Fig. 5. (A) An increase in the fluorescence intensity (F/F) indicating an increase in the intracellular Ca²⁺ concentration of a single caudal heart muscle fibre associated with depolarisation produced by an increase in the external K⁺ concentration to 100 mmol l^-1 at the time marked by arrow. The fibres had been loaded with fluo-3 by immersing in 0Ca²⁺-ASW containing 30 mmol l^-1 Co²⁺ and 2 µmol l^-1 fluo-3-AM for 1.5 h. The bath solution was 0Ca²⁺-ASW containing 30 mmol l^-1 Co²⁺. The dotted line indicates the background fluorescence measured off the cell. (B) Effect of changing the external K⁺ concentration on the membrane potential of caudal heart muscle fibres. Values are means ± S.D. obtained from 10 fibres. The slope was obtained by linear regression to the mean values, and is 35.2 mV per tenfold change in the K⁺ concentration.