diuretics will lower your potassium. Beta blockers blocks beta receptors on heart tissues. Beta blockers are used in the treatment of heart disorders and to reduce the workload of the heart
Beta Blockers themselves (like Toprol or Inderal)cause your heart rate to beat at a slower rate. It will not affect your potassium level.
Potassium levels are effected when you use a non-potassium sparing diuretic--the most common diuretic without the potassium sparing feature is Lasix.
The higher the dosage of the Lasix the greater the need to urinate and thus the higher loss of potassium.
Many doctors will prescribe the "potassium sparing diuretics" to fend off the chance of developing low potassium.
Some of the common side effects of low potassium is muscle spasm.
Beta blockers do not lower potassium. If at all, they should raise it. But this effect is negligible.
Beta blockers also block renin secretion, thereby reducing angiotensin and aldosterone in the cascade (renin activates angiotensin, which increases aldosterone secretion). Aldosterone causes excretion of potassium. So if aldosterone is reduced, potassium level should increase.