We applied the cell-attached and inside-out patch-clamp technique under symmetrical isotonic potassium conditions on single human (and guinea pig) atrial cells. The human cells were isolated by a modified method to that described earlier. Our aim was twofold: 1) to study the single-channel characteristics of potassium channels in human atrial single cells, present under basal conditions (iK1 and iK(ATP] or when stimulated with 10(-5) M acetylcholine; and 2) to calculate the contribution of these three channel types to the total basal potassium conductance in human atrial cells, and to compare the results with data on guinea pig atrial cells under the same conditions. We found that in human cells 58% of the patches (n = 42/74) contained acetylcholine-sensitive potassium channels: their conductance was 42 +/- 1.2 pS and mean open time (tau o) was 1.7 +/- 0.5 msec. They showed sporadic openings in the absence of agonist, and activation by acetylcholine was G-protein dependent. In 16% of the patches (n = 7/44), adenosine (10(-4) M) activated the same channels, but the activity was lower than when stimulated by acetylcholine. In 18% of the patches (n = 9/51), an iK1 channel was present (conductance, 27 pS; tau o, 8.7 msec), whereas in the cell-attached mode, ATP-dependent channels were never seen. However, they were present in half of the inside-out patches on washout of ATPi (conductance, 73 pS; tau o, 1.4 msec). The basal potassium conductance (i.e., in the absence of any exogenous hormone or neurotransmitter) was mainly due to iK1 channels in both human and guinea pig cells, a finding that is in contrast with previous reports. However, the potassium current that is induced by acetylcholine is much higher in guinea pig than in human isolated cells; a fraction of it would suffice to fully determine the resting potassium conductance in guinea pig atrial cells, whereas it can play only a modulatory role in human cells. This difference could be important in species-specific autonomic modulation and antiarrhythmic drug action.