Figure 2.

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), ∆2Tm (B), and ∆3Tm (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 0°C). Before each activation, the muscle fibers were washed in the standard activating solution (at 0°C), 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 0°C. Horizontal bars below the pen trace in C indicate that the temperature was 0°C during relaxation. The active tension in F develops as the result of withdrawal of BDM, and the temperature rises to 25°C. The active tension is relaxed as the result of the addition of 40 mM BDM, and the temperature drops to 0°C. Calibrations are 1 min (abscissa) and 10 kPa (ordinate).