Fluorescence monitoring of rapid changes in membrane potential in heart muscle

The rising phase of rat cardiac action potentials was measured in physiological solutions using the voltage-sensitive dye RH 237. A newly designed optical system and an argon ion laser for excitation allowed measurements without averaging over small areas (20-90 microns diameter) with high time resolution (response time 10-90%, 0.12 ms). The mean value of the fractional change in the fluorescence signal was approximately 3%/100 mV. The signal-to-noise ratio was approximately 60 rms (spot diameter 70 microns) allowing signal differentiation after digital filtering. Multiple measurements within the same spot showed a decrease in the fractional fluorescence change of 20 to 25% after 45 min without changes in the shape of the rising phase and with no measureable phototoxic effects. The optically measured rising phases showed rise times significantly (P less than 0.01) shorter and maximum upstroke velocities equal to or most often greater than those obtained with microelectrode techniques. Comparing simultaneous optical and electrical measurements within the same spot the microelectrode signal was often slightly delayed. This refined system seems well suited to detect fast cellular electrical activities with time and space resolutions comparable or even superior to those obtained using microelectrode techniques.