Mesenteric resistance and brain microvascular angiotensin-converting enzyme in the spontaneously hypertensive rat.

1. Angiotensin-converting enzyme (ACE) concentration was measured in mesenteric and brain microvessels from spontaneously hypertensive rats (SHR) and compared with normotensive controls using a specific radioligand binding assay. 2. Plasma angiotensin-converting enzyme activity was similar in SHR (n = 15) and normotensive controls (n = 21; 58 +/- 1 nmol HL/mL per min, vs 64 +/- 6 nmol HL/mL per min). 3. There was no significant difference between the mesenteric vascular angiotensin-converting enzyme radioligand binding site density (Bmax, fmol/mg protein) of SHR and normotensive controls (954 +/- 77 vs 890 +/- 56, P = 0.5, unpaired Student's t-test), despite significant differences in systolic blood pressure (220 +/- 8 mm Hg vs 120 +/- 6 mm Hg respectively, P less than 0.01) and increased mesenteric wet weight to bodyweight ratio in the hypertensive rats (0.28 +/- 0.02 mg/g, n = 5 vs 0.16 +/- 0.02 mg/g, n = 7, P less than 0.01). 4. Brain vascular angiotensin-converting enzyme radioligand binding site density (Bmax, fmol/mg protein) was also similar in SHR and normotensive controls (467 +/- 62, n = 5 vs 497 +/- 64, n = 5, P = 0.7, unpaired Student's t-test). 5. These results demonstrate that vascular angiotensin-converting enzyme concentration is not altered in the SHR and that vascular ACE is not increased in this form of vascular hypertrophy or regulated by the blood pressure level.