Slow onset of and recovery from Ca blocking action of benidipine in rat aorta and portal vein.

Effects of benidipine, a newly developed 1,4-dihydropyridine derivative, on the high K+-induced contraction in aorta and the spontaneous contraction in portal vein of the rat were examined. When either tissue was treated with a low concentration (0.1-10 nM) of benidipine for 90, 180 min or a longer period, the evoked or spontaneous contraction was reduced in a time- and concentration-dependent manner. The time required for 50% inhibition by 1 nM benidipine was approximately 60 and 120 min in the portal vein and aorta, respectively. Although the exact IC50 of benidipine was not determined because of the unexpectedly slow onset of the effect, it might have been overestimated previously and is apparently lower than 1 nM. After the withdrawal of 1 nM benidipine, its inhibitory action was not removed significantly even by washing for 5 hr. A partial or full recovery from the inhibition was observed in tissues pretreated with 0.3 nM benidipine. The persistent inhibition after withdrawal of 1 nM benidipine was not removed by Bay K 8644 treatment. The results strongly suggest that the slow recovery from the benidipine-induced inhibition is, at least in part, responsible for the long-lasting antihypertensive effects of benidipine.