Effects of relaxin on mast cells. In vitro and in vivo studies in rats and guinea pigs

Modulation of the immunological response of guinea pig mast cells by carbon monoxide

Challenge of guinea pig mast cells with antigen under aerobic conditions induced the expected release of histamine and led to a significant increase in intracellular calcium ([Ca2+]i) and cyclic adenosine monophosphate (cAMP) levels. Prior exposure to CO decreased the immunological histamine release. This effect was accompanied by a decrease in the levels of [Ca2+]i and by an increase in the cyclic guanosine monophosphate (cGMP) levels. The exposure of mast cells to nitrogen (N2) did not modify the release of histamine. The CO-mediated inhibition of the immunological release of histamine was reversed by the soluble guanylate cyclase inhibitor (1 H-[1.2,4]oxadiazolo[4,3-a]quinoxalin-1-one, ODQ) and by oxyhaemoglobin (HbO2). Incubation of mast cells for 4 h with hemin, a heme oxygenase (HO) inducer, resulted in an increase in HO activity, measured as bilirubin production. Hemin abated the immunological release of histamine, in similar fashion to exogenous CO, and increased the cGMP levels. These effects were reversed by ODQ and HbO2. It is proposed that CO from an exogenous or endogenous source stimulates guanylyl cyclase and causes cGMP formation which then induces calcium to be sequestrated so that the [Ca2+]i concentration falls and histamine release is inhibited.

Characterization of gene expression in resting and activated mast cells

To characterize gene expression in activated mast cells more comprehensively than heretofore, we surveyed the changes in genetic transcripts by the method of serial analysis of gene expression in the RBL-2H3 line of rat mast cells before and after they were stimulated through their receptors with high affinity for immunoglobulin E (FcepsilonRI). A total of 40,759 transcripts derived from 11,300 genes were analyzed. Among the diverse genes that had not been previously associated with mast cells and that were constitutively expressed were those for the cytokine macrophage migration inhibitory factor neurohormone receptors such as growth hormone- releasing factor and melatonin and components of the exocytotic machinery. In addition, several dozen transcripts were differentially expressed in response to antigen-induced clustering of the FcepsilonRI. Included among these were the genes for preprorelaxin, mitogen-activated protein kinase kinase 3, and the dual specificity protein phosphatase, rVH6. Significantly, the majority of genes differentially expressed in this well-studied model of mast cell activation have not been identified before this analysis.

Relaxin activates the L-arginine-nitric oxide pathway in vascular smooth muscle cells in culture

The peptide hormone relaxin (RLX) has been shown to elicit a powerful vasodilatory response in several target organs. This response is mediated by the stimulation of intrinsic nitric oxide (NO) generation. The present study was designed to clarify whether RLX directly promotes the relaxation of vascular smooth muscle cells through stimulation of NO generation. Vascular smooth muscle cells from bovine aortas were incubated with RLX at concentrations ranging from 1 nmol/L to 1 micromol/L. The expression and activity of NO synthase, production of NO, and the intracellular levels of cGMP and Ca2+ were determined. The cell morphology and signal transduction mechanisms of these bovine aortic smooth muscle cells in response to RLX were also studied. RLX stimulated the expression of immunoreactive inducible NO synthase and increased significantly and in a concentration-related fashion inducible NO synthase activity, NO generation, and intracellular cGMP levels. Concurrently, RLX significantly decreased cytosolic Ca2+ concentrations and caused changes in cell shape and the actin cytoskeleton that were consistent with cell relaxation. The signal transduction mechanisms leading to the enhanced expression of inducible NO synthase protein and activity caused by RLX involve the activation of tyrosine kinase, phosphatidylcholine-phospholipase C, and the transcription factor nuclear factor-kappaB, similar to bacterial endotoxins and proinflammatory cytokines. This study suggests that RLX is an endogenous agent capable of regulating vascular tone by activation of the L-arginine-NO pathway in vascular smooth muscle cells.

