Mitochondrial regulation of hypoxia-induced increase of adrenomedullin mRNA in HL-1 cells

Avertin®, but Not Volatile Anesthetics Addressing the Two-Pore Domain K+ Channel, TASK-1, Slows Down Cilia-Driven Particle Transport in the Mouse Trachea

RATIONALE

Volatile anesthetics inhibit mucociliary clearance in the airways. The two-pore domain K+ channel, TASK-1, represents one of their molecular targets in that they increase its open probability. Here, we determine whether particle transport speed (PTS) at the mucosal surface of the mouse trachea, an important factor of the cilia-driven mechanism in mucociliary clearance, is regulated by TASK-1.

METHODOLOGY/RESULTS

RT-PCR analysis revealed expression of TASK-1 mRNA in the manually dissected and laser-assisted microdissected tracheal epithelium of the mouse. Effects of anesthetics (isoflurane and Avertin®) and TASK-1 inhibitors (anandamide and A293) on ciliary activity were investigated by assessment of PTS at the mucosal surface of the explanted and opened murine trachea. Neither TASK-1 inhibitors nor isoflurane had any impact on basal and ATP-stimulated PTS. Avertin® reduced basal PTS, and ATP-stimulated PTS decreased in its presence in wild-type (WT) mice. Avertin®-induced decrease in basal PTS persisted in WT mice in the presence of TASK-1 inhibitors, and in two different strains of TASK-1 knockout mice.

CONCLUSIONS/SIGNIFICANCE

Our findings indicate that TASK-1 is expressed by the tracheal epithelium but is not critically involved in the regulation of tracheal PTS in mice. Avertin® reduces PTS independent of TASK-1.

Intrinsic vascular dopamine - a key modulator of hypoxia-induced vasodilatation in splanchnic vessels

Dopamine not only is a precursor of the catecholamines noradrenaline and adrenaline but also serves as an independent neurotransmitter and paracrine hormone. It plays an important role in the pathogenesis of hypertension and is a potent vasodilator in many mammalian systemic arteries, strongly suggesting an endogenous source of dopamine in the vascular wall. Here we demonstrated dopamine, noradrenaline and adrenaline in rat aorta and superior mesenteric arteries (SMA) by radioimmunoassay. Chemical sympathectomy with 6-hydroxydopamine showed a significant reduction of noradrenaline and adrenaline, while dopamine levels remained unaffected. Isolated endothelial cells were able to synthesize and release dopamine upon cAMP stimulation. Consistent with these data, mRNAs coding for catecholamine synthesizing enzymes, i.e. tyrosine hydroxylase (TH), aromatic l-amino acid decarboxylase, and dopamine-β-hydroxylase were detected by RT-PCR in cultured endothelial cells from SMA. TH protein was detected by immunohistochemisty and Western blot. Exposure of endothelial cells to hypoxia (1% O2) increased TH mRNA. Vascular smooth muscle cells partially expressed catecholaminergic traits. A physiological role of endogenous vascular dopamine was shown in SMA, where D1 dopamine receptor blockade abrogated hypoxic vasodilatation. Experiments on SMA with endothelial denudation revealed a significant contribution of the endothelium, although subendothelial dopamine release dominated. From these results we conclude that endothelial cells and cells of the underlying vascular wall synthesize and release dopamine in an oxygen-regulated manner. In the splanchnic vasculature, this intrinsic non-neuronal dopamine is the dominating vasodilator released upon lowering of oxygen tension.

Oxygen Sensing, Cardiac Ischemia, HIF-1α and Some Emerging Concepts

Oxygen plays a critical role in the perpetuation and propagation of almost all forms of life. The primary site of cellular oxygen consumption is the mitochondrial electron transport chain and in addition, oxygen is also used as a substrate for various enzymes involved in cellular homeostasis. Although our knowledge of the biochemistry and physiology of oxygen transport is century old, recent development of sophisticated tools of biophysical chemistry revealed that tissue oxygenation and oxygen sensing is a highly evolved process, especially in mammals. Perturbation of normal oxygen supply is associated with diseases like tumorigenesis, myocardial infarction and stroke. Available information suggests that when tissue oxygen supply is limited, mitochondria emanate signals involving reactive oxygen species generation which in turn stabilizes oxygen sensing transcription factor HIF-1. Upon stabilization, HIF-1 elicits necessary genetic response to cope with the diminished oxygen level. In view of such critical role of HIF-1 in cellular oxygen sensing, recently there has been a heightened interest in understanding the biology of HIF-1 in the context of cardiovascular system. The following review describes some of the recent advances in this regard.

Intermedin/adrenomedullin-2 is a hypoxia-induced endothelial peptide that stabilizes pulmonary microvascular permeability

Accumulating evidence suggests a pivotal role of the calcitonin receptor-like receptor (CRLR) signaling pathway in preventing damage of the lung by stabilizing pulmonary barrier function. Intermedin (IMD), also termed adrenomedullin-2, is the most recently identified peptide targeting this receptor. Here we investigated the effect of hypoxia on the expression of IMD in the murine lung and cultured murine pulmonary microvascular endothelial cells (PMEC) as well as the role of IMD in regulating vascular permeability. Monoclonal IMD antibodies were generated, and transcript levels were assayed by quantitative RT-PCR. The promoter region of IMD gene was analyzed, and the effect of hypoxia-inducible factor (HIF)-1alpha on IMD expression was investigated in HEK293T cells. Isolated murine lungs and a human lung microvascular endothelial cell monolayer model were used to study the effect of IMD on vascular permeability. IMD was identified as a pulmonary endothelial peptide by immunohistochemistry and RT-PCR. Hypoxia caused an upregulation of IMD mRNA in the murine lung and PMEC. As shown by these results, HIF-1alpha enhances IMD promoter activity. Our functional studies showed that IMD abolished the increase in pressure-induced endothelial permeability. Moreover, IMD decreased basal and thrombin-induced hyperpermeability of an endothelial cell monolayer in a receptor-dependent manner and activated PKA in these cells. In conclusion, IMD is a novel hypoxia-induced gene and a potential interventional agent for the improvement of endothelial barrier function in systemic inflammatory responses and hypoxia-induced vascular leakage.

