Inhibition of L-Type Ca2+ Channels by Carbon Monoxide

Inhibition of K+ channels in glomus cells underlies excitation of the carotid body by hypoxia. It has recently been proposed that hypoxic inhibition involves either activation of AMP activated protein kinase (AMPK) or inhibition of carbon monoxide (CO) production by heme oxygenase 2 (HO-2). In the vasculature, L-type Ca2+ channels are also O-2 sensitive. Here, we have investigated the possible involvement of either AMPK or CO in the hypoxic inhibition of L-type Ca2+ channels. Using whole-cell patch clamp recordings from HEK293 cells stably expressing the human cardiac alpha(1C) Ca2+ channel subunit, we found that pre-treatment of cells with AICAR (to activate AMPK) was without effect on Ca2+ currents. CO, applied via the donor molecule CORM-2 caused reversible, voltage-independent Ca2+ channel inhibition of up to ca. 50%, whereas its inactive form (iCORM) was without significant effect. Effects of CO were prevented by the antioxidant MnTMPyP, but not by inhibition of NADPH oxidase (with either apocynin or diphenyleneiodonium), or xanthine oxidase (with allopurinol). Instead, inhibitors of complex III of the mitochondrial electron transport chain and a mitochondrial-targeted antioxidant (Mito Q), prevented the effects of CO. Our data suggest that hypoxic inhibition of L-type Ca2+ channels does not involve AMPK or CO. However, the known cardio-protective effects of HO-1 could arise from an inhibitory action of CO on L-type Ca2+ channels.