Effect of selective angiotensin II receptor antagonism and angiotensin converting enzyme inhibition on the coronary vasculature in vivo. Intravascular two-dimensional and Doppler ultrasound studies.
BACKGROUND
Although angiotensin converting enzyme (ACE) inhibitors have been reported to increase coronary blood flow, the effect of selective angiotensin II (AT1)-receptor antagonism on the coronary circulation has not been defined.


RESULTS
We examined the effects of the AT1-receptor antagonist Losartan (DuP 753, 0.2-3.2 mg/kg) on coronary arteries in vivo in 11 dogs, using a combination of intravascular two-dimensional and Doppler ultrasound. In six dogs, a 30-MHz, 4.3F ultrasound imaging catheter was placed in the midsegment of the circumflex coronary artery to measure cross-sectional area (CSA), and a 0.018-in. Doppler wire was placed alongside to measure coronary flow velocity. At peak effect (1.6 mg/kg), Losartan increased mean coronary CSA from 7.9 +/- 0.5 to 9.5 +/- 0.8 mm2 and average peak velocity (APV) from 32 +/- 10 to 56 +/- 18 cm/sec, resulting in an increase in coronary blood flow from 74 +/- 19 to 151 +/- 36 mL/min. The maximal effect of the ACE inhibitor enalaprilat (5 mg) was an increase in CSA from 7.7 +/- 0.7 to 8.4 +/- 0.8 mm2 and an increase in APV from 36 +/- 10 to 53 +/- 20 cm/sec, with an increase in coronary blood flow from 82 +/- 25 to 122 +/- 41 mL/min. Relative to maximal hyperemia with adenosine (6 mg i.c.), the magnitude of flow increase from baseline was 0.37 with the AT1-receptor antagonist and 0.19 with the ACE inhibitor (p < 0.05). These effects were seen without changes in heart rate or systemic arterial pressure. In an additional five dogs, the ultrasound imaging catheter was introduced directly over a 0.014-in. Doppler wire, and the effects of indomethacin, propranolol, and N omega-nitro-L-arginine methylester (L-NAME) on the vasodilator effect of Losartan (1.6 mg/kg) were examined. Indomethacin and propranolol had no effect on Losartan-induced vasodilation, suggesting that it was not mediated via prostaglandins or beta-adrenoceptors. However, Losartan-induced epicardial vasodilation was partially inhibited by L-NAME, suggesting an action partly dependent on endothelial release of nitric oxide.


CONCLUSIONS
Thus, these acute studies in anesthetized dogs suggest that inhibition of AT1-receptors in the coronary circulation results in vasodilator responses greater in magnitude than ACE inhibition and partly endothelium dependent. The exact role for AT1-receptors in human coronary physiology and pathology remains to be defined.
Sudhir
K
K
Cardiovascular Research Institute, University of California, San Francisco 94143.
MacGregor
J S
JS
Gupta
M
M
Barbant
S D
SD
Redberg
R
R
Yock
P G
PG
Chatterjee
K
K
D000241:Adenosine / Q000494:pharmacology; D057911:Angiotensin Receptor Antagonists; D000806:Angiotensin-Converting Enzyme Inhibitors / Q000494:pharmacology*; D000818:Animals; D001120:Arginine / Q000031:analogs & derivatives / Q000494:pharmacology; D001713:Biphenyl Compounds / Q000494:pharmacology; D001783:Blood Flow Velocity / Q000187:drug effects; D003326:Coronary Circulation / Q000187:drug effects; D003331:Coronary Vessels / Q000000981:diagnostic imaging* / Q000187:drug effects*; D004285:Dogs; D015773:Enalaprilat / Q000494:pharmacology; D005260:Female; D007093:Imidazoles / Q000494:pharmacology; D019808:Losartan; D008297:Male; D019331:NG-Nitroarginine Methyl Ester; D012146:Rest; D013777:Tetrazoles / Q000494:pharmacology; D014463:Ultrasonography; D014664:Vasodilation / Q000187:drug effects
D057911:Angiotensin Receptor Antagonists; D000806:Angiotensin-Converting Enzyme Inhibitors; D001713:Biphenyl Compounds; D007093:Imidazoles; D013777:Tetrazoles; D001120:Arginine; D015773:Enalaprilat; D019808:Losartan; D000241:Adenosine; D019331:NG-Nitroarginine Methyl Ester