Unlike skeletal muscle action potentials, cardiac muscle action potentials unlike skeletal muscle action potentials, cardiac muscle action potentials have a short refractory period do not involve repolarization begin with a rapid depolarization phase involve calcium voltage-gated channels

I think the answer would be that they involve calcium voltage-gated channels.  These channels are found in the membrane of excitable cells with permeability to calcium ions. The cardiac muscle action potential is a brief change in voltage across the cell membrane of heart cells. This is caused by the movement of charged atoms (ions) between the inside and the outside of the cell through the ion channels (proteins).