EXPRESSION OF MYOSIN HEAVY-CHAIN ISOFORMS IN REGENERATED MUSCLE-SPINDLE FIBERS AFTER MUSCLE GRAFTING IN YOUNG AND ADULT RATS - PLASTICITY OF INTRAFUSAL SATELLITE CELLS

Satellite cells are the myogenic precursor cells of postnatal skeletal muscles. After muscle injury they can proliferate, differentiate, fus e and form myofibres. We have analysed regeneration of distinctly diff erent types of intrafusal fibres in rat muscle spindles. We have intro duced the new technique of heterochronous allotransplantation and comp ared it with the previously used standard autografting method. The all otransplantation method enables one to graft muscles from very young a nimals; we have used the extensor digitorum longus (EDL) muscles from 2- to 28-day-old rats, which were grafted into EDL muscles of adult in bred recipients. The regenerated ''intrafusal'' fibres did not express the spindle-specific slow tonic and alpha cardiac-like myosin heavy c hain (MyHC) isoforms and they did not exhibit the dual mATPase reactio n typical of the nuclear bag(2) fibres and the characteristic regional differences in MyHC expression and in the mATPase reaction of nuclear bag(1) and nuclear bag(2) fibres. On the other hand, they expressed e ither fast twitch or slow twitch/beta cardiac MyHC isoforms and exhibi ted an alkali or acid stable mATPase reaction along their whole length , like extrafusal fast type 2 and slow type 1 muscle fibres, respectiv ely. In all regenerated muscle spindles only motor, but no sensory axo ns were found. More than 85% of muscle spindles in our sample containe d regenerated spindle fibres of the same extrafusal fibre type (either type 2 or type 1), in contrast to control muscle spindles, which alwa ys contained intrafusal fibres of three different intrafusal fibre typ es (nuclear bag(1), nuclear bag, and nuclear chain fibres). There were no differences in MyHC expression and mATPase activity between spindl e fibres regenerated in grafts taken from young rats of various ages o r between allotransplanted and autotransplanted EDL muscles. The prese nt results demonstrate that regenerated ''intrafusal'' fibres resemble , according to MyHC expression, extrafusal fast or slow muscle fibres. It can thus be concluded that intrafusal satellite cells derived from distinctly different nuclear bag(1), nuclear bag(2) and nuclear chain fibres show great plasticity, as their MyHC expression can be respeci fied towards the extrafusal muscle fibre phenotype by foreign alpha-mo tor innervation.