Steady-state force–frequency relationship. Representative traces (A) and mean (±SE) twitch tension (B; top) show a flattened force–frequency relationship in R4496C atrial and ventricular trabeculae when compared with WT preparations. Tension is normalized to 1-Hz twitch amplitude. § = P < 0.05 for 4–7-Hz frequencies. In atrial preparations, relative force at 0.5 Hz was also significantly lower in R4496C versus WT trabeculae. (Bottom) No differences were found in the kinetics of twitches at different frequencies of stimulation (Peak, time to peak contraction; RT50, time to 50% relaxation). Error bars represent mean ± SEM.