FILAMENT OVERLAP AFFECTS TNC EXTRACTION FROM SKINNED MUSCLE-FIBERS

Recent studies on calcium regulation of muscle contraction selectively extract troponin C (TnC) from skinned skeletal muscle fibres with a l ow ionic strength rigor solution containing a Ca2+/Mg2+ chelator. As p revious results from this laboratory and others demonstrate a crossbri dge effect, especially rigor, on many of the properties of TnC, the ef fects of filament overlap on TnC extraction from skinned rabbit psoas muscle fibres were investigated. Tension-pCa relationships at a sarcom ere length of 2.7 mum were determined before and after a 5 min TnC ext raction at sarcomere lengths of 2.3, 2.5, 2.7, 3.1, 3.3 or 3.5 mum wit h 20 mm Tris, pH 7.8, 5 mm EDTA. The decrease in the post-extraction m aximum Ca2+ activated tension, an indicator of the amount of TnC extra cted, was linearly related to the overlap of the thick and thin filame nts with decreases in tension being associated with a decrease in fila ment overlap. The smaller fibre diameter at the longer sarcomere lengt h could facilitate diffusion of TnC from fibre segments. However, the wide range of measured diameters, 40-120 mum, accounted for only 14% o f the observed tension decrement and shrinking the fibre with polyviny lpyrrolidone did not increase the tension decrement. Increasing the sa rcomere length before extraction was also found to decrease the TnC co ntent of fibre segments along with the post-extraction maximum tension . Thus, TnC appears to be preferentially extracted from non-overlap th an overlap regions of the sarcomere. These results further indicate th at rigor crossbridges affect TnC other than through increased Ca2+ bin ding and that under the conditions used here, they retard its extracti on.