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1 The effect of probucol on the vascular reactivity of different arteries isolated from rabbits was studied as well as its effects on the development of atherosclerosis in a cholesterol-fed rabbit model. 2 Probucol 1O6-5 x 10-4 M produced a concentration-dependent inhibition of the contractile responses induced by KCl (80 mM), the sequence for the IC50 was: mesenteric artery (5th branch, 4.8+2.6 x 10-5 M) >aorta (8.2+2.3 x l0-M) >femoral artery (>5 x 0-4 M). The response to noradrenaline was: mesenteric artery (5th branch, 4.2+ 1.3 x 10-5 M) >aorta (3.2+ 3.0 x 10-4 M) >femoral (> 5 X 10-4 M). 3 In the aorta, probucol (10-_-10-4 M) shifted the concentration-response curves to Ca2+ downward and to the right. 4 Probucol at 5 x 10' M and 5 x 10-4 M showed a reduction in the 45Ca2+ uptake in resting, nonstimulated aortic rings as well as the uptake induced by both noradrenaline 10-6 M and KC1 80 mM. 5 In experiments in vivo, probucol did not affect lipid profiles; however, drug-treatment significantly decreased the cholesterol content of aortic tissue and the extent of intimal surface covered with atherosclerotic lesions. 6 The vascular reactivity was recovered in femoral arteries from rabbits on the atherogenic diet plus probucol. 7 It is concluded that the effect of probucol in vascular smooth muscle can be attributed to an inhibition of Ca2+ entry through both potential-and receptor-operated pathways. Moreover our findings suggest that the effects of probucol on movement of calcium in vascular smooth muscle may play an important role in the mechanism of antiatherogenic properties of this drug.