Rise in blood glucose with PPID is secondary to the insulin resistance that it causes.  If the insulin resistance is not completely compensated for by increased insulin levels, glucose will be elevated.

Although I agree this level is higher than it should be, it's not high enough to cause any panic.  In fact, the only consequence of elevated BG I've consistently noted in horses is weight loss.  If there are other hyperglycemic consequences occurring they are not being noticed but it's also worth pointing out that human diabetics can run BG considerably higher than what the horses do.  I don't recall any horses being 200 or higher.

Back in the 1980s there were a report of pergolide suppressing insulin release and elevating glucose in rats:

http://www.ncbi.nlm.nih.gov/pubmed/6147289

and another showing no effect of pergolide:

http://www.ncbi.nlm.nih.gov/pubmed/6135620

Durham et al in Equine Veterinary Journal (2009) 41 (9) 924 - 929 reported on 3 horses with type 2 DM and pancreatic beta cell dysfunction (low insulin with hyperglycemia). One horse was diagnosed with PPID and started on pergolide.  After the first dose of pergolide there was a spike in insulin to 35 uIU/L and a drop in glucose from 14.1 mmol/L to 7.5 mmol/L.  Thereafter, glucose remained around 5 and insulin progressively dropped to a high of 5 after feeding.

An abstract in  2013 JEVS found that injection of 5 mg cabergoline in a slow release vehicle in nonPPID mare did not have any effect on IR or insulin response to glucose:

http://www.j-evs.com/article/S0737-0806(13)00198-6/fulltext?mobileUi=0

These two studies suggest that low dose pergolide is not likely to reduce insulin in horses but there is still a possibility that high doses may do so, especially since this study found low dose pergolide to stimulate insulin in rat pancreas (as Durham saw) while high dose suppresses the insulin response to glucose:

http://onlinelibrary.wiley.com/doi/10.1080/15216540600687993/full

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Eleanor in PA

www.drkellon.com

EC Owner 2001