DETERMINANTS OF ACTION-POTENTIAL INITIATION IN ISOLATED RABBIT ATRIALAND VENTRICULAR MYOCYTES

Action potential conduction through the atrium and the ventricle of th e heart depends on the membrane properties of the atrial and ventricul ar cells, particularly with respect to the determinants of the initiat ion of action potentials in each cell type. We have utilized both curr ent-and voltage-clamp techniques on isolated cells to examine biophysi cal properties of the two cell types at physiological temperature. The resting membrane potential, action potential amplitude, current thres hold, voltage threshold, and maximum rate of rise measured from atrial cells (-80 +/- 1 mV, 109 +/- 3 mV, 0.69 +/- 0.05 nA, -59 +/- 1 mV, an d 206 +/- 17 V/s, respectively; means +/- SE) differed significantly ( P < 0.05) from those values measured from ventricular cells (-82.7 +/- 0.4 mV, 127 +/- 1 mV, 2.45 +/- 0.13 nA, -46 +/- 2 mV, and 395 +/- 21 V/s, respectively). Input impedance, capacitance, time constant, and c ritical depolarization for activation also were significantly differen t between atrial (341 +/- 41 M Omega, 70 +/- 4 pF, 23.8 +/- 2.3 ms, an d 19 +/- 1 mV, respectively) and ventricular (16.5 +/- 5.4 M Omega, 99 +/- 4.3 pF, 1.56 +/- 0.32 ms, and 36 +/- 1 mV, respectively) cells. T he major mechanism of these differences is the much greater magnitude of the inward rectifying potassium current in ventricular cells compar ed with that in atrial cells, with an additional difference of an appa rently lower availability of inward Na current in atrial cells. These differences in the two cell types may be important in allowing the atr ial cells to be driven successfully by normal regions of automaticity (e.g., the sinoatrial node), whereas ventricular cells would suppress action potential initiation from a region of automaticity (e.g., an ec topic focus).