[spunkets]

"How can kidney cells sense salt, sweet, sour and bitter? How do they do this without first evolving into taste bud cells? "

These cells are neurons and all that's needed is the appropriate receptor functionality. They're only interested in salty, sweet and umami. Umami is the fifth taste sensation and is the taste of glutamate. This neuron has glutamate receptors and sodium channels. The sodium channels respond to salt. I'm not sure about the sweetness receptors, but it wouldn't be much of a chore to provide that functionality from what's already present.

[spunkets]

... Once the receptors are triggered by the right substance, the cell membrane will be depolarized. That depolarization will be detected by electrodes and be processed and recorded in the electronics.