Pivotal Advance: Up-regulation of acetylcholine synthesis and paracrine cholinergic signaling in intravascular transplant leukocytes during rejection of rat renal allografts

During acute rejection, large numbers of leukocytes accumulate in the blood vessels of experimental renal allografts. About 70% of them are activated, cytotoxic monocytes that appear to be involved in allograft destruction. ACh exerts anti-inflammatory effects upon monocytes/macrophages and has been proposed to be a key player in neuroimmunological interactions. Its short half-life, however, makes it unlikely that neuronal ACh affects blood leukocytes. Renal transplantation was performed in the allogeneic DA to LEW and in the isogeneic LEW to LEW rat strain combination. Intravascular leukocytes were harvested after 4 days, and the expression of CHT1, cChAT, pChAT, and nAChR subunits was investigated by RT-PCR, immunoblotting, and immunohistochemistry. Monocytes were identified by double-labeling with ED1-antibody, directed to a CD68-like antigen. ACh content was measured by HPLC. [Ca(2+)](i) was monitored by Fura-2. Intravascular graft leukocytes express CHT1 and cChAT mRNA and protein and pChAT protein. Their expression is strongly up-regulated in vivo during acute allograft rejection. Immunohistochemistry revealed CHT1, cChAT, and pChAT protein in ED1-positive monocytes. The ACh content of allograft intravascular leukocytes was sixfold higher than that of isografts. Intravascular leukocytes express nAChR subunits, and an ATP-induced increase in [Ca(2+)](i) was augmented in vitro by a nAChR inhibitor in allograft but not isograft leukocytes. Intravascular graft leukocytes, among them monocytes, up-regulate non-neuronal ACh synthesis and develop auto-/paracrine cholinergic attenuation of ATP signaling during acute allograft rejection.

CGRP-alpha responsiveness of adult rat ventricular cardiomyocytes from normotensive and spontaneously hypertensive rats

Calcitonin gene-related peptide (CGRP)-alpha is expressed in heart ventricles in sensory nerves and cardiomyocytes. It modifies inotropism and induces ischaemic preconditioning. This study investigates the effect of CGRP-alpha on the contractile responsiveness of isolated adult ventricular rat cardiomyocytes and the effect of chronic hypertension on this interaction. Cardiomyocytes were isolated and paced at 0.5-2.0 Hz. Cell shortening was recorded via a line camera with a reading frame of 500 Hz. CGRP-alpha exerted a dual effect on cardiomyocytes with a positive contractile effect at 10nM and a negative contractile effect at 10 pM. CGRP-alpha(8-37), a calcitonin receptor-like receptor (CRLR) antagonist, attenuated the positive contractile effect. H89, a protein kinase A antagonist, converted the positive contractile effect into a negative contractile effect. The negative contractile effect was converted again back to a positive contractile effect in the presence of l-nitro arginine. In cardiomyocytes isolated from spontaneously hypertensive rats (SHR) the mRNA expression of CRLR and the receptor-associated modifier protein (RAMP)-2 were lower. However, on the protein level CLRL was up-regulated, RAMP2 expression remained unchanged, and eNOS expression was down-regulated in these cells. These cells responded with a pure positive contractile response. In Langendorff preparations, CGRP-alpha slightly reduced the rate pressure product in hearts from normotensive rats but it caused an increase in hearts from SHR. In conclusion, it is shown that CGRP-alpha exerts dual effects on cardiomyocytes favouring the negative contractile effect at very low concentrations. This effect is compensated in chronic pressure-overloaded hearts and converted into a positive inotropism.

A1 adenosine receptors play an essential role in protecting the embryo against hypoxia

Embryos can be exposed to environmental factors that induce hypoxia. Currently, our understanding of the effects of hypoxia on early mammalian development is modest. Potential mediators of hypoxia action include the nucleoside adenosine, which acts through A(1) adenosine receptors (A(1)ARs) and mediates adverse effects of hypoxia on the neonatal brain. We hypothesized that A(1)ARs may also play a role in mediating effects of hypoxia on the embryo. When pregnant dams were exposed to hypoxia (10% O(2)) beginning at embryonic day (E) 7.5 or 8.5 and continued for 24-96 h, A(1)AR+/+ embryos manifested growth inhibition and a disproportionate reduction in heart size, including thinner ventricular walls. Yet, when dams were exposed to hypoxia, embryos lacking A(1)ARs (A(1)AR-/-) had much more severe growth retardation than A(1)AR+/+ or +/- embryos. When levels of hypoxia-inducible factor 1alpha (HIF1alpha) were examined, A(1)AR-/- embryos had less stabilized HIF1alpha protein than A(1)AR+/- littermates. Normal patterns of cardiac gene expression were also disturbed in A(1)AR-/- embryos exposed to hypoxia. These results show that short periods of hypoxia during early embryogenesis can result in intrauterine growth retardation. We identify adenosine and A(1)ARs as playing an essential role in protecting the embryo from hypoxia.