Vasopressin accelerates protein synthesis in neonatal rat cardiomyocytes

Vasopressin V1a and V1b Receptors: From Molecules to Physiological Systems

The neurohypophysial hormone arginine vasopressin (AVP) is essential for a wide range of physiological functions, including water reabsorption, cardiovascular homeostasis, hormone secretion, and social behavior. These and other actions of AVP are mediated by at least three distinct receptor subtypes: V1a, V1b, and V2. Although the antidiuretic action of AVP and V2 receptor in renal distal tubules and collecting ducts is relatively well understood, recent years have seen an increasing understanding of the physiological roles of V1a and V1b receptors. The V1a receptor is originally found in the vascular smooth muscle and the V1b receptor in the anterior pituitary. Deletion of V1a or V1b receptor genes in mice revealed that the contributions of these receptors extend far beyond cardiovascular or hormone-secreting functions. Together with extensively developed pharmacological tools, genetically altered rodent models have advanced the understanding of a variety of AVP systems. Our report reviews the findings in this important field by covering a wide range of research, from the molecular physiology of V1a and V1b receptors to studies on whole animals, including gene knockout/knockdown studies.

Skeletal muscle regeneration in mice is stimulated by local overexpression of V1a-vasopressin receptor

Skeletal muscle has a remarkable capacity to regenerate after mechanical or pathological injury. We show that the V1a receptor (V1aR) for vasopressin, a potent myogenic-promoting factor that stimulates differentiation and hypertrophy in vitro, is expressed in mouse skeletal muscle and modulated during regeneration after experimental injury. We used gene delivery by electroporation to overexpress the myc-tagged vasopressin V1aR in specific muscles, thus sensitizing them to circulating vasopressin. The correct localization on the surface of the fibers of the recombinant product was demonstrated by confocal immunofluorescence directed against the myc tag. V1aR overexpression dramatically enhanced regeneration. When compared with mock-transfected controls, V1aR overexpressing muscles exhibited significantly accelerated activation of satellite cells and increased expression of differentiation markers. Downstream of V1aR activation, calcineurin was strongly up-regulated and stimulated the expression of IL-4, a potent mediator of myogenic cell fusion. The central role of calcineurin in mediating V1aR-dependent myogenesis was also demonstrated by using its specific inhibitor, cyclosporine A. This study identifies skeletal muscle as a physiological target of hormones of the vasopressin family and reveals a novel in vivo role for vasopressin-dependent pathways. These findings unveil several steps, along a complex signaling pathway, that may be exploited as potential targets for the therapy of diseases characterized by altered muscle homeostasis and regeneration.

Hyperammonaemia in V1a vasopressin receptor knockout mice caused by the promoted proteolysis and reduced intrahepatic blood volume

An analysis of arginine-vasopressin (AVP) V1a receptor-deficient (V1aR-/-) mice revealed that glucose homeostasis and lipid metabolism were altered in the mutant mice. Here, we used V1aR-/- mice to investigate whether the deficiency of the V1a receptor, which led to altered insulin sensitivity, affected protein metabolism. The serum 3-methylhistidine levels were increased in V1aR-/- mice under feeding conditions, indicating that proteolysis was enhanced in muscle tissue from V1aR-/- mice. Furthermore, serum amino acid profiling revealed that the amino acid levels, including glycogenic and branched-chain amino acids, were reduced in V1aR-/- mice. In addition, an alanine-loading test showed that gluconeogenesis was enhanced in V1aR-/- mice. Blood ammonia, which is a by-product of amino acid catabolism, was two times higher in V1aR-/- mice without hepatopathy under the feeding and fasting conditions than in wild-type mice. Amino acid profiling also revealed that the amino acid pattern was not typical of a urea-cycle enzymatic disorder. An ammonia tolerance test and an indocyanine green elimination test showed that V1aR-/- mice had lower ammonia clearance due to a decreased intrahepatic circulating blood volume. Metabolic acidosis, including lactic- and keto-acidosis, was not observed in V1aR-/- mice. These results provide evidence that proteolysis promotes the production of glucose in the muscles of V1aR-/- mice and that hyperammonaemia is caused by promoted protein catabolism and reduced intrahepatic blood volume. Thus, our study with V1aR-/- mice indicates that AVP plays a physiological role via the V1a receptor in regulating both protein catabolism and glucose homeostasis.

