Messenger RNAs encoding the natriuretic peptides and their receptors are expressed in the eye

Interventions in the B-type natriuretic peptide signalling pathway as a means of controlling chronic itch

Chronic itch poses major health care and economic burdens worldwide. In 2013, B-type natriuretic peptide (BNP) was identified as an itch-selective neuropeptide and shown to be both necessary and sufficient to produce itch behaviour in mice. Since then, mechanistic studies of itch have increased, not only at central levels of the spinal relay of itch signalling but also in the periphery and skin. In this review, we have critically analysed recent findings from complementary pharmacological and physiological approaches, combined with genetic strategies to examine the role of BNP in itch transduction and modulation of other pruritic proteins. Additionally, potential targets and possible strategies against BNP signalling are discussed for developing novel therapeutics in itch. Overall, we aim to provide insights into drug development by altering BNP signalling to modulate disease symptoms in chronic itch, including conditions for which no approved treatment exists.

Effects of TAK-639, a novel topical C-type natriuretic peptide analog, on intraocular pressure and aqueous humor dynamics in mice

Primary open-angle glaucoma (POAG) is a leading cause of irreversible blindness, and individuals with ocular hypertension are at risk to develop POAG. Currently, the only modifiable risk factor for glaucoma progression is lowering of intraocular pressure (IOP). A novel mechanism for lowering IOP involves activation of the type B natriuretic peptide receptor (NPR-B), the naturally occurring agonist of which is C-type natriuretic peptide (CNP). Being a cyclic peptide of 22 amino acids, CNP does not readily penetrate the cornea and its ocular hypotensive effect requires intraocular injection. TAK-639 is a synthetic, cornea-permeable, 9-amino acid CNP analog has been studied for the treatment of ocular hypertension and POAG. We assessed TAK-639 in a receptor binding profile and the effects of TAK-639 on NPR-B-mediated cyclic GMP production in cultured transformed human trabecular meshwork (TM) cells (GTM-3). We also evaluated the effects of topical ocular administration of TAK-639 on mouse IOP and aqueous humor dynamics. Among 89 non-natriuretic peptide receptors, transporters, and channels evaluated, TAK-639 at 10 μM displaced ligand binding by more than 50% to only two receptors: the type 2 angiotensin receptor (IC₅₀ = 8.2 μM) and the cholecystokinin A receptor (IC₅₀ = 25.8 μM). In vitro, TAK-639 selectively activates NPR-B (EC₅₀ = 61 ± 11 nM; GTM-3 cells) relative to NPR-A (EC₅₀ = 2179 ± 670 nM; 293T cells). In vivo, TAK-639 lowered mouse IOP by three mechanisms: increase in aqueous humor outflow facility (C), reduction in the aqueous humor formation rate (Fin), and reduction in episcleral venous pressure (Pe). The maximum mean IOP decreases from baseline were -12.1%, -21.0%, and -36.1% for 0.1%, 0.3%, and 0.6% doses of TAK-639, respectively. Maximum IOP-lowering effect was seen at 2 h, and the duration of action was >6 h. With TAK-639 0.6%, at 2 h post-dose, aqueous outflow facility (C) increased by 155.8%, Fin decreased by 41.0%, the uveoscleral outflow rate (Fu) decreased by 52.6%, and Pe decreased by 31.5% (all p < 0.05). No ocular adverse effects were observed. TAK-639 is an efficacious IOP-lowering agent, with a unique combination of mechanisms of action on both aqueous formation and aqueous outflow facility. Further study of this mechanism of treatment may optimize pharmacologic outcomes and provide disease management in patients with POAG and ocular hypertension.

