chains type 1 (LC1F) differing in both apparent molecular mass and pI, one myosin light chain type 2 (LC2F) and one myosin light chain type 3 (LC3F). The fastest-migrating from of LC1F seemed to be predominant during the embryonic and eleutheroembryonic periods. The slowest-migrating form of LC1F was predominant in the 5-year-old fish. Between 1 year and 4 years, both types of LC1F were present in similar amounts. Cardiac as well as red muscle myosin from 3-year-old fish had two types of light chain. The myosin light chains from atria and ventriculi were indistinguishable by two-dimensional electrophoresis, but were different from the myosin light chains from red muscle. Neither the light chains from cardiac nor red muscle were coexpressed with the myosin light chains of white muscle at any of the developmental stages examined. Two myosin heavy chain bands were resolved by SDS/glycerol/polyacrylamide gel electrophoresis of the extract from embryos. One of the bands was presented in minor amounts. The other, and most abundant, band comigrated with the only band found in the extracts of white muscle myosin from older fish. One-dimensional Staphylococcus aureus V8 protease peptide mapping of these bands revealed some differences during development of the white muscle tentatively interpreted as follows. The myosin heavy chain band present in minor amounts in the embryos may represent an early embryonic form that is replaced by a late embryonic or foetal form in the eleutheroembryos. The foetal myosin heavy chain appears to be present until the resorption of the yolk sack and beginning of the free-swimming stage. A new form of myosin heavy chain, termed neonatal and probably expressed around hatching, is present until about 1 year of age. The neonatal myosin heavy chain may be coexpressed with the foetal form and another type, named II-1 myosin heavy chain, whose peptide fragments persisted during the rest of the period under study. A new type of myosin heavy chain, called II-2, is apparently expressed from about 2 years and onwards. A further transition observed between 3 years and 4 years is attributed to the appearance of a new myosin heavy chain, type II-3. New peptides appeared in the V8-protease-treated myosin heavy chain band of 5-year-old charr. These are considered to be due to the expression of yet another type of myosin heavy chain, type II-4. In conclusion, a minimum of six different myosin heavy chains may be sequentially expressed in white skeletal muscle of the Arctic charr during the period under study. The native myosin isoforms from embryos and white muscle from adults differed only in their heavy chain subunits, and seemed to be closely related because of the small differences between their peptide map patterns. In the adult fish a minimum of seven myosin heavy chain types were expressed in cardiac and skeletal muscle displaying five different peptide map patterns: white, red, pink, atrial and ventricular myosin heavy chains.