Developmental differences in delayed rectifying outward current in feline ventricular myocytes

In the present work, we found that the delayed rectifying outward potassium current (I-K) in adult and neonatal cat ventricular myocytes consists of b oth rapid and slow components, I-Kr and I-Ks, respectively, which can be is olated pharmacologically. Thus after complete blockade of I-Kr with dofetil ide, the remaining I-Ks current is homogeneous, as shown by an envelope of tails test. I-Kr maximum tail current density, measured at -40 mV, was simi lar in adult and neonatal myocytes. I-Ks maximum tail current density in ne onatal myocytes, measured at -40 mV, was significantly smaller than in adul t myocytes. Activation kinetics of I-Kr and I-Ks was similar in both age gr oups. However, the I-Kr deactivation time course was significantly faster i n neonatal than in adult myocytes. Developmental differences in the subunit composition of I-Kr that display distinctly different deactivation kinetic s are suggested.