Merocyanine 540 as an optical probe of transmembrane electrical activity in the heart

Frog hearts stained with merocyanine 540 shows a 1.5 to 2.0 percent increase in fluorescence intensity at 585 nanometers during the cardiac action potential when excited with a 540-nonometer light beam. Fluorometric action potentials similar to those recorded with intracellular microelectrodes in pacemaker, atrial, and ventricular tissues were recorded by focusing a 1-millimeter excitation beam on various regions of the heart. The signal-to-noise ratio for a single action potential ranged between 10/1 and 40/1. In spontaneously pacing hearts the slower rate of rise of the fluorescence action potential is due to the slow propagation of the electrical signal. In solutions containing normal calcium concentrations the fluorometric signal is altered by contractions. Merocyanine 540 is biologically inert as it stains the cardiac cell membrane and acts as a sensitive optical probe of the change in transmembrane potential.