Thiol reagents and nitric oxide modulate the gating of BKCa channels from the guinea-pig taenia caeci

1. The site of the direct modulation of the gating of BKCa channels by the nitric oxide donor s-nitroso-l-cysteine (NOCys) was examined in excised membrane patches of the guinea-pig taenia caeci by the use of various thiol (sulphydryl)-specific reagents, including N -ethylmaleimide (NEM) and three charged methanethiosulphonate (MTS) reagents, namely positively charged 2-aminoethyl MTS hydrobromide (MTSEA) and [2-(trimethylammonium)ethyl] MTS bromide (MTSET) and negatively charged sodium (2-sulphonatoethyl) MTS (MTSES), which all specifically convert sulphydryls to a disulphide. 2. At 10 mumol/L, NOCys transiently increased the probability of opening (N.P-o) of the BKCa channels (at 0 mV) after a delay of 1-2 min. 3. Disulphide-reducing agents, such as dithiothreitol (10 mumol/L), increased N.P-o in a manner that was reversed by the sulphide-oxidizing agent thimerosal (10 mumol/L). Both positively charged MTSET (2.5 mmol/L) and negatively charged MTSES (2.5 mmol/L) rapidly increased N.P-o. However, only the MTSES-evoked increase in N.P-o remained after a prolonged washout period. 4. The specific alkylating agent of cysteine thiols NEM (1 mmol/L) and the positively charged, but membrane permeable, MTSEA (2.5 mmol/L) decreased N.P-o (at 0 mV). 5. Pre-exposure of excised membrane patches to NEM or MTSES prevented the excitatory actions of NOCys (10 mumol/L). 6. We conclude that MTSES and NOCys must modify thiols on cysteine residues within basic regions of the channel protein that would electrostatically exclude MTSEA and MTSET. A consensus sequence of various mammalian alpha-subunits of the BKCa channel reveals two pairs of cysteine residues surrounded by basic amino acids that could be the site of action for NOCys and MTSES.