Internal potassium activity in ferret ventricular muscle.

Measurements of the K activity (aiK) in ferret ventricle were made using either single- or double-barrelled K-sensitive micro-electrodes. aiK was also estimated from the Nernst equation by measuring the membrane potential when changing the external K concentration. Micro-electrodes 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 aiK 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 aiK 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 aiK.