Figure 4. Cardiac action potential waveforms (APWs) repeatedly stimulate Ca²⁺ mobilization by potentiation of P2Y receptor signalling.

[Ca²⁺]_i responses in the presence of 1 μm ADP to repeated ventricular (A) and atrial (B) APWs applied at 0.2 Hz. The insets show the shape of the normally paced waveforms on an expanded timescale. C, [Ca²⁺]_i responses to ventricular APWs, applied at both 0.2 Hz and 1 Hz.