Delayed rectifier potassium current (IK) in latent atrial pacemaker cells isolated from cat right atrium

Rapidly and slowly activating components of delayed rectifier K(+) current in guinea-pig sino-atrial node pacemaker cells

The components and properties of the delayed rectifier K(+) current (I(K)) in isolated guinea-pig sino-atrial (SA) node pacemaker cells were investigated using the whole-cell configuration of the patch-clamp technique. An envelope of tails test was conducted by applying depolarizing pulses from a holding potential of -50 mV to +30 mV for various durations ranging from 40 to 2000 ms. The ratio of the tail current amplitude elicited upon return to the holding potential to the magnitude of the time-dependent outward current activated during depolarizing steps was dependent on the pulse duration, while after exposure to the selective I(Kr) inhibitor E-4031 (5 microM) this current ratio became practically constant irrespective of the pulse duration. These observations are consistent with the presence of the E-4031-sensitive, rapidly activating and E-4031-resistant, slowly activating components of I(K) (I(Kr) and I(Ks), respectively) in guinea-pig SA node cells. The activation range for I(Kr), defined as the E-4031-sensitive current (half-maximal activation voltage (V(1/2)) of -26.2 mV) was much more negative than that for I(Ks), defined as the E-4031-resistant current (V(1/2) of +17.2 mV). I(Kr) exhibited a marked inward rectification at potentials positive to -50 mV, whereas I(Ks) showed only a slight rectification. In the current-clamp experiments, bath application of E-4031 (0.5 and 5 microM) initially slowed the repolarization at potentials negative to approximately -30 mV and produced a significant depolarization of the maximum diastolic potential, followed by the arrest of electrical activity, thus indicating that the late phase of the repolarization leading to the maximum diastolic potential at around -60 mV in spontaneous action potentials is primarily produced by I(Kr) in guinea-pig SA node cells. External application of the selective I(Ks) inhibitor 293B (30 microM) also delayed the repolarization process at potentials negative to about -20 mV and induced moderate depolarization of the maximum diastolic potential leading to the arrest of the spontaneous activity. These results provide evidence to suggest that both I(Kr) and I(Ks) are present and play crucial roles in the spontaneous electrical activity of guinea-pig SA node pacemaker cells.

Swelling-induced decrease in spontaneous pacemaker activity of rabbit isolated sino-atrial node cells

The heart responds to an increase in sino-atrial node wall stress with an augmentation in rate of contraction. It has been suggested that swelling-activated ion channels may play a key role in that response. This paper investigates directly the effects of cell swelling on spontaneous activity of rabbit isolated sino-atrial node pacemaker cells. The main finding is that sino-atrial node cells, studied in current clamp mode using amphotericin-permeabilized patches, decrease their spontaneous pacemaker rate by 24.2 +/- 7.8% (P < 0.01, n = 9) during 75% hyposmotic swelling. This response is opposite to the predicted impact of volume-activation of sarcolemmal ion conductances. Computer modelling (OXSOFT Heart v4.8) suggests that swelling-induced dilution of the cytosol, reduction in intracellular potassium concentration, and decrease in the delayed rectifier potassium current, IK, are leading mechanisms in the response. This is supported by voltage-clamp data that show a swelling-induced positive shift in the reversal potential of IK by between 5 and 10 mV (n = 7) and a reduction in amplitude of its rapidly activating component, IKr, (n = 6). Thus, spontaneously active sino-atrial node cells reduce pacemaking rate during swelling. This response cannot be explained by the known volume-activated sarcolemmal ion conductances, but appears to be dictated by other mechanisms including dilution of the cytosol and reduction in IK. The results re-enforce the view that cardiac responses to cell volume changes may be quite different from those to longitudinal stretch.