Does AMP-activated protein kinase couple inhibition of mitochondrial oxidative phosphorylation by hypoxia to calcium signaling in O2-sensing cells?

Specialized O2-sensing cells exhibit a particularly low threshold to regulation by O2 supply and function to maintain arterial pO2 within physiological limits. For example, hypoxic pulmonary vasoconstriction optimizes ventilation-perfusion matching in the lung, whereas carotid body excitation elicits corrective cardio-respiratory reflexes. It is generally accepted that relatively mild hypoxia inhibits mitochondrial oxidative phosphorylation in O2-sensing cells, thereby mediating, in part, cell activation. However, the mechanism by which this process couples to Ca2+ signaling mechanisms remains elusive, and investigation of previous hypotheses has generated contrary data and failed to unite the field. We propose that a rise in the cellular AMP/ATP ratio activates AMP-activated protein kinase and thereby evokes Ca2+ signals in O2-sensing cells. Co-immunoprecipitation identified three possible AMP-activated protein kinase subunit isoform combinations in pulmonary arterial myocytes, with alpha1 beta2 gamma1 predominant. Furthermore, their tissue-specific distribution suggested that the AMP-activated protein kinase-alpha1 catalytic isoform may contribute, via amplification of the metabolic signal, to the pulmonary selectivity required for hypoxic pulmonary vasoconstriction. Immunocytochemistry showed AMP-activated protein kinase-alpha1 to be located throughout the cytoplasm of pulmonary arterial myocytes. In contrast, it was targeted to the plasma membrane in carotid body glomus cells. Consistent with these observations and the effects of hypoxia, stimulation of AMP-activated protein kinase by phenformin or 5-aminoimidazole-4-carboxamide-riboside elicited discrete Ca2+ signaling mechanisms in each cell type, namely cyclic ADP-ribose-dependent Ca2+ mobilization from the sarcoplasmic reticulum via ryanodine receptors in pulmonary arterial myocytes and transmembrane Ca2+ influx into carotid body glomus cells. Thus, metabolic sensing by AMP-activated protein kinase may mediate chemotransduction by hypoxia.

Comparative efficacies and durations of action of phenoxybenzamine, verapamil/nitroglycerin solution, and papaverine as topical antispasmodics for radial artery coronary bypass grafting.

OBJECTIVE: Radial arteries are increasingly used as conduits for coronary artery bypass grafts, but perioperative graft vasospasm continues to be a concern. Phenoxybenzamine, verapamil/nitroglycerin solution, and papaverine have been advocated as topical antispasmodic agents. We compared the relative efficacies and durations of action of these agents. METHODS: Isometric tension developed in response to clinically important vasoconstrictors was measured in 100 radial artery rings (from patients undergoing coronary artery bypass grafting, n = 25) after 15 minutes of ex vivo incubation with phenoxybenzamine, verapamil/nitroglycerin solution, papaverine, or vehicle (control). Duration of action was assessed by measuring responses to vasoconstrictors in antispasmodic pretreated and control rings at intervals through 5 hours. RESULTS: Verapamil/nitroglycerin solution reduced vasoconstriction in response to epinephrine, angiotensin II, prostaglandin F(2alpha), and phenylephrine but its effect had almost completely waned after 5 hours. Phenoxybenzamine prevented vasoconstriction in response to epinephrine, dopamine, and phenylephrine, with its effect lasting at least 5 hours. Papaverine had limited antispasmodic efficacy and prevented vasoconstriction in response to potassium (60 mmol/L) and phenylephrine for only 1 hour at the longest. CONCLUSIONS: Verapamil/nitroglycerin solution has a broad efficacy against a range of vasoconstrictors but a limited duration of action. Papaverine has the shortest duration of action. Phenoxybenzamine is an effective agent with a prolonged duration of action, specifically preventing catecholamine mediated vasospasm of radial artery conduits.

Vasodilation by the calcium-mobilizing messenger cyclic ADP-ribose.

In artery smooth muscle, adenylyl cyclase-coupled receptors such as beta-adrenoceptors evoke Ca(2+) signals, which open Ca(2+)-activated potassium (BK(Ca)) channels in the plasma membrane. Thus, blood pressure may be lowered, in part, through vasodilation due to membrane hyperpolarization. The Ca(2+) signal is evoked via ryanodine receptors (RyRs) in sarcoplasmic reticulum proximal to the plasma membrane. We show here that cyclic adenosine diphosphate-ribose (cADPR), by activating RyRs, mediates, in part, hyperpolarization and vasodilation by beta-adrenoceptors. Thus, intracellular dialysis of cADPR increased the cytoplasmic Ca(2+) concentration proximal to the plasma membrane in isolated arterial smooth muscle cells and induced a concomitant membrane hyperpolarization. Smooth muscle hyperpolarization mediated by cADPR, by beta-adrenoceptors, and by cAMP, respectively, was abolished by chelating intracellular Ca(2+) and by blocking RyRs, cADPR, and BK(Ca) channels with ryanodine, 8-amino-cADPR, and iberiotoxin, respectively. The cAMP-dependent protein kinase A antagonist N-(2-[p-bromocinnamylamino]ethyl)-5-isoquinolinesulfonamide hydrochloride (H89) blocked hyperpolarization by isoprenaline and cAMP, respectively, but not hyperpolarization by cADPR. Thus, cADPR acts as a downstream element in this signaling cascade. Importantly, antagonists of cADPR and BK(Ca) channels, respectively, inhibited beta-adrenoreceptor-induced artery dilation. We conclude, therefore, that relaxation of arterial smooth muscle by adenylyl cyclase-coupled receptors results, in part, from a cAMP-dependent and protein kinase A-dependent increase in cADPR synthesis, and subsequent activation of sarcoplasmic reticulum Ca(2+) release via RyRs, which leads to activation of BK(Ca) channels and membrane hyperpolarization.

Hypoxic pulmonary vasoconstriction: cyclic adenosine diphosphate-ribose, smooth muscle Ca(2+) stores and the endothelium.

Hypoxic pulmonary vasoconstriction (HPV) is unique to pulmonary arteries, and supports ventilation/perfusion matching. However, in diseases such as emphysema, HPV can promote hypoxic pulmonary hypertension (HPH), which ultimately leads to right heart failure. Since it was first described, the mechanisms underpinning HPV have remained obscure, and current therapies for HPH are poor. Previous investigations have suggested that HPV may be mediated by processes intrinsic to the pulmonary artery smooth muscle, and by the release of a vasoconstrictor(s) from the endothelium. It was thought that oxygen-sensitive ion channels in the smooth muscle cell membrane triggered HPV, and it has been argued that the endothelium-derived vasoconstrictor is endothelin-1. However, these proposals remain controversial. This review discusses the regulation by hypoxia of cyclic adenosine diphosphate-ribose production and Ca(2+) release from the sarcoplasmic reticulum in pulmonary artery smooth muscle. The role of these processes in triggering maintained HPV is then related to its subsequent progression due to vasoconstrictor(s) release from the endothelium.