Temperature-dependence of isometric tension and cross-bridge kinetics of cardiac muscle fibers reconstituted with a tropomyosin internal deletion mutant

The effect of temperature on isometric tension and cross-bridge kinetics was studied with a tropomyosin (Tm) internal deletion mutant AS-Delta 23Tm (Ala-Ser-Tm Delta(47-123)) in bovine cardiac muscle fibers by using the thin. lament extraction and reconstitution technique. The results are compared with those from actin reconstituted alone, cardiac muscle-derived control acetyl-Tm, and recombinant control AS-Tm. In all four reconstituted muscle groups, isometric tension and stiffness increased linearly with temperature in the range 5-40 degrees C for fibers activated in the presence of saturating ATP and Ca2+. The slopes of the temperature-tension plots of the two controls were very similar, whereas the slope derived from fibers with actin alone had similar to 40% the control value, and the slope from mutant Tm had similar to 36% the control value. Sinusoidal analysis was performed to study the temperature dependence of cross-bridge kinetics. All three exponential processes A, B, and C were identified in the high temperature range (30-40 degrees C); only processes B and C were identified in the mid-temperature range (15-25 degrees C), and only process C was identified in the low temperature range (5-10 degrees C). At a given temperature, similar apparent rate constants (2 pi a,2 pi b,2 pi c) were observed in all four muscle groups, whereas their magnitudes were markedly less in the order of AS-Delta 23Tm < Actin < AS-Tm approximate to Acetyl-Tm groups. Our observations are consistent with the hypothesis that Tm enhances hydrophobic and stereospecific interactions (positive allosteric effect) between actin and myosin, but Delta 23Tm decreases these interactions (negative allosteric effect). Our observations further indicate that tension/cross-bridge is increased by Tm, but is diminished by Delta 23Tm. We conclude that Tm affects the conformation of actin so as to increase the area of hydrophobic interaction between actin and myosin molecules.