Contrary to previous reports, recent enzymatic assays showed the predominance of chymase-like activity in rat arteries. We determined the existence and significance of such alternative pathways in rat carotid arteries by measuring contraction of arterial rings in organ baths and blood pressure in conscious rats. Hamster aorta served as a positive control for chymase. Temocapril (30 mol/L) inhibited the contractions to angiotensin (Ang) I (10 Ϫ9 to 10 Ϫ5 mol/L) except at high concentrations of

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... g I (Ͼ10 Ϫ7 mol/L). Addition of chymostatin (100 mol/L) to temocapril exerted a synergistic inhibitory effect. Hamster aorta gave similar results, except that temocapril was 30-fold less effective than in rat arteries. [Pro 11 ,D-Ala 12 ]Ang I (10 Ϫ8 to 10 Ϫ5 mol/L), a chymase-specific substrate, provoked similar responses in rat and hamster arteries; chymostatin, but not temocapril, attenuated the responses. CV 11974 (30 mol/L), an Ang II type 1 receptor antagonist, abolished the responses to both peptides. In conscious rats, Ang I (0.03 to 30 g/kg) and [Pro 11 ,D-Ala 12 ]Ang I (7 to 700 g/kg) produced similar pressor responses. Not only CV 11974 (1 mg/kg) but also temocapril (2 mg/kg) abolished Ang I-induced responses in vivo. CV 11974, but not temocapril, inhibited responses to [Pro 11 ,D-Ala 12 ]Ang I. Our results showed the presence of the alternative pathway in rat arteries, but it did not play a major role. Arteries with the opposing characteristics of chymase responded equally to [Pro 11 ,D-Ala 12 ]Ang I. These findings suggest that biochemical and [Pro 11 ,D-Ala 12 ]Ang I-derived results may not reflect the functional significance of chymase. (Hypertension. 1999;34:525-530.)