EFFECTS OF RICIN ON THE ABILITY OF RABBIT ARTERIES TO CONTRACT AND RELAX

Ricin, a toxic lectin from castor beans, reduces blood pressure. The c urrent studies determined the effect of ricin on contractions in respo nse to norepinephrine (NE) of rabbit central ear artery, endothelium-d ependent relaxations to methacholine and ATP of aorta rings and endoth elium-independent relaxations to papaverine in central ear artery and aorta rings. Rabbits were given 0.11 or 0.22 mu g kg(-1) ricin i.v. an d 18 h, 4 days, or 7 days later the arteries were removed and tested. Maximal contractions to NE were increased by 8-23% (P > 0.05) with ric in treatment. The EC(50) was increased in five of six ricin-treated gr oups, although only significantly so at 18 h after a minimum lethal do se of ricin. Maximum relaxations to methacholine were increased by 20- 57% (P > 0.05) at 18 h following both ricin doses but returned to cont rol values at later time points. Relaxations in response to ATP were s ignificantly enhanced in all ricin-treated groups (95-205%) except in the 18 h and 4-day 0.11 mu g kg(-1) dose groups where the increase was 0-46%. Relaxations in response to papaverine were not altered in eith er artery. Thus, ricin decreases the sensitivity of the rabbit central ear artery to NE and increases endothelial-dependent relaxations of t he rabbit aorta. Therefore, decreased blood pressure following ricin a dministration may be due to vasodilation caused by decreased vascular contractions and increased endothelial-dependent vascular relaxation.