We investigated the role of angiotensin II (Ang II) and endothelin-l (ET-I) in transgenic (mREN2)27 rats, a model of the monogenic renin-dependent form of severe hypertension and cardiovascular disease. Four-week-old heterozygous male transgenic (mREN2)27 rats (n=24) were matched according to body weight (BW) and blood pressure (BP) and randomly allocated to receive a placebo (group P), the mixed endothelin type A and B receptor antagonist bosentan (100 mg/kg BW PO, group B), the Ang II type I-specific receptor antagonist irbesartan (50 mg/kg BW PO, group I), or the endothelin type A-selective antagonist EMS-182874 (52 mg/kg BW PO, group EMS). After 4 weeks of treatment, during which BW and BP were measured weekly, animals were euthanized, and the heart, left ventricle, right ventricle, adrenal gland, brain, and kidney were weighed. The plasma levels of adrenocortical steroids were measured by high-performance liquid chromatography. The tension responses of ET-free segments of the thoracic aorta to 5X10(-6) mmol/L phenylephrine, 60 mmol/L KCl, and cumulative doses of ET-1 were assessed. The density of ET-1 receptor subtypes in the aorta and vascular structural changes in the mesenteric arterioles (100 to 200 mu m ID) were also measured with autoradiography and myography, respectively. Compared with all other groups, group I rats showed significantly (P<0.001) lower systolic BP (group I, 161+/-8 mmHg; group P, 269+/-23 mmHg; group B, 275+/-17 mm Hg; and group EMS, 254+/-21 mm Hg), left ventricular weight (2.2+/-0.15 versus 3.71+/-0.26, 3.38+/-0.27, and 3.96+/-0.51 mg/g BW, respectively), tension responses to vasoconstrictors, and normalized media thickness of the mesenteric arterioles (22.3+/-0.6 versus 25.3+/-0.5, :15.5+/-0.7, and 24.1+/-1.5 mu m, respectively). Compared with levels in group P (78+/-25 pmol/mL), plasma aldosterone levels were significantly decreased in group B (51+/-11 pmol/ml) and group I (40+/-16 pmol/mL). Thus, endogenous ET-B and Ang II contribute to the regulation of aldosterone, but only Ang II is crucial for the development of hypertension, and related target organ damage via the Ang II type 1 receptor. Endogenous Ang II does not appear to enhance cardiovascular production of ET-I in this model of hypertension within the time span of our experiment.