Augmented vasoconstrictor responses to serotonin precede development of atherosclerosis in aorta of WHHL rabbit

Abstract

Watanabe heritable hyperlipidemic (WHHL) rabbits have elevated concentrations of plasma cholesterol and develop progressive atherosclerosis. The present investigation was undertaken to evaluate the vascular responses to vasoactive compounds of aorta from WHHL rabbits and normal New Zealand White (NZW) rabbits at 1 and 6 months of age. Rings of distal thoracic aorta were suspended under isometric tension in oxygenated Krebs buffer. Developed tension was measured in response to graded concentrations of agonists. Maximal responses to KCI (40 mM) were the same in aortas from the 1-month-old and 6-month-old WHHL and NZW rabbits. Aortas from 1-month-old animals were more sensitive to serotonin than aortas from 6-month-old animals. Aortas from WHHL rabbits exhibited an increased maximal response to serotonin when compared with NZW controls. In contrast, the constrictor responses to norepinephrine were reduced in WHHL rabbits compared with NZW rabbits at both age groups. Methacholine decreased tension development in serotonin-contracted vessels. This relaxation was greatest in aortas from NZW rabbits. In 1-month-old NZW rabbits fed a high cholesterol diet, the constrictor responses to serotonin and the relaxation responses to metacholine did not differ from NZW rabbits ingesting a normal diet. However, the responses to norepinephrine were markedly attenuated in the hypercholesterolemic NZW rabbits. Microscopic evaluation of the aortas revealed occasional adherent leukocytes and irregularities in the vascular endothelium in 1-month-old WHHL animals. These changes were greater in aortas from 6-month-old WHHL animals, with more adherent leukocytes, adherent platelets, and severe irregularities in the endothelial surface. These results indicate that the vasoconstrictor responses to serotonin are augmented in WHHL rabbits, and this enhanced response precedes the development of gross, but not microscopic, atherosclerotic changes in the blood vessel. The enhancement of vasoconstrictor responses to serotonin in the WHHL rabbit cannot be attributed to hypercholesterolemia.