Relaxin protects against myocardial injury caused by ischemia and reperfusion in rat heart

Myocardial injury caused by ischemia and reperfusion comes from multiple pathogenic events, including endothelial damage, neutrophil extravasation into tissue, platelet and mast cell activation, and peroxidation of cell membrane lipids, which are followed by myocardial cell alterations resulting eventually in cell necrosis. The current study was designed to test the possible cardioprotective effect of the hormone relaxin, which has been found to cause coronary vessel dilation and to inhibit platelet and mast cell activation. Ischemia (for 30 minutes) was induced in rat hearts in vivo by ligature of the left anterior descending coronary artery; reperfusion (for 60 minutes or less if the rats died before this predetermined time) was induced by removal of the ligature. Relaxin (100 ng) was given intravenously 30 minutes before ischemia. The results obtained showed that relaxin strongly reduces 1) the extension of the myocardial areas affected by ischemia-reperfusion-induced damage, 2) ventricular arrhythmias, 3) mortality, 4) myocardial neutrophil number, 5) myeloperoxidase activity, a marker of neutrophil accumulation, 6) production of malonyldialdehyde, an end product of lipid peroxidation, 7) mast cell granule release, 8) calcium overload, and 9) morphological signs of myocardial cell injury. This study shows that relaxin can be regarded as an agent with a marked cardioprotective action against ischemia-reperfusion-induced myocardial injury.

Evidence that relaxin's effects on growth and softening of the cervix are not mediated through prostaglandins in the rat

Relaxin plays a major role in promoting the growth and softening of the cervix that occurs during the second half of pregnancy in the rat. There is limited evidence that prostaglandins play a role in cervical softening in mammalian species. Accordingly, this study was conducted to determine if prostaglandins mediate relaxin's effects on the rat cervix. To attain that objective, indomethacin was used to inhibit cyclooxygenase, the key enzyme in the conversion of arachidonic acid to prostaglandins. Twenty-six nonpregnant female rats were ovariectomized when they were 78 days old (day 1 of treatment). At ovariectomy (O), each rat was fitted with silicon tubing implants containing progesterone (P) and estrogen (E) in doses that provided blood levels similar to those during late pregnancy in rats. Rats were randomly assigned to three treatment groups. Group OPE controls (n = 8 rats) received 2 ml indomethacin vehicle (0.5% methyl cellulose, 0.025 Tween 80 in water) via gavage at 0900 h on days 8 and 9 and 0.5 ml relaxin vehicle (0.9% NaCl) s.c. at 6-h intervals from 1200 h on day 8 through 0600 h on day 10. Group OPER (n = 9 rats) was treated as group OPE except that 20 microg highly purified porcine relaxin was administered. Group OPERI (n = 9 rats) was treated as group OPER except that indomethacin was administered at a dose (20 mg/kg BW) that reduced cervical PGE2 levels by more than 90%. Between 0800 h and 1000 h on day 10, the cervices were removed, trimmed of fat, weighed, and placed in ice-cold Krebs-Ringer bicarbonate buffer, pH 7.5. Cervical extensibility (degree of softening) was determined within 4 h of tissue collection. Both the mean cervical wet weight and the mean cervical extensibility in the relaxin-treated group OPER rats were markedly greater (P < 0.01) than in the group OPE controls. Treatment with indomethacin did not diminish relaxin's effects on either cervical wet weight or cervical extensibility. In conclusion, this study provides evidence that relaxin's effects on cervical growth and softening in the rat are not mediated through prostaglandins.

