The size of the phase 1 notch varies among cardiac cells. It is prominent in myocytes in the epicardial and midmyocardial regions of the left ventricular wall and in ventricular Purkinje fibers. However, the notch is negligible in myocytes from the endocardial region of the left ventricle. What's the reason?
Between at least the endocardial and epicardial regions, I believe the transient outward current is facilitated much more in epicardial cells. This article demonstrates the differences in Ito between the two cell types in dogs.
I'm also recommending the following article about Ito phenotypes: "Transient outward potassium current, ‘Ito’, phenotypes in the mammalian left ventricle: underlying molecular, cellular and biophysical mechanisms"