Relaxin induces rapid, transient vasodilation in the microcirculation of hamster skeletal muscle

In vitro effect of relaxin in the rat corpus cavernosum under hyperglycemic and normoglycemic conditions

Relaxin, an endogenous peptide hormone, elicits vascular relaxation by its direct effect or by modulating the endothelium-dependent relaxation response and is clinically evaluated for the treatment of coronary artery disease. However, its effect on penile tissue has not been explored yet. This study aimed to investigate the effect of serelaxin, recombinant human relaxin-2, on rat corpus cavernosum (CC) under healthy and hyperglycemic conditions. Strips of CC obtained from thirty-nine male Wistar rats weighing 300-350 g were used in organ baths for isometric tension studies to investigate the serelaxin-mediated relaxation (10-12-10-7 M) under normoglycemic conditions and the effect of serelaxin on endothelium-dependent [nitric oxide (NO)- and prostacyclin-mediated] relaxation responses under hyperglycemic conditions. The in vitro hyperglycemia model was created by 3 h of incubation with 44 mM glucose monohydrate +120 μM methylglyoxal. NO-dependent relaxation responses were evaluated by cumulative acetylcholine (10-9-10-4 M) administration in the presence of indomethacin (10-6 M). Prostacyclin-mediated relaxation was evaluated by cumulative administration of iloprost (10-9-10-6 M), a prostacyclin analog. Maximum relaxation responses to serelaxin were not significantly different compared to the time-control (p = 0.480). Three hours of incubation of rat CC in hyperglycemic conditions impaired NO- and prostacyclin-mediated relaxation responses (p = 0.032 and p = 0.047, respectively). Serelaxin coincubation worsened NO-mediated relaxation responses (p = 0.016) but did not affect prostacyclin-mediated responses (p = 0.425). Together, our results demonstrate that in vitro administration of serelaxin does not cause relaxation in penile tissue and short-term in vitro serelaxin treatment in hyperglycemic conditions mimicked diabetes modulates endothelium-dependent responses by worsening NO-mediated responses. Serelaxin exerts different effects via different mechanism on endothelium-dependent responses depending on the dose and duration of exposure. Therefore, proper timing and dosing of serelaxin administration in the penile tissue need to be investigated in further studies in diabetic animal models.

Effects of pregnancy and lactation on muscle-tendon morphology

Pregnancy and lactation hormones have been shown to mediate anatomical changes to the musculoskeletal system that generates animal movement. In this study, we characterize changes in the medial gastrocnemius muscle, its tendon and aponeuroses that are likely to have an effect on whole animal movement and energy expenditure, using the rat model system, Rattus norvegicus. We quantified muscle architecture (mass, cross-sectional area, and pennation angle), muscle fiber type and diameter, and Young's modulus of stiffness for the medial gastrocnemius aponeuroses as well as its contribution to Achilles tendon in three groups of three-month-old female rats: virgin, primiparous pregnant, and primiparous lactating animals. We found that muscle mass drops by 23% during lactation but does not change during pregnancy. We also found that during pregnancy muscle fibers switch from Type I to IIa and during lactation from Type IIb to Type I. The stiffness of connective tissues that has a demonstrated role in locomotion, the aponeurosis and tendon, also changed. Pregnant animals had a significantly less stiff aponeurosis. However, tendon stiffness was most affected during lactation, with a significant drop in stiffness and interindividual variation. We propose that the energetic demands of locomotion may have driven the evolution of these anatomical changes in muscle-tendon units during pregnancy and lactation to ensure more energy can be allocated to fetal development and lactation.

