Heme is required for carbon monoxide activation of mitochondrial BKCa channel

Carbon monoxide (CO) is an endogenously synthesized gaseous mediator and is involved in the regulation of numerous physiological processes. Mitochondria, in which hemoproteins are abundant, are among the targets for CO action. Large-conductance calcium-activated (mitoBK(C)(a)) channels in the inner mitochondrial membrane share multiple biophysical similarities with the BKCa channels of the plasma membrane and could be a potential target for CO. To test this hypothesis, the activity of the mitoBK(C)(a) channels in human astrocytoma U-87 MG cell mitochondria was assessed with the patch-clamp technique. The effects of CO-releasing molecules (CORMs), such as CORM-2, CORM-401, and CORM-Al, were compared to the application of a CO-saturated solution to the mitoBK(C)(a) channels in membrane patches. The applied CORMs showed pleiotropic effects including channel inhibition, while the CO-containing solution did not significantly modulate channel activity. Interestingly, CO applied to the mitoBK(C)(a) channels, which were inhibited by exogenously added heme, stimulated the channel. To summarize, our findings indicate a requirement of heme binding to the mitoBK(C)(a) channel for channel modulation by CO and suggest that CORMs might have complex unspecific effects on mitoBK(C)(a) channels.