Furosemide is a much, much better diuretic than spironolactone (Prilactone) and is the only treatment for cats and dogs in a congestive heart failure crisis. Furosemide is NOT toxic to the kidneys, it causes elevated BUN and creat because it uses dehydration to pull fluid from the lungs. It is not a benign drug, though, and is only used as-needed.
Spironolactone is not a substitute for furosemide. If your cardiologist doesn't think your cat needs furosemide any longer, that's one thing. If this low dose of furosemide is keeping your cat from CHF, that's another. When was your cat's last echo and cardio checkup? That will give you more info on whether or not it's safe to stop the furosemide.
Hope this helps,