Fig. 2.

Amplitude variation of the transient Ca²⁺ rise. Shown are fractional changes in the amplitude of the transient Ca²⁺ rise, Δ[Ca²⁺]_tr, during a train of 40 action potentials at the indicated stimulation frequencies (●, 1 Hz; ○, all other frequencies), normalized to the first transient of each train. Three different patterns of facilitation and depression emerge. At low frequencies (f_Stim ≤ 1 Hz), Δ[Ca²⁺]_tr amplitudes are essentially constant throughout the train (●). At moderate frequencies (1 Hz < f_Stim ≤ 15 Hz), Δ[Ca²⁺]_tr amplitudes facilitate during the initial stimuli but deteriorate later in the train. Finally, at higher frequencies, depression of Δ[Ca²⁺]_tr amplitudes dominates throughout the train, resulting in Δ[Ca²⁺]_tr amplitudes deteriorating to <20% of their initial values at the end of 40 stimuli.