Outward potassium currents in freshly isolated smooth muscle cell of dog coronary arteries

Outward membrane currents were characterized in single coronary smooth muscle cells of adult beagle dogs. The cells averaged 96.4 x 7.1 microns and had a resting potential of -50.7 mV, an input resistance of 307.9 M omega, a capacitance of 32.3 pF, and a calculated membrane surface area of 4,037 microns2. The cells contracted in response to external application of acetylcholine or high K+. In voltage clamp by use of the suction pipette method, outward current began to appear at -50 mV and reached 15.2 nA at 50 mV with a current density of 376.5 microA/cm2. The current was reduced by external tetraethylammonium, Ba2+, and internal Cs+, and its reversal potential had a Nernst relation to external K+ concentration. Elevation of external Ca2+ (Ca2+o) from 0 to 0.3 mM increased total K+ current by up to 300%; elevation of internal Ca2+ (Ca2+i) to 5 x 10(-7) M by internal perfusion increased total outward current to a similar extent, suggesting a large difference in Ca2+ transmembrane sensitivity. Total whole-cell K+ current consisted of two components: an initial time-independent current (Ii) followed by a time-dependent current (It). Ii and It were through separate K+ channels based on differences in a) sensitivity to Ca2+09b) modulation by an inward Ca2+ current, c) current amplitudes and activation kinetics, and d) responses to pharmacological agents. It was the largest component, measuring 4.5 nA in 0 mM Ca2+o but increasing to 11.9 nA in 0.3 mM Ca2+o with a steep 2.5 power function. It activated with a biexponential time course; in Ca2+o-free solution, its time course was relatively insensitive to voltage changes but became voltage sensitive in the presence of Ca2+o. Further, such sensitivity was abolished or enhanced by Co2+ or Bay K 8644, respectively. We concluded that there are two types of Ca2+-sensitive K+ currents, Ii and It, in coronary smooth muscle cells. Via an inward Ca2+ channel Ca2+o strongly modulates It, both in amplitude and kinetics.