The calcium current may become the dominant depolarizing current in these cells if the sodium current is inhibited by a decrease in the resting transmembrane potential, by drugs which block the sodium channel or by diseases, particularly myocardial ischemia. The transformation of the sodium dependent upstroke into a calcium dependent upstroke is facilitated by beta-adrenergic agonists which enhance calcium conductance through the L-type calcium channels. When this occurs, the cells then acquire the characteristics of AV nodal cells. They depolarize very slowly and impulse conduction through this region is slowed.