Rabbit sino‐atrial node cells: isolation and electrophysiological properties.

1. A method has been developed for isolating calcium‐tolerant, single rabbit sinoatrial node cells which maintain their natural shape following isolation. The majority of viable, spontaneously active cells were elongated and measured about 100 microns in length. 2. Staining fixed cells with Haematoxylin‐Eosin revealed that a ‘cell’ with projections was usually an aggregate of more than one cell. 3. Single, elongated, spontaneously active cells were current and voltage clamped using the whole‐cell configuration of the patch‐clamp recording technique. The spontaneous activity and time‐dependent currents recorded were similar to those reported previously in multicellular nodal preparations and in single cells. 4. An assessment was made of the time course of L‐type calcium current run‐down: a stable period of between 10 and 20 min followed by a rapid run‐down (over about 2 min) was typically observed. 5. In most cells, a fast, TTX‐sensitive Na+ current component was seen. A few cells showed a transient outward K+ current (iA). 6. The activation range for the hyperpolarization‐activated current, if, varied from cell to cell. In the majority of actively beating cells, the threshold for if was near the maximum diastolic potential (about ‐65 mV in most cells) but in other cells, no if could be recorded within the pacemaker range. 7. Millimolar concentrations of MnCl2 caused a marked increase in if, but only when the pipette solution did not contain EGTA. Inclusion of EGTA (to buffer Ca2+ to about pCa 8) significantly reduced the effect of Mn2+ which therefore probably occurs through inhibition of Na(+)‐Ca2+ exchange and consequent rise in intracellular Ca2+ concentration.