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Arginine vasopressin (AVP), in addition to being an antidiuretic hormone, might also have pressor effects relevant to the maintenance of hypertension. Results from several experimental and clinical studies suggested that the pressor function of AVP is more important in low-renin hypertension and in the salt-loaded state and that it might be further maximized under sympathetic suppression.To assess whether selective vasopressin receptor inhibition lowers the blood pressure in a racially diverse group of low-renin hypertensive subjects.Thirty-nine hypertensive subjects (16 Caucasian, 23 African-American) eating a 200 mmol/day sodium diet were administered a single intravenous dose of a selective vasopressin receptor antagonist and their blood pressure was monitored constantly for the ensuing 3 h. The protocol was repeated 3 days later after treatment with a single oral dose of 0.4 mg clonidine.Of these patients, 54% had their blood sampled for determination of hormone profiles. African-Americans with hypertension had higher baseline plasma AVP levels than did Caucasians (1.13 ± 0.05 versus 0.37 ± 0.06 pg/ml, respectively, P < 0.05), and lower plasma renin activity (0.34 ± 0.07 versus 1.03 ± 0.08 ng/ml per h, respectively, P < 0.05). Selective vasopressin receptor inhibition lowered the mean arterial pressure in African-Americans but not that in Caucasians (lowering by 28 ± 4 mmHg in African-Americans versus lowering by 5 ± 3 mmHg in Caucasians, P < 0.05). Moreover, vasopressin receptor blockade further reduced the arterial pressure in African-Americans but not that in Caucasians after pretreatment with clonidine.AVP seems to play a more important role as a pressor hormone in maintaining the elevation of arterial pressure in African-American hypertensives than it does in Caucasian hypertensives.