Simultaneous Vm and Cai recordings from the heart of different species. Simultaneous recordings of Vm and Cai were obtained with PGH1 and Rhod-2 from guinea pig (A), rabbit (B), and mouse (C) hearts. The heart was excited at 540 nm, the emission for Rhod-2 was measured at 585 ± 30 nm, and the PGH I emission was measured above 700 nm. In rabbit hearts, Vm and Cai are shown at slow (a) and fast (b) sweep speeds, after the addition of an IKr blocker (E4031) (c), and PGH1 was measured at long excitation wavelengths (λex = 690 nm and >750 nm).