Study on rats... Just wanted to highlight this conclusion in the abstract: "In contrast, acute caffeine administration decreased significantly Glut4 expression at 0.5mM of caffeine"
With all the evidence on caffeine's possible negative effect on insulin sensitivity, this study seems like a concern to me as well. I bring it up since I was just reading Bill Roberts 'Change Your Body Fat Set Point' article", and one of the goals in gene expression he mentions is: "Increase in GLUT-4 to increase nutrient transport into muscle".
It doesn't seem like a good idea anymore to consume a significant amount of caffeine pre-workout, then have a bunch of MAG-10 and Plazma before lifting. Just my opinion... may stick to coffee instead because of its health properties (I'll also be having less caffeine overall in my day).
There is a couple things to take into consideration with this. One being the dosage of caffeine. I am not sure what the conversion would be for that amount into a rat compared with a similar dosage in humans. Typically, in research, the dosage is plenty high to illicit a particular effect. If it does translate to a relatively high dosage of caffeine for humans, then it stands to probably show the same pattern of reduced GLUT4 expression via decreased insulin sensitivity.
With a high enough caffeine dosage, there tends to be a significant cortisol response which could decrease insulin sensitivity and thus GLUT4 expression. This may not be the case with a more "normal" dosage of caffeine though, or at least it would contribute to a response that wasn't very significant.
Another thing to take into consideration when referring to caffeine and working out is that there are two mechanisms by which GLUT4 can be expressed. One is insulin mediated, and the other is non-insulin mediated. Insulin mediated GLUT4 expression, as you probably guessed is elicited by insulin. Non-insulin mediated GLUT4 expression is elicited mechanically, via exercise.
Point being, if you are consuming carbs and then exercising you harnessing both insulin and non-insulin mediated glucose uptake, potentially diminishing what drawbacks caffeine may have on GLUT4 expression.
Schanz's take on this is relevant. I hate rat studies so much. Dosing is ALWAYS a problem when converting to human consumption. I'd also add that if any sort of pre-workout contributes significantly to the quality of your workout, the benefits may very well outweigh the drawbacks regarding GLUT-4. That's always something to consider.