What studies are there showing the effectiveness of any substance on canine hypertension?
Check out this consensus statement (PMID 17552466):

J Vet Intern Med. 2007 May-Jun;21(3):542-58.
Guidelines for the identification, evaluation, and management of systemic hypertension in dogs and cats.
Brown S1, Atkins C, Bagley R, Carr A, Cowgill L, Davidson M, Egner B, Elliott J, Henik R, Labato M, Littman M, Polzin D, Ross L, Snyder P, Stepien R; American College of Veterinary Internal Medicine.

It includes 185 references, at least a dozen of which are related to your question. Also, if you look to the right of the article on PubMed, there are links to other relevant articles. 

FYI: I located this article using search string  

(canine hypertension) NOT pulmonary Filters: Review


There are quite a few others. (Canine pulmonary hypertension is apparently a popular research topic, so I found it useful to exclude those.)