A method for estimation of drug affinity constants to the open conformational state of calcium channels

The affinity of D600 to calcium channels in the open state has been examined in isolated smooth muscle cells of the rabbit ear artery. Calcium channel currents were measured in high external barium solution by means of the patch-clamp technique. The current inhibition in various D600 concentrations (3-100 microM) on application of trains of short test pulses (20-80 ms) has been studied in nonmodified calcium channels and in cells where the calcium channels were modified by the agonist dihydropyridine (+) 202,791 (100 nM). The kinetics of the peak current decay has been analyzed with a mathematical model which is based on the experimental finding that D600 interacts primarily with calcium channels in the open conformational state. The model approach allows the estimation of drug affinity constants of D600 to the calcium channel in the open conformation. An association rate constant to the open conformational state of D600 of 6.16 x 10(4) M-1 s-1 was estimated. The association rate of the drug was not significantly changed after the calcium channels have been modified with 100 nM (+) 202,791. A method for correction of rate constants for possible drug trapping is discussed.