Fig. 3.

A: correlation of the maximal speeds of contraction and relaxation in an isolated murine trabecula (wild-type C57 mouse), assessed under conditions where length, frequency, or concentration of the β-adrenergic agonist isoproterenol was varied. Data recorded at 37°C. dF/dt_max and dF/dt_min, maximum positive and negative derivatives of force, respectively. B: correlation of speed of relaxation and speed of contraction in rat trabeculae (average data of n = 10 trabeculae) at various conditions. FS, Frank-Starling relationship, muscle contracted at 4 different lengths (0.5, 1, or 2 stimulation frequency); RT, room temperature (held at 22.5°C); BT, body temperature (held at 37.5°C); Temp, 1 Hz, optimal length at various temperature (in 2.5°C steps) between RT and BT; Iso, various concentrations of isoproterenol; Temp Up ISO, maximal isoproterenol concentration at different temperatures; FFR, force-frequency relationship. Because the slowest data (low dF/dt) are bundled and to convey that the correlation holds true for timing parameters in general rather than only dF/dt, we plotted the same data now correlating time from stimulation to peak tension to time from peak tension to RT₅₀. All data except for those at low preload, 0.5 Hz, and at the lowest temperature (●) deviated significantly (P < 0.05, analysis of variance) from the mean correlation. Data modified from Janssen (22).