Kinetics of binding of membrane-active drugs to receptor sites. Diffusion-limited rates for a membrane bilayer approach of 1,4-dihydropyridine calcium channel antagonists to their active site.

Using the model of 1,4-dihydropyridine calcium channel-blocking drug binding to receptors in the cardiac sarcolemmal membrane, diffusion-limited rates of association were calculated for two distinct approaches. In the "aqueous approach," the drug reaches the receptor by diffusion through the bulk solvent, whereas in the "membrane approach," the drug partitions into the membrane bilayer and then diffuses laterally to a specific receptor site. Calculated rates for the membrane approach were approximately 3 orders of magnitude greater than those for the aqueous approach. The membrane approach diffusion-limited rate depends weakly on the sizes of the binding site, the drug molecule, and the vesicle, but depends strongly on ligand asymmetry. Although the measured binding rates for several 1,4-dihydropyridines were all slower than the calculated diffusion-limited rates for either model, other experimental data (such as very high partition coefficients and specific positions of these drugs in the membrane bilayer) suggest that the membrane approach is the most likely. These results have important implications for specifying critical characteristics of active 1,4-dihydropyridines.