K+, Na+, and Cl- activities in ventricular myocytes isolated from rabbit heart

Intracellular K+, Na+, and Cl- activities (aiK, aiNa and aiCl) were measured in ventricular myocytes enzymatically isolated from adult rabbit heart. The activities in normal Tyrode solution containing 2.5 mM Ca2+ were the following (in mM): aiK = 100.0 +/- 3.5 (n = 9); aiNa = 8.4 +/- 1.5 (n = 6); and aiCl = 17.9 +/- 1.5 (n = 11) (mean +/- SE). Membrane potential was -81.6 +/- 0.7 mV (n = 26). These values were determined after correction for changes of junction and tip potential at the reference electrode, estimated to be 4.9 +/- 0.6 mV (n = 7) for 0.15 M KCl-filled electrodes; and intracellular interference detected by the Cl- ion-selective electrode, 11.2 +/- 0.6 mM (n = 4). Extended-tip shunting was avoided by fabricating Na+ ion-selective microelectrodes from aluminosilicate rather than borosilicate glass. These results show that isolated cardiac cells can maintain normal intracellular ion activities. Diffusion of electrolyte from the reference electrode can rapidly alter the intracellular milieu, however. After 10 min of impalement with 0.15 M KCl-filled microelectrodes (resistance approximately equal to 25 M omega), aiK increased by 8.7 +/- 2.0 mM and aiCl by 10.3 +/- 3.1 mM. In contrast, aiNa did not significantly change during the double impalement.