EFFECTS OF REMOVAL OF WEIGHT-BEARING FUNCTION ON CONTRACTILITY AND MYOSIN ISOFORM COMPOSITION IN SINGLE HUMAN SKELETAL-MUSCLE CELLS

The purpose of this study was to investigate the effects of a 6-week p eriod without weight bearing, achieved by bed rest, on the contractile behaviour, myosin isoform expression and myofibrillar protein content of single human muscle fibres. Percutaneous biopsied specimens of the quadriceps muscle were taken from three healthy male volunteers befor e and at the end of the experimental period. Maximum force normalised to cross-sectional area (specific tension), maximum velocity of unload ed shortening (V-0), and myosin heavy chain (MyHC) and light chain (My LC) isoform composition were measured in single membrane-permeabilised muscle cells obtained from these specimens. At the end of the experim ental period, specific tension was reduced (P < 0.001) by 40% and ther e was a parallel decline in myofibrillar protein content per muscle ce ll volume. V-0 did not change significantly in response to bed rest wh en data from all muscle cells were pooled. In two of the subjects, how ever, V-0 decreased (P < 0.01-0.001) in muscle cells expressing the be ta/slow (type I) MyHC isoform, but there was no change in fibres expre ssing type IIA or a combination of type IIA and IIB MyHCs. The slowing in type I MyHC fibres was associated with a change in the isoform com position of the regulatory MyLC.