DETERMINANTS OF ACTION-POTENTIAL DURATION IN NEONATAL RAT VENTRICLE CELLS

Study objective - The aim was to study the currents that determine the action potential duration in ventricular cells from neonatal rats. Design - Microelectrode measurements of action potentials from ventricle strips were compared with action potentials obtained from isolated myocytes with the whole cell patch clamp method in current clamp mode. Ionic currents were studied in myocytes in voltage clamp mode using recognised modulators of channel activity. Experimental material - Neonatal rats (2 d old) were decapitated and myocytes were prepared from the apical third of collagenase treated hearts. Measurements and main results - Modification of the action potential by 1.8-5.0 mM Ca, 2.0 mM Co, 8 mM 4-aminopyridine, 1.8 mM Sr, and 20 mM tetraethylammonium suggested the presence of the slowly inactivating Ca current I(Ca,L), an early outward current I(eo), and at least one other K current. Action potentials from myocytes and ventricular strips were comparable. Voltage clamp experiments were confirmatory and revealed currents with the following properties: (1) I(Ca,L): a Ca current with a current density of 21.7-mu-A.cm-2, activated between -30 and -20 mV, saturated at 1.8 Ca(o), inactivated faster at 5 than at 1.8 mM Ca(o), more permeable to Ba and Sr than to Ca, and with Sr as charge carrier blocked by Ca; (2) I(eo): the peak current had a linear I/V relation between 0 and 70 mV and was abolished by 4 mM 4-aminopyridine; (3) I(K1): the current was an inward rectifier that showed a relaxation at potentials negative to -90 mV. Conclusions - Action potentials obtained from neonatal rat ventricle with microelectrodes are comparable with those measured in myocytes in current clamp mode. The action potential duration is mainly determined by I(Ca,L), I(eo), and I(K1), and there is no evidence for the presence of a delayed rectifier.