Developmental differences in delayed rectifying outward current in feline ventricular myocytes.

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Abstract
In the present work, we found that the delayed rectifying outward potassium current ( I K) in adult and neonatal cat ventricular myocytes consists of both rapid and slow components, I Kr and I Ks, respectively, which can be isolated pharmacologically. Thus after complete blockade of I Kr with dofetilide, 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 similar in adult and neonatal myocytes. I Ks maximum tail current density in neonatal myocytes, measured at −40 mV, was significantly smaller than in adult myocytes. Activation kinetics of I Kr and I Ks was similar in both age groups. However, the I Kr deactivation time course was significantly faster in neonatal than in adult myocytes. Developmental differences in the subunit composition of I Kr that display distinctly different deactivation kinetics are suggested.