Rate-related changes of the transmembrane potential in the rabbit atrial myocardium.

We studied changes of the transmembrane resting and steady state action potentials in the rabbit atrial trabecular preparations (length 2-4 mm, diameter 150-800 micrometers, standard microelectrode techniques). Following cessation of rapid drive the transmembrane potential shows an initial depolarization and a terminal hyperpolarization. Increasing the stimulation frequencies before the cessation of driving the initial depolarization and the terminal hyperpolarization are more accentuated. When the stimulation frequency is abruptly reduced, the enddiastolic ("resting") potential increases (steady state resting potential - 82 +/- 3 mV at 1.0 s, -73 +/- 6 mV at 0.25 s driving interval). Decreasing the rate of stimulation the duration of the steady state action potential shortens at 25% and prolongs at 90% repolarization. A calculated background conductivity is diminished using a slower drive. From the experimental findings ensue some considerations about a K+ accumulation, the rate dependent sodium pump and the rate related shifts of the steady state outward current-voltage relationships.