The slowly activating delayed rectifier K+ current, IKs, is an important modulator of cardiac action potential repolarization. Here, we describe a novel benzodiazepine, [L-364,373 [(3-R)-1, 3-dihydro-5-(2-fluorophenyl)-3-(1H-indol-3-ylmethyl)-1-methyl-2H- 1,4-benzodiazepin-2-one] (R-L3), that activates IKs and shortens action potentials in guinea pig cardiac myocytes. These effects were additive to isoproterenol, indicating that channel activation by R-L3 was independent of beta-adrenergic receptor stimulation. The increase of IKs by R-L3 was stereospecific; the S-enantiomer, S-L3, blocked IKs at all concentrations examined. The increase in IKs by R-L3 was greatest at voltages near the threshold for normal channel activation, caused by a shift in the voltage dependence of IKs activation. R-L3 slowed the rate of IKs deactivation and shifted the half-point of the isochronal (7.5 sec) activation curve for IKs by -16 mV at 0.1 microM and -24 mV at 1 microM. R-L3 had similar effects on cloned KvLQT1 channels expressed in Xenopus laevis oocytes but did not affect channels formed by coassembly of KvLQT1 and hminK subunits. These findings indicate that the association of minK with KvLQT1 interferes with the binding of R-L3 or prevents its action once bound to KvLQT1 subunits.