Fig. 2. Physiologically higher extracellular Ca²⁺ compensates for cardiac function at decreased heart rate of ex vivo mouse working heart.

Increasing of Ca²⁺ in perfusion medium from 2.25 mM to 2.75 mM significantly increased both systolic and diastolic functions as shown by the higher LVP max, lower LVPmin (A), higher ±dP/dt (B) and stroke volume (C) of ex vivo mouse working hearts at heart rate of ~260 bpm, compensating the decrease from lowering heart rate from 480 bpm at 2.25 mM Ca²⁺. The test was done at 10 mmHg preload and 55 mmHg afterload. N = 6 hearts. Values are presented as mean ± SE. *P<0.05 vs. 2.25 mM Ca²⁺ control in Student’s t test.