Hypoxic regulation of cardiac Ca²⁺ channel: possible role of haem oxygenase

Key points Lack of oxygen to the heart, hypoxia, triggers a cascade of events that within minutes leads to irreversible tissue damage. One of the initial steps at the onset of hypoxia is suppression of the cellular calcium channels that control the strength of the heartbeat. The role of oxygen in energy production is well known, but it is not known how lack of oxygen within seconds may suppress the calcium channels. We tested the involvement of different regulatory pathways that are known to modulate the calcium channel in response to various stimuli, and succeeded to link it to a haem oxygenase protein that may act as the primary oxygen sensor. Our findings provide a better understanding of the initial events that occur during episodes of acute hypoxia induced by coronary thrombosis and bring into focus a new set of therapeutic considerations. Abstract Acute and chronic hypoxias are common cardiac diseases that lead often to arrhythmia and impaired contractility. At the cellular level it is unclear whether the suppression of cardiac Ca 2+ channels (Ca V 1.2) results directly from oxygen deprivation on the channel protein or is mediated by intermediary proteins affecting the channel. To address this question we measured the early effects of hypoxia (5–60 s, ) on Ca 2+ current ( I Ca ) and tested the involvement of protein kinase A (PKA) phosphorylation, Ca 2+ /calmodulin‐mediated signalling and the haem oxygenase (HO) pathway in the hypoxic regulation of Ca V 1.2 in rat and cat ventricular myocytes and HEK‐293 cells. Hypoxic suppression of I Ca and Ca 2+ transients was significant within 5 s and intensified in the following 50 s, and was reversible. Phosphorylation by cAMP or the phosphatase inhibitor okadaic acid desensitized I Ca to hypoxia, while PKA inhibition by H‐89 restored the sensitivity of I Ca to hypoxia. This phosphorylation effect was specific to Ca 2+ , but not Ba 2+ or Na + , permeating through the channel. CaMKII inhibitory peptide and Bay K8644 reversed the phosphorylation‐induced desensitization to hypoxia. Mutation of CAM/CaMKII‐binding motifs of the α 1c subunit of Ca V 1.2 fully desensitized the Ca 2+ channel to hypoxia. Rapid application of HO inhibitors (zinc protoporphyrin (ZnPP) and tin protoporphyrin (SnPP)) suppressed the channel in a manner similar to acute hypoxia such that: (1) I Ca and I Ba were suppressed within 5 s of ZnPP application; (2) PKA activation and CaMKII inhibitors desensitized I Ca , but not I Ba , to ZnPP; and (3) hypoxia failed to further suppress I Ca and I Ba in ZnPP‐treated myocytes. We propose that the binding of HO to the CaM/CaMKII‐specific motifs on Ca 2+ channel may mediate the rapid response of the channel to hypoxia.