Identification and properties of ATP-sensitive potassium channels in myocytes from rabbit Purkinje fibres

Our goal was to identify the ATP-sensitive potassium (KATP) channels in cardiac Purkinje cells and to document the functional properties that might distinguish them from KATP channels in other parts of the heart.

Single Purkinje cells and ventricular myocytes were isolated from rabbit heart. Standard patch-clamp techniques were used to record action potential waveforms. and whole-cell and single-channel currents.

The KATP channel opener levcromakalim (10 microM) caused marked shortening of the Purkinje cell action potential. Under whole-cell voltage-clamp, levcromakalim induced an outward current, which was blocked by glibenclamide (5 microM), in both Purkinje cells and ventricular myocytes. Metabolic poisoning of Purkinje cells with NaCN and 2-deoxyglucose caused a significant shortening of the action potential (control 376 +/- 51 ms; 6 min NaCN/2-deoxyglucose 153 +/- 21 ms). This effect was reversed with the application of glibenclamide. Inside-out membrane patches from Purkinje cells showed unitary current fluctuations which were inhibited by cytoplasmic ATP with an IC50 of 119 microM and a Hill coefficient of 2.1. This reflects approximately five-fold lower sensitivity to ATP inhibition than for KATP channels from ventricular myocytes under the same conditions. The slope conductance of Purkinje cell KATP channels, with symmetric, 140 mM K+, was 60.1 +/- 2.0 pS (mean +/- SEM). Single-channel fluctuations showed mean open and closed times of 3.6 +/- 1.5 ms and 0.41 +/- 0.2 ms, respectively, at -60 mV and approximately 21 degrees C. At positive potentials. KATP channels exhibited weak inward rectification that was dependent on the concentration of internal Mg2+. Computer simulations, based on the above results, predict significant shortening of the Purkinje cell action potential via activation of KATP channels in the range 1-5 mM cytoplasmic ATP.

Purkinje cell KATP channels may represent a molecular isoform distinct from that present in ventricular myocytes. The presence of KATP channels in the Purkinje network suggests that they may have an important influence on cardiac rhythm and conduction during periods of ischemia.