A RAPIDLY ACTIVATING AND SLOWLY INACTIVATING POTASSIUM CHANNEL CLONEDFROM HUMAN HEART - FUNCTIONAL-ANALYSIS AFTER STABLE MAMMALIAN-CELL CULTURE EXPRESSION

The electrophysiological properties of HK2 (Kv 1.5), a K+ channel clon ed from human ventricle, were investigated after stable expression in a mouse Ltk - cell line. Cell lines that expressed HK2 mRNA displayed a current with delayed rectifier properties at 23-degrees-C, while sha m transfected cell lines showed neither specific HK2 mRNA hybridizatio n nor voltage-activated currents under whole cell conditions. The expr ession of the HK2 current has been stable for over two years. The depe ndence of the reversal potential of this current on the external K+ co ncentration (55 mV/decade) confirmed K+ selectivity, and the tail enve lope test was satisfied, indicating expression of a single population of K+ channels. The activation time course was fast and sigmoidal (tim e constants declined from 10 ms to < 2 ms between 0 and +60 mV). The m idpoint and slope factor of the activation curve were E(h) = - 14 +/- 5 mV and k = 5.9 +/- 0.9 (n = 31), respectively. Slow partial inactiva tion was observed especially at large depolarizations (20 +/- 2% after 250 ms at +60 mV, n = 32), and was incomplete in 5 s (69 +/- 3%, n = 14). This slow inactivation appeared to be a genuine gating process an d not due to K+ accumulation, because it was present regardless of the size of the current and was observed even with 140 mM external K+ con centration. Slow inactivation had a biexponential time course with lar gely voltage-independent time constants of approximately 240 and 2,700 ms between -10 and +60 mV. The voltage dependence of slow inactivatio n overlapped with that of activation: E(h) = -25 +/- 4 mV and k = 3.7 +/- 0.7 (n = 14). The fully activated current-voltage relationship dis played outward rectification in 4 mM external K+ concentration, but wa s more linear at higher external K+ concentrations, changes that could be explained in part on the basis of constant field (Goldman-Hodgkin- Katz) rectification. Activation and inactivation kinetics displayed a marked temperature dependence, resulting in faster activation and enha nced inactivation at higher temperature. The current was sensitive to low concentrations of 4-aminopyridine, but relatively insensitive to e xternal TEA and to high concentrations of dendrotoxin. The expressed c urrent did not resemble either the rapid or the slow components of del ayed rectification described in guinea pig myocytes. However, this cha nnel has many similarities to the rapidly activating delayed rectifyin g currents described in adult rat atrial and neonatal canine epicardia l myocytes. Therefore human Kv 1.5 may contribute to the initial fast repolarization and to the K+ conductance during the plateau phase of t he cardiac action potential.