Slow pen trace of isometric tension in the standard activating solution (pCa 4.66, 5 mM MgATP, 8 mM Pi, ionic strength 200 mM, 15 mM CP, 320 units/ml CK, pH 7.00) at different temperatures (indicated). Experiments were performed in thin filament-reconstituted cardiac muscle fibers, in which nfTm (A), (B), and (C) were added back, together with bovine cardiac Tn. D, activation of native fibers; E, activation of thin filament-extracted fibers after gelsolin treatment for 50–90 minutes; F, activation of actin filament-reconstituted fibers; and G, activation of Tm- and Tn-reconstituted fibers. H, activation at eight different temperatures, as indicated. Between D, E, F, and G, the pen recorder was stopped so that extraction and reconstitution could be performed (at ). Before each activation, the muscle fibers were washed in the standard activating solution (at ), which did not induce tension. Tension was induced by switching the fibers to a bath of the same solution at the higher temperature. The fibers were relaxed in the Rx solution that contained 40 mM BDM at . Horizontal bars below the pen trace in C indicate that the temperature was during relaxation. The active tension in F develops as the result of withdrawal of BDM, and the temperature rises to . The active tension is relaxed as the result of the addition of 40 mM BDM, and the temperature drops to . Calibrations are 1 min (abscissa) and 10 kPa (ordinate).