The chemical modification of K-Ca channels by carbon monoxide in vascular smooth muscle cells

The chemical modification of big conductance calcium-activated potassium (K-Ca) channels in rat tail artery smooth muscle cells by carbon monoxide (CO) was investigated using the cell-free single channel recording technique. Exposure of the internal surface of cell membranes to diethyl pyrocarbonate (DEPC) neither affected the characteristics of K-Ca channels nor modified the stimulatory effect of CO on K-Ca channels. However, when DEPC was applied to the external surface of cell membranes, the open probability of K-Ca channels was reduced. The pH and concentration dependence of the effect of DEPC indicated the specific modification of histidine residues. Kinetic analysis suggested that one externally located histidine residue was modified by DEPC. Treatment of the external surface of cell membranes with DEPC abolished the CO-induced increase in the open probability of K-Ca channels. Likewise, the presence of CO partially protected K-Ca channels from inhibition by DEPC. Moreover, photooxidation of the histidine residue located on the external membrane surface abolished the CO-induced activation of K-Ca channels. Our study demonstrates that the CO-induced increase in the open probability of K-Ca channels may rely specifically on the structure and topological locations of histidine residues.