The measurement of free nitrendipine in human serum by an equilibrium dialysis - radioreceptor assay.

A radioreceptor assay using [3H]nitrendipine and rat cerebral cortical membranes, in conjunction with equilibrium dialysis, measures the unbound (free) level of nitrendipine in human sera. The sensitivity of the assay is 0.1-0.2 picomoles/ml and is linear from 4 X 10(-11) to 4 X 10(-9) M nitrendipine. Other dihydropyridine calcium channel antagonists may be measured using this assay if these compounds are used to generate the standard curve. Blank serum interferes with specific [3H]nitrendipine binding (24 percent inhibition per 20 microliter serum) whereas serum dialysates do not. Total serum nitrendipine levels may be measured, but the sensitivity of the assay is decreased due to interference by serum. Nitrendipine is highly protein bound in serum (93 - 99 percent). This protein binding is essentially unchanged over a serum concentration from 1 to 100 ng/ml. This assay is suitable for pharmacokinetic and pharmacodynamic studies.