It is well understood in the research community that hyper dosing of exogenous GH (or excessive natural secretion) causes insulin resistence. There really is no argument on this. I can post several dozen studies that make this finding. This is not a problem with minimal doses (00.0017mg/kg body weight), but bodybuilder's and elite athletes typically exceed this dose.
This insulin resistence occurs immediatly, not hours after the injection. That's the point of injecting in conjunction with the exoGH. Those who use GH without insulin often complain of hypoglycemia.
What we hope to do, is to have GH, insulin, and IGF-1 all circulating in serum at the same time so as to achieve maximum results. And this ain't easy, because the body fights us all the way. GH negatively impacts insulin, IGF-1 negatively impacts GH production, and GH causes more IGF-1 to be bound and not free to attach to it's recepters........damn!
But it is possible through proper and timely supplementation, to over-ride the system. One look at today's pro's will tell you that.