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, P(O(2)) < 5 mmHg) 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 ICa) 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.