In Vitro: (R)-(+)-Bay-K-8644, a conventional racemic mixture of Bay K 8644, is widely used as an L-type Ca2+ channel agonist. Each optical isomer possesses opposite effects on IBa, (R)-(+)-Bay-K-8644 as an antagonist and S(-)-Bay K 8644 as an agonist. (R)-(+)-Bay-K-8644 inhibits Ba2+ currents (IBa) (IC50=975 nM). When (R)-(+)-Bay-K-8644 (0.5 μM) is applied, IBa is suppressed to 71±10% of control. In the presence of (R)-(+)-Bay-K-8644IBa. (R)-(+)-Bay-K-8644 is a calcium channel inhibitor.