Emerging therapies for the management of decompensated heart failure: from bench to bedside

While pharmaceutical innovation has been highly successful in reducing mortality in chronic heart failure, this has not been matched by similar success in decompensated heart failure syndromes. Despite outstanding issues over definitions and end points, we argue in this paper that an unprecedented wealth of pharmacologic innovation may soon transform the management of these challenging patients. Agents that target contractility, such as cardiac myosin activators and novel adenosine triphosphate-dependent transmembrane sodium-potassium pump inhibitors, provide inotropic support without arrhythmogenic increases in cytosolic calcium or side effects of more traditional agents. Adenosine receptor blockade may improve glomerular filtration and diuresis by exerting a direct beneficial effect on glomerular blood flow while vasopressin antagonists promote free water excretion without compromising renal function and may simultaneously inhibit myocardial remodeling. Urodilatin, the renally synthesized isoform of atrial natriuretic peptide, may improve pulmonary congestion via vasodilation and enhanced diuresis. Finally, metabolic modulators such as perhexiline may optimize myocardial energy utilization by shifting adenosine triphosphate production from free fatty acids to glucose, a unique and conceptually appealing approach to the management of heart failure. These advances allow optimism not only for the advancement of our understanding and management of decompensated heart failure syndromes but for the translational research effort in heart failure biology in general.

Ca2+ dynamics of thrombin-stimulated rat heart-derived embryonic myocytes: relationship to protein synthesis and cell growth

Various cell types respond to the serum protease, thrombin, with increased proliferation rates. In non-dividing postnatal mammalian cardiomyocytes, however, thrombin induces cellular hypertrophy. Both growth responses are associated with early Ca2+ signaling. The present study was conducted to characterize Ca2+ dynamics in thrombin stimulated, dividing embryonic cardiomyocytes, and to ascertain whether such dynamics support hypertrophic or hyperplastic growth. H9c2 rat cardiomyoblasts responded to thrombin with immediate, large increments in free Ca2+ that arose principally from the release of S(E)R sequestered Ca2+ and that persisted for only a few min. Ca2+ stores were refilled within 1h. Thrombin also increased rates of overall protein synthesis for several hours. This translational up-regulation, which required gene transcription, was abolished if cells were incubated at low extracellular Ca2+ during the first hour with thrombin. The protease conferred protective effects against toxicity resulting from serum deprivation and doxorubicin treatment. However, thrombin induced neither cellular hypertrophy, as is seen with arginine vasopressin, nor hyperplasia, as is observed with platelet-derived growth factor (PDGF-BB), in H9c2 cardiomyocytes. In comparison with vasopressin or PDGF-BB, thrombin promoted brief Ca2+ signaling, little cation movement to the extracellular fluid, and more rapid refilling of the S(E)R. It is concluded that the Ca2+ signaling generated by thrombin and the translational stimulation shown in this report to depend on this Ca2+ signaling are insufficient to sustain a major growth response in these embryonic cardiomyocytes.

Effects of arginine vasopressin on growth of rat cardiac fibroblasts: role of V1 receptor

The abnormal proliferation of cardiac fibroblasts is involved in the pathophysiologic process of left ventricular hypertrophy (LHV) associated with essential hypertension. Arginine vasopressin (AVP) has been reported to contribute significantly to the pathogenesis of hypertension. In this study, the authors investigated the effects of AVP and its V1 receptor antagonist [d(CH2)5Tyr2(Me)]AVP on the growth of rat cardiac fibroblasts. Cardiac fibroblasts of neonatal Sprague-Dawley rats were isolated, and growth-arrested cardiac fibroblasts were stimulated with 2.5% fetal calf serum in the presence and absence of AVP (0.001, 0.01, 0.1, and 1 microM) and [d(CH2)5Tyr2(Me)]AVP (0.1 microM). DNA synthesis was measured by [3H]thymidine incorporation. Thiazolyl blue assay and flow cytometry techniques were adopted to measure cell numbers and analyze cell cycle, respectively. Arginine vasopressin (0.1 and 1 microM) significantly increased DNA synthesis in cardiac fibroblasts. Moreover, AVP (0.1 and 1 microM) significantly increased the number of cardiac fibroblasts. Analysis of cell cycle showed that AVP (0.1 microM) increased S-stage percentage and proliferation index (PI). The V1 receptor antagonist [d(CH2)5Tyr2(Me)]AVP (0.1 microM) significantly inhibited DNA synthesis in cardiac fibroblasts. The cell number, S-stage percentage, and PI induced by AVP (0.1 microM) were significantly decreased by [d(CH2)5Tyr2(Me)]AVP (0.1 microM). These findings suggest that AVP might promote the proliferation of rat cardiac fibroblasts, which seems to be mediated via the V1 receptor. Arginine vasopressin may be involved in the pathophysiologic process of LVH by promoting cardiac fibroblast proliferation.

