Intracellular potassium activity in rabbit sinoatrial node. Evaluation during spontaneous activity and arrest.

Measurement of intracellular potassium activity (ak) is important in furthering our understanding of the basis of the low resting and maximum diastolic potentials and the membrane currents underlying pacemaker activity in the sinoatrial node. K-selective microelectrodes have been successfully applied to measure ak in Purkinje fibers, ventricular muscle, and atrium. Similar measurements in the sinoatrial node are difficult, however, because of the cellular heterogeneity, small cell size, and spontaneous activity. We measured ak in shortened, electrically homogeneous strips of rabbit sinoatrial node. Transmembrane potential was recorded with microelectrodes of 2030 MSI resistance. Ion-sensitive signal was measured with similar microelectrodes filled with a Kselective liquid ion exchanger. All experiments were performed with simultaneous impalements with ion-sensitive and conventional microelectrodes. During spontaneous activity, cycle length was 341 ± 12 msec, maximum diastolic potential (MDP) was —62.0 ± 1.4 mV, and ion-sensitive signal (VK) was 27.0 ± 1.2 mV. Apparent ak calculated from the maximum diastolic potentials and VK was 151 ± 6 mM. Replacement of chloride ion by thiocyanate ion led to rapid reversible arrest with a maximum diastolic potential of —49 ± 3.3 mV and VK 25.0 ± 2.7 mV. Calculated ak was 85 ± 3 mM (n = 12, [K]o = 4 mM). ak measured after sequential arrest by thiocyanate ion replacement and 1 mg/liter verapamil agreed within 2 mM. The close agreement between values obtained in preparations arrested by thiocyanate ion and verapamil suggests that, over the period of time required for arrest, the agency of arrest does not change ak. We also measured a value of 80 ± 3.3 mM for ak in quiescent right atrial strips. We conclude that (1) ak is similar in the sinoatrial node and right atrium, (2) the slow frequency response of the ion sensitive microelectrode may lead to a considerable overestimate of ak, (3) the low MDP in SA nodal cells compared with the atrium is not the result of a low ak, and (4) the net driving force on potassium ions is outward throughout the range of pacemaker potentials at normal [K]o. (C/rc Res 51: 271-279, 1982)