Positively charged but membrane permeable MTS reagent, MTSEA decreased BKCa channel activity but did not prevent the excitatory actions of NOCys. Application of MTSEA (2.5 mM for 5 min), to the cytosolic surface decreased BKCa channel activity (N.Po) recorded at either 0 mV (a, b) or during ramped depolarizations between −80 and +80 mV (ci, ii). This reduction in N.Po remained upon washout of the MTSEA (b). The subsequent application of NOCys (10 μM) still increased N.Po at 0 mV (a, b) as well as the number of channel openings evoked by a ramp depolarization (ciii). The significant effects of all agents were established using Student's paired t-tests. *denotes P< 0.05.