Fig. 11. Serotonin increases the strength and frequency of the heartbeat in isolated hearts (N=7). (A) Means of ratios of the amplitude of the contraction in the presence and absence of serotonin (5-HT) for each preparation are shown as a function of temperature. There are no statistically significant differences in amplitude across temperature (P=0.40). (B) Means of ratios of the frequency of the heartbeat in the presence and absence of serotonin for each preparation are shown as a function of temperature. There are no significant differences in heart rate across temperature (P=0.69). (C) Phase plots for heartbeats recorded in the absence (control) and presence of serotonin show similar shapes. Each trace shows the rate of change of force (y-axis) as a function of the force of the contraction (x-axis) for heartbeats recorded over 1 min at 2°C. In this experiment serotonin increased contraction amplitude at 2°C by 39.6%.