Does anyone have a succesful and detailed method of how to do the experiment above?

I used 3H-OMG and 14C-mannitol radioactives, krebs buffer and etc. The steps involved pre-incubation, rinse and incubation and finally measured the glucose uptake/ transport using scintillation counter. As ones would expect the treatment with insulin will increase glucose uptake, however, the result I've got was negative.

I don't know if the incubation time, the buffer or concentration of insulin might be the cause. Can someone shed some light?