Interaction of Selective Amino Acid Residues of KCa Channels with Carbon Monoxide

The activation of big-conductance KCa channels in vascular smooth muscle cells by carbon monoxide (CO) has been demonstrated previously. One specific target of CO on KCa channel proteins is the histidine residue. The roles of other amino acid residues on the functionality of KCa channels, as well as their reactions to CO, have been unclear. In the present study, the cell-free single channel recording technique was used to investigate the chemical modification of KCa channels by CO and other chemical agents. The modification of negatively charged carboxyl groups and the ε-amino group of lysine did not affect the open probability, but decreased single-channel conductance of KCa channels. When sulfhydryl groups of cysteine were modified with N-ethylmaleimide, the open probability of KCa channels was decreased, but single-channel conductance was not affected. None of the above chemical modifications affected the CO-induced increase in the open probability of KCa channels. However, N-ethylmaleimide treatment reduced the stimulatory effect of nitric oxide (NO) on KCa channels. Finally, pretreatment of smooth muscle cells with NO abolished the effects of subsequently applied CO on KCa channel proteins. Our study demonstrates that CO and NO acted on different amino acid residues of KCa channel proteins. The interaction of CO and NO determines the functional status of KCa channels in vascular smooth muscle cells