title: Effect of length and cross-bridge attachment on Ca2+ binding to cardiac troponin C.
abstract: The sensitivity of skinned cardiac muscle bundles to Ca2+ is a function of sarcomere length. Ca2+ sensitivity is increased as fiber length is extended along the ascending limb of the force-length curve and it has been suggested that this phenomenon makes a major contribution to the steep force-length relationship that exists in living cardiac muscle. To gain greater insight into the mechanism behind the length dependence of Ca2+ sensitivity isotopic measurements of Ca2+ binding to detergent-extracted bovine, ventricular muscle bundles were made under conditions in which troponin C was the only major Ca2+ binding species. Experiments were designed to determine whether 1) Ca2+-troponin C affinity varies in the sarcomere length range corresponding to the ascending limb of the force-length curve, and 2) Ca2+ binding correlates with length per se or with changes in the number of length-dependent cross-bridge attachments. Measurements were made of Ca2+ binding in the rigor and relaxed states. The latter state was produced by suppressing actin-myosin interaction with the phosphate analogue, sodium vanadate. After vanadate treatment it is possible to obtain a complete Ca2+ saturation curve in the presence of physiological MgATP concentrations and at constant sarcomere length. The results show that the binding of Ca2+ to the regulatory site of cardiac troponin C is length dependent but this length dependence is actually a dependence on the number of attached cross bridges.
mesh_terms: D000251:Adenosine Triphosphatases / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D002417:Cattle; D004492:Edetic Acid / Q000494:pharmacology; D009200:Myocardial Contraction; D009206:Myocardium / Q000201:enzymology / Q000378:metabolism* / Q000648:ultrastructure; D009210:Myofibrils / Q000648:ultrastructure*; D011188:Potassium / Q000494:pharmacology; D012518:Sarcomeres / Q000648:ultrastructure*; D014336:Troponin / Q000378:metabolism*; D019209:Troponin C
chemical_list: D014336:Troponin; D019209:Troponin C; D004492:Edetic Acid; D000251:Adenosine Triphosphatases; D011188:Potassium; D002118:Calcium