Science Review: Vasopressin and the cardiovascular system part 2 - clinical physiology

Vasopressin is emerging as a rational therapy for vasodilatory shock states. In part 1 of the review we discussed the structure and function of the various vasopressin receptors. In part 2 we discuss vascular smooth muscle contraction pathways with an emphasis on the effects of vasopressin on ATP-sensitive K+ channels, nitric oxide pathways, and interaction with adrenergic agents. We explore the complex and contradictory studies of vasopressin on cardiac inotropy and coronary vascular tone. Finally, we summarize the clinical studies of vasopressin in shock states, which to date have been relatively small and have focused on physiologic outcomes. Because of potential adverse effects of vasopressin, clinical use of vasopressin in vasodilatory shock should await a randomized controlled trial of the effect of vasopressin's effect on outcomes such as organ failure and mortality.

Endothelin in the perinatal circulation

During the fetal period, blood is oxygenated through the placenta, and most of the cardiac output bypasses the lung through the ductus arteriosus. At birth, pulmonary vascular resistance falls with the initiation of ventilation. Coincidentally, the ductus arteriosus constricts. Endothelin-1 (ET-1) appears to play an important role during that transition period and postnatally. ET-1 can dramatically increase resistance in the placental microcirculation and may be involved in blood flow redistribution with hypoxia. At birth, the increase in oxygen tension is important in triggering ductus vasoconstriction. It is proposed that oxygen triggers closure of the ductus arteriosus by activating a specific, cytochrome P450-linked reaction, which in turn stimulates the synthesis of ET-1. On the neonatal heart, ET-1 has a positive chronotropic but negative inotropic effect. In the newborn piglet and the fetal lamb, both term and preterm, ET-1 causes a potent, long-lasting pulmonary vasoconstriction. Furthermore, a transient dilator response has been identified, and it is ascribed to nitric oxide formation. ET receptors are abundant in the piglet pulmonary vasculature. They are predominantly of the ETA constrictor subtype, though ETB2 constrictor receptors may also be present in certain species. The dilator response is linked to the ETB1 receptor, and the number of ETB1 receptors is reduced in hypoxia-induced pulmonary hypertension. ET-1 appears to be a causative agent in the pathogenesis of hypoxia- and hyperoxia-induced pulmonary hypertension as demonstrated by reversal of hemodynamic and morphological changes with treatment with an ETA receptor antagonist. Findings are amenable to practical applications in the management of infants with pulmonary hypertension or requiring persistent patency of the ductus arteriosus.

Endothelin ETA receptor antagonism does not attenuate angiotensin II-induced cardiac hypertrophy in vivo in rats

1. Angiotensin (Ang) II causes cardiac hypertrophy in vitro and in vivo. It also stimulates the release of endothelin (ET)-1. Endothelin-1 induces hypertrophy of cardiomyocytes in vitro. 2. In the present study, we examined whether the cardiac hypertrophic action of AngII in vivo was mediated by ET-1 via ETA receptors. We also determined whether arginine vasopressin (AVP), another ET-1 stimulator, could cause cardiac hypertrophy in vivo through an ET-1-dependent pathway. 3. In Sprague-Dawley rats (n = 8 per group), we determined whether the orally administered ETA receptor antagonist BMS 193884 could attenuate the cardiac hypertrophic effect of: (i) i.v. AngII infusion at either 100 or 200 ng/kg per min, i.v., for 1 week; (ii) AngII infusion at 100 ng/kg per min, i.v., for 2 weeks; and (iii) AVP infusion at either 2 or 10 ng/kg per min, i.v., for 1 week. Mean arterial pressure and heart rate were also measured. 4. Infusion with AngII for both 1 and 2 weeks increased left ventricular weight. Only AngII infusion at 200 ng/kg per min for 1 week increased blood pressure. Endothelin ETA receptor blockade did not attenuate the left ventricular hypertrophy, even though it reduced the hypertensive effect of AngII. Arginine vasopressin increased blood pressure, but did not cause cardiac hypertrophy. 5. We showed that AngII can cause cardiac hypertrophy through a direct, blood pressure-independent effect on the heart. Endothelin-1 did not mediate the cardiac hypertrophic effect of AngII through ETA receptors. This may indicate the involvement of ETB receptors in this model of cardiac hypertrophy. Arginine vasopressin did not cause cardiac hypertrophy in vivo.

