Large conductance Ca(2+)-activated K(+) (BK) channels are known to be regulated by both intracellular Ca(2+) and voltage. Although BK channel modulators have been identified, there is a paucity of information regarding the molecular entities of this channel that govern interaction with blockers and activators. Using both whole-cell and single-channel electrophysiological studies we have characterized the possible role that a threonine residue in the pore region of the channel has on function and interaction with BK channel modulators. A threonine-to-serine substitution at position 352 (T352S) resulted in a 59-mV leftward shift in the voltage-dependent activation curve. Single-channel conductance was 236 pS for the wild-type channel and 100 pS for the T352S mutant, measured over the range -80 mV to +80 mV. In addition, there was an almost 10-fold reduction in the potency of the BK channel inhibitor 1-[1-hexyl-6-(methyloxy)-1H-indazol-3-yl]-2-methyl-1-propanone (HMIMP), the IC(50) values being 4.3 +/- 0.3 and 38.2 +/- 3.3 nM for wild-type and mutant channel, respectively. There was no significant difference between wild type and the mutant channel in response to inhibition by iberiotoxin. The IC(50) was 8.1 +/- 0.3 nM for the wild type and 7.7 +/- 0.3 nM for the mutant channel. Here, we have identified a residue in the pore region of the BK channel that alters voltage sensitivity and reduces the potency of the blocker HMIMP.