Active force (S) and rate of change of force (S) throughout the time of isometric contraction in electrically stimulated, circumferentially cut segments of dog anterior tibial artery were analyzed for dynamic indices of the contractile state. Previous works have determined indices that change with temperature but not with length (L) or calcium (Ca) and were designated indices of crossbridge dynamics. They assumed that changes in L and Ca change only the number of crossbridges between contractile filaments and temperature affects only the dynamics of individual crossbridges. We have shown: (1) specific ratios of S/S to be independent of L; (2) S/S at the same instant of time (V*CE) is related to an index from previous work which was defined as a velocity of internal shortening; (3) specific indices of S/S were decreased with lower temperature but increased with time in a Ca-free solution. Our results for a Ca-free solution do not agree with previous work but can be explained by changes of the smooth muscle cell membrane. This suggests that L-independent indices are useful for characterizing changes in the overall dynamic state of the tissue but are not necessarily representative of a change in crossbridge dynamics.
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