Relationship of alpha receptor types to hypotension and renal vasodilation caused by alpha blockers in conscious dogs.

In conscious instrumented normotensive and two-kidney, one clip Goldblatt hypertensive dogs, we compared the effects of the alpha-receptor blocking agent, urapidil, on blood pressure, renal vascular resistance, heart rate, and plasma renin activity with those of prazosin and phentolamine. Urapidil (2 mg/kg) and prazosin (0.25 mg/kg) decreased blood pressure and renal vascular resistance in both groups of animals, and urapidil caused a small increase in renal blood flow. Heart rate, but not plasma renin activity, was increased at the peak of the hypotension. Phentolamine had no significant effect on any of these parameters. All three agents markedly inhibited the renal vasoconstrictor responses to intraarterially administered phenylephrine and norepinephrine, and thus exhibited an alpha 1-receptor blocking action. Only urapidil significantly antagonized the response to B-HT 933, a selective alpha 2-receptor agonist, indicating that it also interacts at alpha 2-receptor sites. Since both normotensive and hypertensive animals exhibited similar hypotensive responses after both urapidil and prazosin, the degree of alpha-receptor blockade achieved did not reveal greater sympathetic tone in renal hypertension.