Isolated bovine ventricular myocytes

1. Bovine ventricular myocytes isolated according to isenberg and Klockner (1982a) were superfused with Tyrode solution ([Ca]0-3.6 mM, 35°C) and investigated with intracellular microelectrodes. The properties of the action potential (AP) were compared to those published for multicellular ventricular tissue. 2. At 1 Hz stimulation, the AP had 4 clearly discernible phases with a trapezoidal configuration and a duration of 276 ±38 ms. At [Ca]0=1.8 mM the 4 phases were less clear. Following a 1 min rest period, 1 Hz stimulation shortened the AP by 28% ([Ca]0=3.6 mM) or by 12% ([Ca]0=1.8 mM). 3. The AP started with a sodium spike which rose with\(\dot V_{\max }\) of 250±60 V/s to an overshoot of +40±5 mV. The sodium spike was blocked by 60 μM TTX.\(\dot V_{\max }\) was modified with 2s conditioning pre-pulses; the resultingh∞ curve showed half maximal inactivation atV0.5=−65 mV and a slope factork=5.5 mV. Reduction of [Ca]0 to 1.8 mM shiftedh∞ by −5 mV but did not modifyk. However, large shifts inh∞ curves were measured in myocytes depolarized by 20 mM or 0 mM [K]0. 4. Slow AP's were studied when the sodium spike was inactivated by high [K]0 or blocked by TTX. The slow AP's rose with\(\dot V_{\max }\) of 11±5 V/s to a maximum plateau potential of 36±4 mV;\(\dot V_{\max }\) increased up to 24 V/s when the latency was shortened by increased stimulus strength.\(\dot V_{\max }\), plateau and duration were increased by 0.1 μM adrenaline, reduced by 1.8 mM [Ca]0 and blocked by Ni, Co or D600. 5. Changes in [K]0 modified the resting potential (Vm) as discribed in multicellular ventricular tissue.Vmax was −84 ±4 mV at [K]0=5.4 mM and −54±3 mV at [K]0=20 mM; the latter value suggests [K]i=152±8 mM. The increase in [K]0 abolished the sodium spike and shortened the AP, but did not change the maximum plateau potential. In voltage clamp experiments the i-v relation of the repolarizing late outward currents was N-shaped. The sensitivity of these currents to [Cs]0 and [K]0 indicated that the inward rectifying potassium conductance declines with [K]0. 6. The experimental interventions used in the present study changed the AP of the isolated myocyte in a way similar to that reported for multicellular ventricular tissue, i.e. no electrophysiological peculiarity was observed. We conclude that the enzymatic isolation procedure did not significantly alter the properties of the Na-, Ca- or K-channels.