Evidence that agonist and antagonist enantiomers of the dihydropyridine PN 202–791 act at different sites on the voltage‐dependent calcium channel of vascular muscle

S(+)-PN 202-791, a calcium channel agonist, and its optical isomer R(-)-PN 202-791, a calcium channel antagonist, respectively increased and decreased inward current carried by barium through voltage-dependent calcium channels in isolated ear artery cells of the rabbit in a concentration-dependent manner. The EC50 or IC50 derived from the concentration-response relationship was unaltered in the presence of the other enantiomer indicating that no competitive antagonism exists between these enantiomers and suggesting that the actions of these two enantiomers involve two separate binding sites.