Final answer:
The plateau phase of myocardial contractile cells is due to increasing Ca2+ ions and decreasing K+ ions.
Step-by-step explanation:
The flattening of the action potentials of myocardial contractile cells, known as the plateau phase, is due to a combination of increasing Ca2+ ions and decreasing K+ ions.
In the plateau phase, the slow Ca2+ channels open, allowing Ca2+ ions to enter the cell, while few K channels are open, which prevents K+ ions from exiting the cell. This combination of ions results in the flattening of the action potential.