Capillary communication: the role of capillaries in sensing the tissue environment, coordinating the microvascular, and controlling blood flow

Historically, capillaries have been viewed as the microvascular site for flux of nutrients to cells and removal of waste products. Capillaries are the most numerous blood vessel segment within the tissue, whose vascular wall consists of only a single layer of endothelial cells and are situated within microns of each cell of the tissue, all of which optimizes capillaries for the exchange of nutrients between the blood compartment and the interstitial space of tissues. There is, however, a growing body of evidence to support that capillaries play an important role in sensing the tissue environment, coordinating microvascular network responses, and controlling blood flow. Much of our growing understanding of capillaries stems from work in skeletal muscle and more recent work in the brain, where capillaries can be stimulated by products released from cells of the tissue during increased activity and are able to communicate with upstream and downstream vascular segments, enabling capillaries to sense the activity levels of the tissue and send signals to the microvascular network to coordinate the blood flow response. This review will focus on the emerging role that capillaries play in communication between cells of the tissue and the vascular network required to direct blood flow to active cells in skeletal muscle and the brain. We will also highlight the emerging central role that disruptions in capillary communication may play in blood flow dysregulation, pathophysiology, and disease.

RXFP1 Receptor Activation by Relaxin-2 Induces Vascular Relaxation in Mice via a Gαi2-Protein/PI3Kß/γ/Nitric Oxide-Coupled Pathway

Background: Relaxins are small peptide hormones, which are novel candidate molecules that play important roles in cardiometablic syndrome. Relaxins are structurally related to the insulin hormone superfamily, which provide vasodilatory effects by activation of G-protein-coupled relaxin receptors (RXFPs) and stimulation of endogenous nitric oxide (NO) generation. Recently, relaxin could be demonstrated to activate G_i proteins and phosphoinositide 3-kinase (PI3K) pathways in cultured endothelial cells in vitro. However, the contribution of the G_i-PI3K pathway and their individual components in relaxin-dependent relaxation of intact arteries remains elusive.

Methods: We used Gα_i2- (Gnai2^-/-) and Gα_i3-deficient (Gnai3^-/-) mice, pharmacological tools and wire myography to study G-protein-coupled signaling pathways involved in relaxation of mouse isolated mesenteric arteries by relaxins. Human relaxin-1, relaxin-2, and relaxin-3 were tested.

Results: Relaxin-2 (∼50% relaxation at 10^-11 M) was the most potent vasodilatory relaxin in mouse mesenteric arteries, compared to relaxin-1 and relaxin-3. The vasodilatory effects of relaxin-2 were inhibited by removal of the endothelium or treatment of the vessels with N (G)-nitro-L-arginine methyl ester (L-NAME, endothelial nitric oxide synthase (eNOS) inhibitor) or simazine (RXFP1 inhibitor). The vasodilatory effects of relaxin-2 were absent in arteries of mice treated with pertussis toxin (PTX). They were also absent in arteries isolated from Gnai2^-/- mice, but not from Gnai3^-/- mice. The effects were not affected by FR900359 (Gα_q protein inhibitor) or PI-103 (PI3Kα inhibitor), but inhibited by TGX-221 (PI3Kβ inhibitor) or AS-252424 (PI3Kγ inhibitor). Simazine did not influence the anti-contractile effect of perivascular adipose tissue.

Conclusion: Our data indicate that relaxin-2 produces endothelium- and NO-dependent relaxation of mouse mesenteric arteries by activation of RXFP1 coupled to G_i2-PI3K-eNOS pathway. Targeting vasodilatory G_i-protein-coupled RXFP1 pathways may provide promising opportunities for drug discovery in endothelial dysfunction and cardiometabolic disease.

Luteal and hypophyseal expression of the canine relaxin (RLN) system during pregnancy: Implications for luteotropic function

