Fig. 5. The rates of contraction and relaxation of the heartbeat depend on temperature. (A) Tension recordings averaged over 12 successive heartbeats show the shape of the heartbeat at the indicated temperatures. The inset shows heartbeats at 2°C and 22°C normalized to the maximal amplitude of the heartbeat. (B) Phase plots of the rate of change of force (y-axis) as a function of the force of contraction (x-axis) of the heartbeat. Plots represent all heartbeats recorded during 1 min at each indicated temperature. (C) Rates of contraction and of relaxation measured as the mean (±s.d.) of the maximum rising and falling slopes (dF/dt) of the heartbeat at indicated temperatures. (D) Acute Q₁₀ values for the contraction and relaxation of the heartbeat calculated from the change in dF/dt as a function of temperature (see Materials and methods). (E) Phase plots of B normalized for contraction amplitude at each temperature. (F) Rates of contraction and relaxation from C normalized for contraction amplitude at each temperature. (G) Acute Q₁₀ values calculated from normalized data in F. All data are from a single isolated heart.