1. Whole-cell voltage-clamp measurements were made of the time- and voltage-dependent properties of the inwardly rectifying background potassium current I(K1), in single myocytes from rabbit ventricle. The main goal of these experiments was to define the role of I(K1) in the plateau and repolarization phases of the action potential (AP). 2. Action potentials from single ventricular myocytes were used as the command signals for voltage-clamp measurements. In these 'action potential voltage-clamp' experiments, I(K1) was isolated from other membrane currents by taking the difference between control currents and currents in K+-free bathing solution. The results show that I(K1) is small during the plateau, but then rapidly increases during repolarization and declines in early diastole. 3. Evidence of an important functional role for I(K1) in AP repolarization was obtained by comparing the magnitude of I(K1) and the rate of change of membrane potential (dV(m)/dt) in the same cell during the AP. The time courses of I(K1) and dV(m)/dt during the AP were closely correlated, indicating that I(K1) was the principal current responsible for final repolarization. 4. Rectangular voltage-clamp steps were used to study time- and voltage-dependent changes in I(K1) at membrane potentials corresponding to the repolarization phase of the AP. 'Slow' relaxations or tail currents, lasting 100-300 ms, were consistently recorded when the cell was repolarized to potentials in the range -30 to -70 mV, following depolarizations between +10 and -10 mV. 5. The close correlation between the magnitude of the steady-state I(K1) (in an external K+ concentration of 5.4 mM), which was outward for membrane potentials in the range -30 to -70 mV, and the magnitude of the tail currents, suggests that they resulted from a slow increase, or reactivation, of I(K1). 6. The component of the slow tails due to reactivation of I(K1) can be separated from a previously described component due to Na+-Ca2+ exchange since the I(K1) component: (i) does not depend on the presence of the calcium current, I(Ca); (ii) can be recorded when internal EGTA (5 mM) suppresses large changes in [Ca2+]i; (iii) does not depend on the Na+ electrochemical gradient; (iv) is abolished in K+-free external solution; and (v) is not present in rabbit atrial myocytes, in which I(K1) is very small. 7. The time- and voltage-dependent properties of I(K1) revealed by these tail current experiments suggest that the measured magnitude of I(K1) will be dependent on the voltage-clamp protocol. Application of a 'conditioning' depolarization preceding a ramp reduces the magnitude of I(K1) recorded during the ramp. The magnitude of I(K1) also depends on the rate of change of membrane potential during the ramp; 'faster' ramps produced less outward I(K1). 8. These results show that I(K1) is 'inactivated' during the upstroke and plateau phases of an AP and consequently the amount of I(K1) available for repolarization is less than the maximal I(K1) present in the cell. Thus, heart rate-induced changes in I(K1) may contribute to alterations in the plateau and/or the duration of the action potential, as well as the threshold for firing Ca2+-dependent action potentials, or slow responses.