By acting through its receptors (RXFP1, RXFP2), relaxin (RLN) exerts species-specific effects during pregnancy; possible luteotropic effects through stimulation of prolactin (PRL) release have been suggested. In the domestic dog (Canis lupus familiaris) serum PRL increases in pregnant bitches shortly after RLN appears in the circulation, and a possible functional relationship between the RLN and the PRL systems in regulating progesterone secretion has been implied. Therefore, here (Study 1) the luteal expression and localization of the RLN system was investigated by immunohistochemistry using custom-made antibodies and semi-quantitative PCR, at selected time points during gestation: pre-implantation (d. 8–12), post-implantation (d. 18–25), mid-gestation (d. 35–40) and at normal and antigestagen-induced luteolysis. Further, (Study 2) hypophyseal expression of the RLN system and its spatial association with PRL was assessed. Luteal expression of RLN, but not of its receptors, was time-dependent: it increased significantly following implantation towards mid-gestation and decreased at prepartum. Antigestagen treatment resulted in downregulation of RLN and RXFP2. Whereas RLN was localized in steroidogenic cells, RXFP1 and RXFP2 also stained strongly in macrophages and vascular endothelial cells. The RLN system was detected in the canine adenohypophysis and was co-localized with PRL in hypophyseal lactotrophs. The intraluteal RLN seems to be involved in regulating the canine corpus luteum (CL) in a time-dependent manner. The presence of RLN family members in the adenohypophysis implies their possible involvement in regulating the availability of PRL and other pituitary hormones.

Relaxin-2 in Cardiometabolic Diseases: Mechanisms of Action and Future Perspectives

Despite the great effort of the medical community during the last decades, cardiovascular diseases remain the leading cause of death worldwide, increasing their prevalence every year mainly due to our new way of life. In the last years, the study of new hormones implicated in the regulation of energy metabolism and inflammation has raised a great interest among the scientific community regarding their implications in the development of cardiometabolic diseases. In this review, we will summarize the main actions of relaxin, a pleiotropic hormone that was previously suggested to improve acute heart failure and that participates in both metabolism and inflammation regulation at cardiovascular level, and will discuss its potential as future therapeutic target to prevent/reduce cardiovascular diseases.

Orchidectomy attenuates high-salt diet-induced increases in blood pressure, renovascular resistance, and hind limb vascular dysfunction: role of testosterone

Sex hormone-dependent vascular reactivity is an underlying factor contributing to sex differences in salt-dependent hypertension. This study evaluated the role of androgens (testosterone) in high salt-induced increase in blood pressure (BP) and altered vascular reactivity in renal blood flow and perfused hind limb preparation. Weanling male rats (8 weeks old, 180-200 g) were bilaterally orchidectomised or sham operated with or without testosterone replacement (Sustanon 250, 10 mg/kg intramuscularly once in 3 weeks) and placed on a normal (0.3%) or high (4.0%) NaCl diet for 6 weeks. The high-salt diet (HSD) increased arterial BP, renal vascular resistance (RVR) and positive fluid balance (FB). These changes were accompanied by decreased plasma nitric oxide levels. The increased BP, RVR and FB observed in the rats fed a HSD were reversed by orchidectomy while testosterone replacement prevented the reversal. Phenylephrine (PE)-induced increased vascular resistance in the perfused hind limb vascular bed was enhanced by HSD, the enhanced vascular resistance was prevented by orchidectomy and testosterone replacement reversed orchidectomy effect. Vasorelaxation responses to acetylcholine (ACh) and sodium nitroprusside (SNP) were impaired in HSD groups, orchidectomy attenuated the impairment, while testosterone replacement prevented the orchidectomy attenuation. These data suggested that eNOS-dependent and independently-mediated pathways were equally affected by HSD in vascular function impairment and this effect is testosterone-dependent in male Sprague-Dawley rats.

Smooth Muscle Ion Channels and Regulation of Vascular Tone in Resistance Arteries and Arterioles

