Can the binding of Ca2+ to two regulatory sites on troponin C determine the steep pCa/tension relationship of skeletal muscle?
The relationship between tension and Ca2+ concentration in single skinned muscle fibers has been determined with a high density of experimental points and the data have been fitted by a least squares method to the Hill equation. We find that the mean Hill coefficient for the slope of the tension/Ca2+ relationship is between 5 and 6, and the pKd is about 5.9. Because there are four Ca2+ binding sites on troponin C, and only two of these regulate hydrolysis of MgATP, we conclude that the regulation of tension by Ca2+ binding is greatly modified by other factors. One important factor is the time required for a cross-bridge to complete a cycle once initiated, relative to the time Ca2+ remains bound to troponin C. The pCa/tension relationship will shift to higher pCa values as the ratio of cross-bridge cycle time to the Ca2+ bound time increases. For example, the pCa/tension curve may progressively shift to the left with increase in tension because strain in the myofilament lattice progressively increases the cycle time. This left shift will produce a pCa/tension relationship that is steeper than the actual Ca2+ binding curve. The anticipated shift of the pCa/tension curve with cycle time also bears on interpretations of earlier experiments on the "active state" and on the effects of Ca2+ on the maximal velocity of shortening.
D000199:Actins / Q000378:metabolism; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D009119:Muscle Contraction; D009124:Muscle Proteins / Q000378:metabolism*; D009132:Muscles / Q000378:metabolism*; D009218:Myosins / Q000378:metabolism; D011817:Rabbits; D014336:Troponin / Q000378:metabolism*
D000199:Actins; D009124:Muscle Proteins; D014336:Troponin; D009218:Myosins; D002118:Calcium