Relaxin counteracts myocardial damage induced by ischemia-reperfusion in isolated guinea pig hearts: evidence for an involvement of nitric oxide

Relaxin was previously shown to cause coronary vasodilation and to inhibit mast cell activation through a stimulation of endogenous nitric oxide production. This suggests that relaxin may have beneficial effects on ischemia-reperfusion-induced myocardial injury, which is triggered by endothelial damage and impaired nitric oxide generation. In this study, we tested the effect of relaxin on isolated and perfused guinea pig hearts subjected to ischemia and reperfusion. Ischemia was induced by ligature of the left anterior descending coronary artery; removal of the ligature induced reperfusion. Relaxin, at the concentration of 30 ng/ml of perfusion fluid, causes: a significant increase in coronary flow and in nitric oxide generation; a significant decrease in malonyldialdehyde production and in calcium overload, both markers of myocardial injury; an inhibition of mast cell granule exocytosis and histamine release, which are known to contribute to myocardial damage; a reduction of ultrastructural abnormalities of myocardial cells; an improvement of heart contractility. The beneficial effects of relaxin were blunted by the NO synthase inhibitor L-NMMA. The current study provides first experimental evidence that relaxin has a powerful protective effect on the heart undergoing ischemia and reperfusion acting through a nitric oxide-driven mechanism.

Relaxin counteracts asthma-like reaction induced by inhaled antigen in sensitized guinea pigs

In previous studies, the peptide hormone relaxin (RLX) was found to inhibit mast cell secretion and platelet activation. It has been established that the release of mediators from these cells plays a central pathogenic role in allergic asthma. This prompted us to ascertain whether RLX may counteract the respiratory and histopathological abnormalities of the asthma-like reaction to inhaled antigen in sensitized guinea pigs. Guinea pigs were sensitized with ovalbumin and challenged with the same antigen given by aerosol. Some animals received RLX (30 microg/kg BW, twice daily for 4 days) before antigen challenge. Other animals received inactivated RLX in place of authentic RLX. Respiratory abnormalities, such as cough and dyspnea, were analyzed as were light and electron microscopic features of lung specimens. RLX was shown to reduce the severity of respiratory abnormalities, as well as histological alterations, mast cell degranulation, and leukocyte infiltration in sensitized guinea pigs exposed to ovalbumin aerosol. RLX was also found to promote dilation of alveolar blood capillaries and to reduce the thickness of the air-blood barrier. This study provides evidence for an antiasthmatic property of RLX and raises the possibility of new therapeutic strategies for allergic asthma in humans.

The effect of ketotifen on nitric oxide synthase activity

1. We studied the effect of ketotifen, a second generation H1-receptor antagonist on nitric oxide synthase (NOS) activity in colonic mucosa and in renal tissues, and on rat renal haemodynamics in vivo. 2. Ketotifen (100 micrograms ml-1) increased human colonic NOS activity from 3.7 +/- 0.6 to 14.5 +/- 1.3 nmol g-1 min-1 (P < 0.005, ANOVA). In rat renal cortical and medullary tissues ketotifen increased NOS activity by 55% and 86%, respectively (P < 0.001). The stimulation of NOS activity was attenuated by NADPH deletion and by the addition of N omega nitro-L-arginine methyl ester (L-NAME) or aminoguanidine, but not by [Ca2+] deprivation. NOS activity was unaffected by two other H1-antagonists, diphenhydramine and astemizole, or by the structurally related cyproheptadine. Renal cortical NOS activity was also significantly stimulated 90 min after intravenous administration of ketotifen to anaesthetized rats. 3. Ketotifen administration to anaesthetized rats induced modest declines in blood pressure and reduced total renal, cortical and outer medullary vascular resistance. This is in contrast to diphenhydramine, which did not induce renal vasodilatation. 4. We conclude that ketotifen stimulates NOS activity by mechanisms other than H1-receptor antagonism. The association of this effect with therapeutic characteristics of ketotifen and the clinical implications of these findings are yet to be defined.