Vascular tone of resistance arteries and arterioles determines peripheral vascular resistance, contributing to the regulation of blood pressure and blood flow to, and within the body's tissues and organs. Ion channels in the plasma membrane and endoplasmic reticulum of vascular smooth muscle cells (SMCs) in these blood vessels importantly contribute to the regulation of intracellular Ca2+ concentration, the primary determinant of SMC contractile activity and vascular tone. Ion channels provide the main source of activator Ca2+ that determines vascular tone, and strongly contribute to setting and regulating membrane potential, which, in turn, regulates the open-state-probability of voltage gated Ca2+ channels (VGCCs), the primary source of Ca2+ in resistance artery and arteriolar SMCs. Ion channel function is also modulated by vasoconstrictors and vasodilators, contributing to all aspects of the regulation of vascular tone. This review will focus on the physiology of VGCCs, voltage-gated K+ (KV) channels, large-conductance Ca2+-activated K+ (BKCa) channels, strong-inward-rectifier K+ (KIR) channels, ATP-sensitive K+ (KATP) channels, ryanodine receptors (RyRs), inositol 1,4,5-trisphosphate receptors (IP3Rs), and a variety of transient receptor potential (TRP) channels that contribute to pressure-induced myogenic tone in resistance arteries and arterioles, the modulation of the function of these ion channels by vasoconstrictors and vasodilators, their role in the functional regulation of tissue blood flow and their dysfunction in diseases such as hypertension, obesity, and diabetes. © 2017 American Physiological Society. Compr Physiol 7:485-581, 2017.

G-Protein-coupled receptors as potential drug candidates in preeclampsia: targeting the relaxin/insulin-like family peptide receptor 1 for treatment and prevention

BACKGROUND

Important roles for G-protein-coupled receptors (GPCRs) have been identified in the maternal physiological adaptations to pregnancy and in the pathogenesis of preeclampsia. On this basis, GPCRs are potential therapeutic targets for preeclampsia.

OBJECTIVES AND RATIONALE

In this review, vasopressin and apelin are initially considered in this context before the focus on the hormone relaxin and its cognate receptor, the relaxin/insulin-like family peptide receptor 1 (RXFP1). Based on both compelling scientific rationale and a promising safety profile, the relaxin ligand-receptor system is comprehensively evaluated as a potential therapeutic endpoint in preeclampsia.

SEARCH METHODS

The published literature relating to the topic was searched through January 2016 using PubMed.

OUTCOMES

Relaxin is a peptide hormone secreted by the corpus luteum; it circulates in the luteal phase and during pregnancy. Activation of RXFP1 is vasodilatory; thus, relaxin supplementation is expected to at least partly restore the fundamental vasodilatory changes of normal pregnancy, thereby alleviating maternal organ hypoperfusion, which is a major pathogenic manifestation of severe preeclampsia. Specifically, by exploiting its pleiotropic hemodynamic attributes in preeclampsia, relaxin administration is predicted to (i) reverse robust arterial myogenic constriction; (ii) blunt systemic and renal vasoconstriction in response to activation of the angiotensin II receptor, type 1; (iii) mollify the action of endogenous vasoconstrictors on uterine spiral arteries with failed remodeling and retained smooth muscle; (iv) increase arterial compliance; (v) enhance insulin-mediated glucose disposal by promoting skeletal muscle vasodilation and (vi) mobilize and activate bone marrow-derived angiogenic progenitor cells, thereby repairing injured endothelium and improving maternal vascularity in organs such as breast, uterus, pancreas, skin and fat. By exploiting its pleiotropic molecular attributes in preeclampsia, relaxin supplementation is expected to (i) enhance endothelial nitric oxide synthesis and bioactivity, as well as directly reduce vascular smooth muscle cytosolic calcium, thus promoting vasodilation; (ii) improve the local angiogenic balance by augmenting arterial vascular endothelial and placental growth factor (VEGF and PLGF) activities; (iii) ameliorate vascular inflammation; (iv) enhance placental peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PCG1α) expression, and hence, peroxisome proliferator-activated receptor gamma (PPAR-γ) activity and (v) confer cytotrophoblast and endothelial cytoprotection. Insofar as impaired endometrial maturation (decidualization) predisposes to the development of preeclampsia, relaxin administration in the late secretory phase and during early pregnancy would be anticipated to improve decidualization, and hence trophoblast invasion and spiral artery remodeling, thereby reducing the risk of preeclampsia. Relaxin has a favorable safety profile both in the non-pregnant condition and during pregnancy.

