COORDINATE CHANGES IN THE EXPRESSION OF TROPONIN SUBUNIT AND MYOSIN HEAVY-CHAIN ISOFORMS DURING FAST-TO-SLOW TRANSITION OF LOW-FREQUENCY-STIMULATED RABBIT MUSCLE

The purpose of this study was to follow the time course of changes in the expression of myosin heavy chain (HC) and troponin (Tn) subunit is oforms during fast-to-slow transition as induced in rabbit fast-twitch muscle by low-frequency stimulation. The evaluation of changes in the relative concentrations of myosin and troponin subunit isoforms were supplemented by measurements of relative protein synthesis rates using an in situ labeling technique. Changes in the amounts of mRNA encodin g fast troponin C (TnC) were followed by Northern blot analysis, those for fast and slow troponin I (TnI) by in vitro translation of total R NA. The various fast myosin heavy chain (HC) and fast troponin T (TnT) isoforms were exchanged sequentially. Myosin HCIId which is the predo minant fast isoform in rabbit tibialis anterior, was exchanged with HC IIa and, finally, the latter was replaced by the slow myosin HCI. The replacement of HCIId by HCIIa was accompanied by an exchange of TnT1f and TnT2f with TnT3f. The expression of HCI was accompanied by an exch ange of TnT3f with the slow TnT isoforms, TnT1s and TnT2s. The changes in the relative concentrations of the TnT isoforms were preceded by s imilar changes of their relative synthesis rates. Pronounced decreases in the fast TnI and TnC isoforms occurred only with prolonged stimula tion and were preceded by changes of the specific mRNAs and decreases in relative synthesis rates. The parallel time courses of the sequenti al transitions in both the myosin heavy chain and troponin T isoforms suggest the existence of coordinate programs of expression serving spe cific functional requirements.