Transformation of structural, functional, metabolic and molecular characteristics is induced by stimulating fast-twitch muscles with the frequency pattern of a motoneuron normally innervating a slow-twitch muscle. These changes correspond to a transition of a "fast-white" into a "slow-red" muscle. Intermittent stimulation (8 h/d) does not affect the system but causes alterations of the sarcoplasmic reticulum which lead to changes in time to peak and half relaxation time. Continuous stimulation (24 h/d) induces a transformation of the muscle also at the level of the myosin system.

Animals↗

PubMedDate not supplied

Characterization of cardiac microsomes from spontaneously hypertonic rats.

Capacity of Ca2+ sequestration was found to be significantly lowered in microsomal preparations of hearts from spontaneously hypertonic rats. A decrease to 40% of the control level was found for basal and extra ATPase. A similar reduction existed in initial and total Ca2+ uptake. These findings are correlated with a lower concentration of the Ca2+ transport ATPase in SDS gel electrophoresis, a lower density of the 7-9 nm intramembraneous particles and higher half-lives of phosphoprotein. Altered contractility of hypertrophied myocardium may thus be partially explained by the dysfunction of the Ca2+ sequestering system.

Adenosine Triphosphatases↗

PubMedDate not supplied

Ca2+ -ions as coupling agents in enzymatic differentiation and carbohydrate metabolism of cultured muscle cells.

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Animals↗

PubMedDate not supplied

Fiber transformation and fiber replacement in chronically stimulated muscle.

Chronic low-frequency stimulation (CLFS) of fast-twitch muscles induces fast-to-slow fiber-type transitions that differ in their extent in rat and rabbit. Fast-twitch muscle of the normal rat responds to CLFS with sequential transitions in myosin heavy chain (HC) expression in the order HCIIb --> HCIId --> HCIIa. However, in rabbit muscle the changes proceed beyond the state of HCIIa and include, as a final step, the expression of the slow myosin HCI. The time course for the transitions in myosin HC expression at both the messenger ribonucleic acid and protein levels suggests that fiber-type conversions occur asynchronously in a sequential manner. Thus type IIB fibers convert first to type IID fibers; these fibers then transform into type IIA fibers, which in rabbit muscle ultimately change into type I fibers. This sequence and the related changes in myosin isoforms point to different thresholds of the genes encoding the various fast and slow myosin HC isoforms. Although transformation of existing fast fibers into slower fibers is the major process that underlies the stimulation-induced fast-to-slow conversion, fiber replacement may also contribute. In rabbit muscle CLFS may cause metabolic exhaustion and subsequent deterioration in a fraction of the fast-twitch glycolytic fibers. For the most part these are replaced by satellite cell-derived, newly formed slow-twitch fibers.

Animals↗