WIDER IMPLICATIONS

There is a strong scientific rationale for RXFP1 activation in severe preeclampsia by administration of relaxin, relaxin analogs or small molecule mimetics, in order to mollify the disease pathogenesis for safe prolongation of pregnancy, thus allowing time for more complete fetal maturation, which is a primary therapeutic endpoint in treating the disease. In light of recent data implicating deficient or defective decidualization as a potential etiological factor in preeclampsia and the capacity of relaxin to promote endometrial maturation, the prophylactic application of relaxin to reduce the risk of preeclampsia is a plausible therapeutic approach to consider. Finally, given its pleiotropic and beneficial attributes particularly in the cardiovascular system, relaxin, although traditionally considered as a 'pregnancy' hormone, is likely to prove salutary for several disease indications in the non-pregnant population.

Relaxin and insulin-like peptide 3 in the musculoskeletal system: from bench to bedside

Skeletal muscles and bones form a joined functional unit sharing a complex mechanical, biochemical and hormonal crosstalk. A number of factors, including sex hormones, physiologically regulate the musculoskeletal system. Striking gender differences in muscle and bone mass, and function are mainly caused by distinct actions exerted by oestrogens and androgens. However, relaxin and relaxin-related peptides, such as insulin-like peptide 3 (INSL3), might contribute to these sex-associated differences in physiological and pathological conditions (such as osteoporosis and sarcopenia). Relaxin is a 'pregnancy' hormone, but it is also produced from the prostate gland, and has recently attracted attention as a potential drug for cardiovascular disorders and fibrosis. In contrast, INSL3 is a male-specific hormone produced by the Leydig cells of the testis with a fundamental role in testicular descent during fetal life. Recent evidence suggests that both hormones have interesting roles in the musculoskeletal system. Relaxin and INSL3, by finely tuning bone formation and resorption, are involved in bone remodelling processes, and relaxin contributes to the healing of injured ligaments and promotes skeletal muscle regeneration. Here, we review the most recent findings on the effects of relaxin and INSL3 on skeletal muscle and the cell components of bone. In the light of the experimental evidence available and animal models, their clinical implications are also discussed.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Serelaxin: A Novel Therapeutic for Vascular Diseases

Vascular dysfunction is an important hallmark of cardiovascular disease. It is characterized by increased sensitivity to vasoconstrictors, decreases in the endothelium-derived vasodilators nitric oxide (NO) and prostacyclin (PGI2), and endothelium-derived hyperpolarization (EDH). Serelaxin (recombinant human relaxin) has gained considerable attention as a new vasoactive drug, largely through its beneficial therapeutic effects in acute heart failure. In this review we first describe the contribution of endogenous relaxin to vascular homeostasis. We then provide a comprehensive overview of the novel mechanisms of serelaxin action in blood vessels that differentiate it from other vasodilator drugs and explain how this peptide could be used more widely as a therapeutic to alleviate vascular dysfunction in several cardiovascular diseases.

Upregulation of relaxin after experimental subarachnoid hemorrhage in rabbits

BACKGROUND

Although relaxin causes vasodilatation in systemic arteries, little is known about its role in cerebral arteries. We investigated the expression and role of relaxin in basilar arteries after subarachnoid hemorrhage (SAH) in rabbits.

METHODS

Microarray analysis with rabbit basilar artery RNA was performed. Messenger RNA expression of relaxin-1 and relaxin/insulin-like family peptide receptor 1 (RXFP1) was investigated with quantitative RT-PCR. RXFP1 expression in the basilar artery was investigated with immunohistochemistry. Relaxin concentrations in cerebrospinal fluid (CSF) and serum were investigated with an enzyme-linked immunosorbent assay. Using human brain vascular smooth muscle cells (HBVSMC) preincubated with relaxin, myosin light chain phosphorylation (MLC) was investigated with immunoblotting after endothelin-1 stimulation.

RESULTS

After SAH, RXFP1 mRNA and protein were significantly downregulated on day 3, whereas relaxin-1 mRNA was significantly upregulated on day 7. The relaxin concentration in CSF was significantly elevated on days 5 and 7. Pretreatment with relaxin reduced sustained MLC phosphorylation induced by endothelin-1 in HBVSMC.

