"Couldn't help thinking my control preparations were already "hyperglycemic"."

If glucose concentration is a concern, rather than wondering about it, simply use a buffer or medium that contains the normal level of glucose. Osmolarity to isotonic level can be easily attained by any number of solutes that do not directly effect whatever is it you are studying.

"Never had a satisfactory answer to this question."

How bizarre!

Tausif,

I was aware you could top up the osmolarity by other means. I just wondered why a standard buffer recipe would use sugar levels this high.

Regarding my own use of this buffer, I didn't have a choice. I was doing my PhD at the time and wasn't allowed to change the buffer for control experiments

Hello

I was all the time believing that such high glucose concentration is required to keep the osmotic pressure so that the cells do not burst. If you come to find another explanation, kindly put it as answer to your question here. 

Thanks