Effects of Mebudipine and Dibudipine, Two New Calcium-channel Blockers, on Rat Left Atrium, Rat Blood Pressure and Human Internal Mammary Artery

Mebudipine and dibudipine are two new dihydropyridine calcium‐channel blockers that have been synthesized in our laboratory. In a previous study, they showed considerable relaxant effect on vascular and ileal smooth muscle. Here, the pharmacological effects of mebudipine and dibudipine on isolated rat left atrium, rat blood pressure and isolated human internal mammary artery are described. Results are compared with those obtained for nifedipine. Mebudipine and dibudipine reduced contraction force of rat left atrium (pIC30 values: 5·37 ± 0·13 and 5·49 ± 0·15, respectively) but their negative inotropic effects were significantly weaker than that of nifedipine (pIC30 value: 6·63 ± 0·11). Mebudipine and dibudipine lowered rat blood pressure. The hypotensive effect of mebudipine was similar to that of nifedipine while dibudipine was weaker than nifedipine. It was found that the half‐life of the hypotensive action of dibudipine (41·91 ± 3·77 min, 31·13 ± 2·26 min and 28·20 ± 4·37 min at 2, 4 and 8 mg kg−1 orally administered doses, respectively) was longer than that of nifedipine (11·85 ± 2·88 min, 16·65 ± 2·42 min and 14·03 ± 0·10 min at the same doses, respectively). Also, it appeared that mebudipine had a slower rate of absorption compared with nifedipine (the time to reach peak hypotensive action at 2, 4 and 8 mg kg−1 orally administered doses were, respectively, 24·00 ± 6·96 min, 23·75 ± 2·39 min and 15·00 ± 2·04 min for mebudipine and 7·80 ± 0·86 min, 13·75 ± 3·15 min and 8·33 ± 0·88 min for nifedipine). The two new compounds, as well as nifedipine, relaxed KCl‐treated isolated human internal mammary artery (pEC50 values; 7·87 ± 0·12, 7·22 ± 0·24 and 7·67 ± 0·12 for mebudipine, dibudipine and nifedipine, respectively). The relaxant effects of mebudipine and dibudipine did not show any significant difference compared with that of nifedipine. It is concluded that these new compounds are weak cardiodepressants and, with due attention to its significant vasorelaxant action, mebudipine is a vasoselective compound. In addition, these two compounds have potent blood pressure lowering effects. Also, their vasorelaxant action can be reproduced in human vascular preparations.