Biphasic variation of beating rate as a function of NCX density. At high NCX density, LCR events are suppressed, giving a low beating rate. However, for very low NCX densities, the calcium clock becomes desynchronized from the membrane, leading to diastolic release that peaks too early to trigger the action potential. This results in a characteristic “kink” in the NCX phase-plane loop and a lower beating rate. Note that I_NCX depends on calcium and voltage and so does not vary directly with k_NCX. I_f was included in the model for this and later simulations examining rate regulation.