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1 Institut für
Pharmakologie,
In rat ventricle, two Ca2+-insensitive components of K+ current have been distinguished kinetically and pharmacologically, the transient, 4-aminopyridine (4-AP)-sensitive Ito and the sustained, tetraethylammonium (TEA)-sensitive IK. However, a much greater diversity of depolarization-activated K+ channels has been reported on the level of mRNA and protein. In the search for electrophysiological evidence of further current components, the whole cell voltage-clamp technique was used to analyze steady-state inactivation of outward currents by conditioning potentials in a wide voltage range. Peak (Ipeak) and late (Ilate) currents during the test pulse were analyzed by Boltzmann curve fitting, producing three fractions each. Fractions a and b had different potentials of half-maximum inactivation (V0.5); the third residual fraction, r, did not inactivate. Fractions a for Ipeak and Ilate had similar relative amplitudes and V0.5 values, whereas size and V0.5 of fractions b differed significantly between Ipeak and Ilate. Only b of Ipeak was transient, suggesting a relation with Ito, whereas a, b, and r of Ilate appeared to be three different sustained currents. Therefore, four individual outward current components were distinguished: Ito (b of Ipeak), IK (a), the steady-state current Iss (r), and the novel current IKx (b of Ilate). This was further supported by differential sensitivity to TEA, 4-AP, clofilium, quinidine, dendrotoxin, heteropodatoxin, and hanatoxin. With the exception of Ito, none of the currents exhibited a marked transmural gradient. Availability of IK was low at resting potential; nevertheless, IK contributed to action potential shortening in hyperpolarized subendocardial myocytes. In conclusion, on the basis of electrophysiological and pharmacological evidence, at least four components contribute to outward current in rat ventricular myocytes.
isolated myocytes; rat ventricle; transient current; sustained current; heteropodatoxin; hanatoxin; dendrotoxin; cloned channels
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