Looks like the (not-so) little guy will be okay and able to come home tomorrow. So long as he continues doing well. His potassium and sodium levels are down to almost normal again, and the vet took out the catheter. He's keeping Tiny overnight because his BUN and creatinin levels are still pretty high, though. But, if all goes well tonight, he'll come home tomorrow and be on antibiotics for 2 weeks.
The cause of all of this? Urinary tract crystals. His tract was so full of crystals, the vet found them on his penis. When the emergency vet inserted the catheter last night, Tiny's urine was coming out blood red. He was one terribly sick kitty, and they weren't sure he'd survive the surgery to insert the catheter.
Apparently, it's something in his diet that caused it. So, tomorrow, the vet is going to give us some recommendations for changes to his food. I'll post what he had to say tomorrow when we go get Tiny. (Yes, I am being optimistic that my kitty is coming home. Deal.) Maybe what he says will be helpful in preventing this from happening to someone else's cat.
Thanks for the good wishes, all. I'll be sure to give Tiny extra cuddles for you all. :)
~Bev (& Tiny, too!)