There is a spontaneously hypertensive rat, Kown as SHR may be you could read, and there are a lot of studys about that.. I think you could check that out :)
its an open question as it depends on the strain e.g. you could claim WKY are hypertensive compared to Sprague Dawley. In general hypertension studies are done with WKY as control and SHR (spontaneously hypertensive rat) as the hypertensive group. There is a wealth of literature on that and the numbers you would expect and the major animal suppliers such as Harlan and Charles River also give you a wealth of background and information.
you can also study induced hypertension in normal rats, sprague-dawley or WKY. This can be done with angiotensin II infusion or DOCA salt. You could also study dahl salt sensitive rats. There are a ton of studies using this procedure. Good luck