Thats because Dig has 2 MOA:
1) Blocks Na/K channels>decreases Na/Ca exhange >more Ca inside the cell> increased contractility. (positive inotropy)
2) Reduces heart rate by via vagal nerve stimulation>M2 receptors>decrease AV nodal conduction(negative chronotropy)
An interesting note is that in overdose: Patients complain of diarrhea, nausea,.etc. Why? Due to blockade of Na/K channels elsewhere in the body (including the gut, nerves).
