Ionic currents in single isolated bullfrog atrial cells

Enzymatic dispersion has been used to yield single cells from segments of bullfrog atrium. Previous data (Hume and Giles, 1981) have shown that these individual cells are quiescent and have normal resting potentials and action potentials. The minimum DC space constant is approximately 920 microns. The major goals of the present study were: (a) to develop and refine techniques for making quantitative measurements of the transmembrane ionic currents, and (b) to identify the individual components of ionic current which generate different phases of the action potential. Initial voltage-clamp experiments made using a conventional two-microelectrode technique revealed a small tetrodotoxin (TTX)-insensitive inward current. The small size of this current (2.5-3.0 X 10(-10)A) and the technical difficulty of the two-microelectrode experiments prompted the development of a one-microelectrode voltage-clamp technique which requires impalements using a low-resistance (0.5-2 M omega) micropipette. Voltage-clamp experiments using this new technique in isolated single atrial cells reveal five distinct ionic currents: (a) a conventional transient Na+ current, (b) a TTX-resistant transient inward current, carried mainly by Ca++, (c) a component of persistent inward current, (d) a slowly developing outward K+ current, and (e) an inwardly rectifying time-independent background current. The single suction micropipette technique appears well-suited for use in the quantitative study of ionic currents in these cardiac cells, and in other small cells having similar electrophysiological properties.