Human vascular to cardiac tissue selectivity of L‐ and T‐type calcium channel antagonists

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Abstract
1. Voltage-operated calcium channel (VOCC) antagonists are effective antihypertensive and antianginal agents but they also depress myocardial contractility. 2. We compared four L-type calcium channel antagonists, felodipine, nifedipine, amlodipine and verapamil and a relatively T-type selective calcium channel antagonist, mibefradil, on human and rat isolated tissue assays to determine their functional vascular to cardiac tissue selectivity (V/C) ratio. 3. The V/C ratio was calculated as the ratio of the IC50 value of the antagonist that reduced (by 50%) submaximally contracted (K+ 62 mM) human small arteries from the aortic vasa vasorum (vascular, V) mounted in a myograph and the IC50 value of the antagonist that reduced (-)-isoprenaline (6 nM) submaximally stimulated human right atrial trabeculae muscle (cardiac, C) mounted in organ chambers. 4. The average pIC50 values (-log IC50 M) for the human vascular preparations were felodipine 8.30, nifedipine 7.78, amlodipine 6.64, verapamil 6.26 and mibefradil 6.22. The average pIC50 values for the cardiac muscle were felodipine 7.21, nifedipine 6.95, verapamil 6.91, amlodipine 5.94, and mibefradil 4.61. 5. The V/C ratio calculated as antilog [pIC50V-pIC50C] is thus mibefradil 41, felodipine 12, nifedipine 7, amlodipine 5 and verapamil 0.2. 6. In rat small mesenteric arteries the pIC50 values for the five drugs were similar to the values for human vasa vasorum arteries contracted by K+ 62 mM. However for methoxamine (10 microM) contraction in the rat arteries the pIC50 values were lower for felodipine 7.24 and nifedipine 6.23, but similar for verapamil 6.13, amlodipine 6.28 and mibefradil 5.91. 7. In conclusion. in the human tissue assays, the putative T-channel antagonist mibefradil shows the highest vascular to cardiac selectivity ratio; some 3 fold higher than the dihydropyridine, felodipine, and some 200 fold more vascular selective than the phenylalkylamine, verapamil. This favourable vascular to cardiac selectivity for mibefradil, from a new chemical class of VOCC antagonist, may be explained by its putative T-channel selectivity.