INTERNAL POTASSIUM ACTIVITY IN FERRET VENTRICULAR MUSCLE

Measurements of the K activity ( a i K ) in ferret ventricle were made using either single‐ or double‐barrelled K‐sensitive micro‐electrodes. a i K was also estimated from the Nernst equation by measuring the membrane potential when changing the external K concentration. Microelectrodes filled with the Corning K sensor gave unreliable results most probably due to interference from some substance(s) in the heart cells. Reproduceable measurements were obtained using a valinomycin cocktail as the K sensor. The mean value ± S.D. for a i K from four experiments which met strict criteria for calibration, electrode penetration and drift was 104 mmol/l ± 9 mmol/l. If all nine experiments were taken the value was 101 mmol/l ± 8 mmol/l. Estimation from the Nernst equation gave values of a i K that were on average 20 mmol/l higher than the measured values with the valinomycin cocktail. It is recommended that a valinomycin cocktail be used to measure a i K .