EXPRESSION OF TITIN ISOFORMS IN RED AND WHITE MUSCLE-FIBERS OF CARP (CYPRINUS-CARPIO L) EXPOSED TO DIFFERENT SARCOMERE STRAINS DURING SWIMMING

Titin (also known as connectin) is a striated-muscle-specific protein that spans the distance between the Z- and M-lines of the sarcomere. T he elastic segment of the titin molecule in the I-band is thought to b e responsible for developing passive tension and for maintaining the c entral position of thick filaments in contracting sarcomeres. Differen t muscle types express isoforms of titin that differ in their molecula r mass. To help to elucidate the relation between the occurrence of ti tin isoforms and the functional properties of different fibre types, w e investigated the presence of different titin isoforms in red and whi te fibres of the axial muscles of carp. Gel electrophoresis of single fibres revealed that the molecular mass of titin was larger in red tha n in white fibres. Fibres from anterior and posterior axial muscles we re also compared. For both white and red fibres the molecular mass of titin in posterior muscle fibres was larger than in anterior muscle fi bres. Thus, the same fibre type can express different titin isoforms d epending on its location along the body axis. The contribution of titi n to passive tension and stiffness of red anterior and posterior fibre s was also determined. Single fibres were skinned and the sarcomere le ngth dependencies of passive tension and passive stiffness were determ ined. Measurements were made before and after extracting thin and thic k filaments using relaxing solutions with 0.6 mol.l(-1) KCl and 1 mol. l(-1) KI. Tension and stiffness measured before extraction were assume d to result from both titin and intermediate filaments, and tension af ter extraction from only intermediate filaments. Compared to mammalian skeletal muscle, intermediate filaments developed high levels of tens ion and stiffness in both posterior and anterior fibres. The passive t ension-sarcomere length curve of titin increased more steeply in red a nterior fibres than in red posterior fibres and the curve reached a pl ateau at a shorter sarcomere length. Thus, the smaller titin isoform o f anterior fibres results in more passive tension and stiffness for a given sarcomere strain. During continuous swimming, red fibres are exp osed to larger changes in sarcomere strain than white fibres, and post erior fibres to larger changes in strain than anterior fibres. We prop ose that sarcomere strain is one of the functional parameters that mod ulates the expression of different titin isoforms in axial muscle fibr es of carp.