Relaxin induces an extracellular matrix-degrading phenotype in human lung fibroblasts in vitro and inhibits lung fibrosis in a murine model in vivo

Pulmonary fibrosis is the common end stage of a number of pneumopathies. In this study, we examined the ability of the human cytokine, relaxin, to block extracellular matrix deposition by human lung fibroblasts in vitro, and to inhibit lung fibrosis in a bleomycin-induced murine model. In vitro, relaxin (1-100 ng/ml) inhibited the transforming growth factor-beta-mediated over-expression of interstitial collagen types I and III by human lung fibroblasts by up to 45% in a dose-dependent manner. Relaxin did not affect basal levels of collagen expression in the absence of TGF-beta-induced stimulation. Relaxin also blocked transforming growth factor-beta-induced upregulation of fibronectin by 80% at the highest relaxin dose tested (100 ng/ml). The expression of matrix metalloproteinase-1, or procollagenase, was stimulated in a biphasic, dose-dependent manner by relaxin. In vivo, relaxin, at a steady state circulating concentration of approximately 50 ng/ml, inhibited bleomycin-mediated alveolar thickening compared with the vehicle only control group (P < 0.05). Relaxin also restored bleomycin-induced collagen accumulation, as measured by lung hydroxyproline content, to normal levels (P < 0.05). In summary, relaxin induced a matrix degradative phenotype in human lung fibroblasts in vitro and inhibited bleomycin-induced fibrosis in a murine model in vivo. These data indicate that relaxin may be efficacious in the treatment of pathologies characterized by lung fibrosis.

Relaxin binds to and elicits a response from cells of the human monocytic cell line, THP-1

Relaxin is a 6-kDa peptide of the insulin family that is present at increased levels in the circulation during pregnancy. Its functions at that time are thought to include maintenance of myometrial quiescence, regulation of plasma volume, and release of neuropeptides, such as oxytocin and vasopressin. The protein also promotes connective tissue remodeling, which allows cervical ripening and separation of the pelvic symphysis in various mammalian species. In this report, we provide evidence for a novel target of relaxin, the human monocytic cell line, THP-1. Relaxin bound with high affinity (Kd = 102 pM) to a specific receptor on THP-1 cells. Receptor density was low ( approximately 275 receptors/cell), but binding of relaxin triggered intracellular signaling events. Receptor density was not modulated by pretreatment with estrogen, progesterone, or a number of other agents known to induce differentiation of THP-1 cells. Cross-linking studies showed radiolabeled relaxin bound primarily to cell surface proteins with an apparent molecular mass of >200 kDa. Other members of the insulin-like family of proteins (insulin, insulin-like growth factors I and II, and relaxin-like factor) were unable to displace the binding of relaxin to THP-1 cells, suggesting that a distinct receptor for relaxin exists on this monocyte/macrophage cell line.

Relaxin-induced increased coronary flow through stimulation of nitric oxide production

1. Relaxin (RLX) is a multifunctional hormone which, besides its role in pregnancy and parturition, has also been shown to influence the cardiovascular system. In this study, we investigated the effect of RLX on coronary flow of rat and guinea-pig hearts, isolated and perfused in a Langendorff apparatus. RLX was either added to the perfusion fluid at a concentration of 5 x 10(-9) M for a 20-min perfusion, or given as a bolus into the aortic cannula at concentrations of 10(-9) M, 5 x 10(-8) M dissolved in 1 ml of perfusion fluid. 2. RLX, given either for a 20-min perfusion or as a bolus in the aortic cannula to guinea-pig and rat isolated hearts, increased the coronary flow and the amount of nitrite, a stable end-product of nitric oxide (NO) metabolism, that appeared in the perfusates in a concentration-dependent fashion. 3. The increase in coronary flow and in nitrite in the perfusates induced by RLX was significantly reduced by pretreatment with the nitric oxide synthase (NOS) inhibitor, NG-monomethyl-L-arginine (L-NMMA, 10(-4) M). 4. The effects of RLX on coronary flow and nitrite amounts in the perfusates were compared with those induced by the endothelium-dependent vasodilator agent, acetylcholine (ACh, 10(-8)-10(-7) M), and by the endothelium-independent vasodilator agent, sodium nitroprusside (SNP, 10(-7)-10(-6) M). The results obtained show that RLX is more effective than ACh and SNP in increasing coronary flow. 5 The results of this study show that RLX increases coronary flow through stimulation of NO production; hence this hormone should be regarded as a novel agent capable of improving myocardial perfusion.