Macrophages require different nucleoside transport systems for proliferation and activation

To evaluate the mechanisms involved in macrophage proliferation and activation, we studied the regulation of the nucleoside transport systems. In murine bone marrow-derived macrophages, the nucleosides required for DNA and RNA synthesis are recruited from the extracellular medium. M-CSF induced macrophage proliferation and DNA and RNA synthesis, whereas interferon gamma (IFN-gamma) led to activation, blocked proliferation, and induced only RNA synthesis. Macrophages express at least the concentrative systems N1 and N2 (CNT2 and CNT1 genes, respectively) and the equilibrative systems es and ei (ENT1 and ENT2 genes, respectively). Incubation with M-CSF only up-regulated the equilibrative system es. Inhibition of this transport system blocked M-CSF-dependent proliferation. Treatment with IFN-gamma only induced the concentrative N1 and N2 systems. IFN-gamma also down-regulated the increased expression of the es equilibrative system induced by M-CSF. Thus, macrophage proliferation and activation require selective regulation of nucleoside transporters and may respond to specific requirements for DNA and RNA synthesis. This report also shows that the nucleoside transporters are critical for macrophage proliferation and activation.

Regulated expression of GRP78 during vasopressin-induced hypertrophy of heart-derived myocytes

Although the development of cellular hypertrophy is widely believed to involve Ca(2+) signaling, potential supporting roles for sequestered Ca(2+) in this process have not been explored. H9c2 cardiomyocytes respond to arginine vasopressin with an initial mobilization of Ca(2+) stores and reduced rates of mRNA translation followed by repletion of Ca(2+) stores, up-regulation of translation beyond initial rates, and the development of hypertrophy. Rates of synthesis of the endoplasmic reticulum (ER) chaperones, GRP78 and GRP94, were found to increase preferentially at early times of vasopressin treatment. Total GRP78 content increased 2- to 3-fold within 8 h after which the chaperone was subject to post-translational modification. Preferential synthesis of GRP78 and the increase in chaperone content both occurred at pM vasopressin concentrations and were abolished at supraphysiologic Ca(2+) concentrations. Co-treatment with phorbol myristate acetate decreased vasopressin-dependent Ca(2+) mobilization and slowed appearance of new GRP78 molecules in response to the hormone, whereas 24 h pretreatment with phorbol ester prolonged vasopressin-dependent Ca(2+) mobilization and further increased rates of GRP78 synthesis in response to the hormone. Findings did not support a role for newly synthesized GRP78 in translational up-regulation by vasopressin. However up-regulation, which does not depend on Ca(2+) sequestration, appeared to expedite chaperone expression. This report provides the first evidence that a Ca(2+)-mobilizing hormone at physiologic concentrations signals increased expression of GRP78. Translational tolerance to depletion of ER Ca(2+) stores, typifying a robust ER stress response, did not accompany vasopressin-induced hypertrophy.

Vasopressin-induced hypertrophy in H9c2 heart-derived myocytes

Protein synthesis in H9c2 heart-derived myocytes responds biphasically to arginine vasopressin (1 microM). An initial 50% inhibition attributable to Ca(2+) mobilization from the sarcoplasmic/endoplasmic reticulum is followed by a recovery that subsequently converts to a 1.5-fold stimulation. This study was undertaken to ascertain whether vasopressin programs H9c2 cells to undergo hypertrophy or to proliferate and whether early translational inhibition is required for programming. Translational suppression was observed only at vasopressin concentrations (>1 nM) causing extensive (>50%) depletion of Ca(2+) stores and was diminished at supraphysiologic extracellular Ca(2+) concentrations. Stimulation of protein synthesis, by contrast, was unaffected by changes in extracellular Ca(2+), depended on gene transcription, was suppressed by a protein kinase C pseudosubstrate sequence (peptide 19-27), and was observed at pM vasopressin concentrations. Activation of MAP kinases, phosphoinositide 3-kinase, calcineurin, S6 kinase, or eIF4 could not be implicated in the stimulation, which persisted for 24 h. Vasopressin-treated H9c2 cells underwent hypertrophy by standard criteria. Cellular protein accumulation occurred at pM hormone concentrations, was blocked by peptide 19-27, was observed regardless of retinoic acid pretreatment to prevent myogenic transdifferentiation, and preceded full repletion of Ca(2+) stores. It is proposed that H9c2 cells, which possess all basic features of V1-vasopressin receptor signaling, provide a convenient model for investigating vasopressin-induced myocyte hypertrophy. Early translational suppression is not needed for vasopressin-induced H9c2 myocyte hypertrophy whereas activation of protein kinase C appears essential.