Physioex 8.0 Cardiovascular Physiology Exercise 6 Activity 5 - Effects of Ions

Hey guys,

I just finished by Physioex assignments, but I've been having a lot of trouble on 3 questions

What was the effect of calcium ions on heart rate? Why?

What was the effect of sodium ions on heart rate? Why?

What was the effect of potassium ions on heart rate? Why

To me it looked like they all decreased heart rate, but im not sure if it's true. Or if it is true I'm not sure why.
Maybe calcium prolongs the plateu region during heart contraction thus delaying the onset of another beat. Maybe adding potassium ruins the concentration gradient thereby slowing down repolarizing thereby delaying the onset of another beat.

These are the only questions I have left, and any help would be appreciated.

Thanks guys