CONCLUSION

Upregulation of relaxin and downregulation of RXFP1 after SAH may participate in development of cerebral vasospasm. Downregulation of RXFP1 may induce a functional decrease in relaxin activity during vasospasm. Understanding the role of relaxin may provide further insight into the mechanisms of cerebral vasospasm.

Therapeutic effects of serelaxin in acute heart failure

Over the past few decades, research on the peptide hormone, relaxin, has significantly improved our understanding of its biological actions under physiological and diseased conditions. This has facilitated the conducting of clinical trials to explore the use of serelaxin (human recombinant relaxin). Acute heart failure (AHF) is a very difficult to treat clinical entity, with limited success so far in developing new drugs to combat it. A recent phase-III RELAX-AHF trial using serelaxin therapy given during hospitalization revealed acute (ameliorated dyspnea) and chronic (improved 180-day survival) effects. Although these findings support a substantial improvement by serelaxin therapy over currently available therapies for AHF, they also raise key questions and stimulate new hypotheses. To facilitate the development of serelaxin as a new drug for heart disease, joint efforts of clinicians, research scientists and pharmacological industries are necessary to study these questions and hypotheses. In this review, after providing a brief summary of clinical findings and the pathophysiology of AHF, we present a working hypothesis of the mechanisms responsible for the observed efficacy of serelaxin in AHF patients. The existing clinical and preclinical data supporting our hypotheses are summarized and discussed. The development of serelaxin as a drug provides an excellent example of the bilateral nature of translational research. 

Arteriolar diameter and spontaneous vasomotion: importance of potassium channels and nitric oxide

Arterioles display cyclic variations in diameter, termed vasomotion initiated by smooth muscle cells (SMCs), but the endothelium should also be evaluated due to its modulatory role on vessel tone. Since nitric oxide (NO) and prostacyclin (PGI2) regulate SMC tone and activate K(+) currents, we have investigated their role on vasomotion, by observing effects of topical application of N(ω)-nitro-l-arginine (L-NA, NO synthesis inhibitor), glibenclamide (KATP channel inhibitor), sodium nitroprusside (SNP, NO donor), iloprost (PGI2 analogue) and methylene blue (MB, cGMP production inhibitor) on the cheek pouch preparation of anesthetized male hamsters. L-NA (10(-10)-10(-6)M) induced vasoconstriction, reduction and abolition of vasomotion. MB (10(-7) to 10(-5)M) reduced mean arteriolar diameter with no changes on vasomotion. In the presence of 10(-6)M of MB, addition of 10(-6)L-NA totally abolished vasomotion without further constriction. Glibenclamide (10(-6)M) in the presence of L-NA at equimolar concentration restored both vasomotion frequency and amplitude. This effect was not observed in the presence of TEA 5mM. SNP (10(-10)-10(-6)M) induced a dose-dependent increase of arteriolar diameter and decreased vasomotion. Iloprost (10(-12)-10(-6)M) induced a concentration dependent increase of arteriolar diameter, reduced vasomotion frequency, but in lower concentrations (10(-12)-10(-10)M) increased its amplitude and in higher concentrations (10(-9)-10(-6)M) decreased it. SNP and iloprost inhibited vasomotion at 10(-7)M; however, at this concentration SNP and iloprost induced an increment of 35% and 50% of the initial arteriolar diameter, respectively. In the presence of L-NA (10(-6)M), vasomotion was restored by SNP at 10(-10)M and iloprost 10(-12)M, which corresponded to 80% of the initial diameter value. Around the initial (control) arteriolar diameter value, vasomotion presented its highest frequencies and amplitudes. Cessation of vasomotion occurred with L-NA (10(-6)M) in the presence of SNP (10(-6)M) and iloprost (10(-7)M) when arteriolar diameter reached 150% and 120% of its initial value, respectively. In conclusion, the present study strongly suggests that vasomotion (1) is not solely related to vascular tone, (2) needs an interplay between vascular tone and membrane currents and (3) could be modulated by NO (but not cGMP) and KATP channels. In addition, our results point to the existence of dissociation between vasomotion frequency and amplitude.