Dissociation of the effects of forskolin and dibutyryl cAMP on force of contraction and phospholamban phosphorylation in human heart failure

Endogenous sulfur dioxide is a novel inhibitor of hypoxia-induced mast cell degranulation

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Endogenous SO₂/AAT pathway exists in mast cells (MCs).

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Endogenous SO₂ is a novel MC membrane stabilizer under hypoxic circumstance.

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MC-derived SO₂ upregulates cAMP level, thereby suppressing MC degranulation.

Introduction

Mast cell (MC) degranulation is an important step in the pathogenesis of inflammatory reactions and allergies; however, the mechanism of stabilizing MC membranes to reduce their degranulation is unclear.

Methods

SO₂ content in MC culture supernatant was measured by HPLC-FD. The protein and mRNA expressions of the key enzymes aspartate aminotransferase 1 (AAT1) and AAT2 and intracellular AAT activity were detected. The cAMP level in MCs was detected by immunofluorescence and ELISA. The release rate of MC degranulation marker β-hexosaminidase was measured. The expression of AAT1 and cAMP, the MC accumulation and degranulation in lung tissues were detected.

Objectives

To exam whether an endogenous sulfur dioxide (SO₂) pathway exists in MCs and if it serves as a novel endogenous MC stabilizer.

Results

We firstly show the existence of the endogenous SO₂/AAT pathway in MCs. Moreover, when AAT1 was knocked down in MCs, MC degranulation was significantly increased, and could be rescued by a SO₂ donor. Mechanistically, AAT1 knockdown decreased the cyclic adenosine monophosphate (cAMP) content in MCs, while SO₂ prevented this reduction in a dose-independent manner. Pretreatment with the cAMP-synthesizing agonist forskolin or the cAMP degradation inhibitor IBMX significantly blocked the increase in AAT1 knockdown-induced MC degranulation. Furthermore, in hypoxia-stimulated MCs, AAT1 protein expression and SO₂ production were markedly down regulated, and MC degranulation was activated, which were blunted by AAT1 overexpression. The cAMP synthesis inhibitor SQ22536 disrupted the suppressive effect of AAT1 overexpression on hypoxia-induced MC degranulation. In a hypoxic environment, mRNA and protein expression of AAT1 was significantly reduced in lung tissues of rats. Supplementation of SO₂ elevated the cAMP level and reduced perivascular MC accumulation and degranulation in lung tissues of rats exposed to a hypoxic environment in vivo.

Conclusion

SO₂ serves as an endogenous MC stabilizer via upregulating the cAMP pathway under hypoxic circumstance.

Inotropic effects of L-lysine in the mammalian heart

We studied the effects of L-lysine in cardiac preparations of mice and men. Of note, L-lysine increased force of contraction in a concentration- and time-dependent manner in isolated electrically paced left atrium of mouse and in human right atrium. It further increased heart rate and left ventricular pressure in the isolated perfused mouse heart. In isolated adult mouse cardiomyocytes, the contractility as assessed by edge detection was increased as well as the Ca(2+) transients after electrically pacing by field stimulation. However, using the patch clamp technique, no effect of L-lysine on action potential duration from a constant holding potential or on current through L-type calcium channels could be observed. However, L-lysine led to a depolarization of unclamped cells. Furthermore, effects of L-lysine were stereospecific, as they were not elicited by D-lysine. The inotropic effects of L-lysine were not abrogated by additionally applied L-ornithine or L-arginine (known inhibitors of lysine transport). However, L-lysine (5 mM) shifted the concentration-response curve for a positive inotropic effect of 5-hydroxytryptamine (5-HT; serotonin) in atrium of transgenic mice (with cardiac specific overexpression of 5-HT(4) receptors) to higher concentrations. In summary, we describe a novel positive inotropic effect of an essential amino acid, L-lysine, in the mammalian heart. One might speculate that L-lysine treatment under certain conditions could sustain cardiac performance. Moreover, L-lysine is able to block, at least in part, cardiac 5-HT(4) receptors.