Natriuretic Peptides and Normal Body Fluid Regulation

Natriuretic peptides are structurally related, functionally diverse hormones. Circulating atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are delivered predominantly by the heart. Two C-type natriuretic peptides (CNPs) are paracrine messengers, notably in bone, brain, and vessels. Natriuretic peptides act by binding to the extracellular domains of three receptors, NPR-A, NPR-B, and NPR-C of which the first two are guanylate cyclases. NPR-C is coupled to inhibitory proteins. Atrial wall stress is the major regulator of ANP secretion; however, atrial pressure changes plasma ANP only modestly and transiently, and the relation between plasma ANP and atrial wall tension (or extracellular volume or sodium intake) is weak. Absence and overexpression of ANP-related genes are associated with modest blood pressure changes. ANP augments vascular permeability and reduces vascular contractility, renin and aldosterone secretion, sympathetic nerve activity, and renal tubular sodium transport. Within the physiological range of plasma ANP, the responses to step-up changes are unimpressive; in man, the systemic physiological effects include diminution of renin secretion, aldosterone secretion, and cardiac preload. For BNP, the available evidence does not show that cardiac release to the blood is related to sodium homeostasis or body fluid control. CNPs are not circulating hormones, but primarily paracrine messengers important to ossification, nervous system development, and endothelial function. Normally, natriuretic peptides are not powerful natriuretic/diuretic hormones; common conclusions are not consistently supported by hard data. ANP may provide fine-tuning of reno-cardiovascular relationships, but seems, together with BNP, primarily involved in the regulation of cardiac performance and remodeling. © 2017 American Physiological Society. Compr Physiol 8:1211-1249, 2018.

Association of NT-proANP Level in Plasma and Humor Aqueous with Primary Open-Angle Glaucoma

PURPOSE

The aim of this study is to determine differences in the levels of NT-proANP in the plasma and aqueous humor of glaucoma and cataract patients and to evaluate whether any relationships are present.

METHODS

The study group consisted of 58 patients with primary-open glaucoma (POAG) undergoing trabeculectomy surgery. The control group was comprised of 32 age-matched cataract patients. The concentration of the N-terminal fragment of the proatrial natriuretic peptide (NT-proANP, 1-98) in the aqueous humor and blood plasma samples was measured using an immunochemical method (ELISA).

RESULTS

The plasma NT-proANP concentration was significantly increased in patients with POAG compared to that in the control group (7.00 vs. 4.65 nmol/L, P = 0.0054). Similarly, the NT-proANP concentration in the aqueous humor was significantly higher in the POAG patients (0.47 vs. 0.09 nmol/L, P = 0.0112). However, there was no correlation between the NT-proANP values in the aqueous humor and the plasma of the POAG patients, as well as between the NT-proANP values in the aqueous humor and the intraocular pressure.

CONCLUSIONS

We identified an association between the levels of NT-proANP in the plasma and the aqueous humor with POAG. Our data support the idea of the involvement of NP system in the development of POAG and highlight ANP as a possible biomarker of glaucoma.

Regulation of intraocular pressure by soluble and membrane guanylate cyclases and their role in glaucoma

Glaucoma is a progressive optic neuropathy characterized by visual field defects that ultimately lead to irreversible blindness (Alward, 2000; Anderson et al., 2006). By the year 2020, an estimated 80 million people will have glaucoma, 11 million of which will be bilaterally blind. Primary open-angle glaucoma (POAG) is the most common type of glaucoma. Elevated intraocular pressure (IOP) is currently the only risk factor amenable to treatment. How IOP is regulated and can be modulated remains a topic of active investigation. Available therapies, mostly geared toward lowering IOP, offer incomplete protection, and POAG often goes undetected until irreparable damage has been done, highlighting the need for novel therapeutic approaches, drug targets, and biomarkers (Heijl et al., 2002; Quigley, 2011). In this review, the role of soluble (nitric oxide (NO)-activated) and membrane-bound, natriuretic peptide (NP)-activated guanylate cyclases that generate the secondary signaling molecule cyclic guanosine monophosphate (cGMP) in the regulation of IOP and in the pathophysiology of POAG will be discussed.

Ghrelin in ocular pathophysiology: from the anterior to the posterior segment

Ghrelin is a 28 amino acid acylated peptide produced in several organs that binds the growth hormone secretagogues receptor type 1a (GHSR-1a). It acts over a wide range of systems, e.g. the endocrine, cardiovascular, musculoskeletal and immune systems and the eye. The aim of this work is to review the physiologic and pathologic implications of the ghrelin-GHSR-1a in the eye. A systematic revision of studies published between 2000 and 2013 in English, Spanish or Portuguese in MEDLINE, EMBASE and Scopus was performed. Search words used included: ghrelin, GHSR-1a, ocular production, iris muscular kinetics, ciliary body, glaucoma, retinopathy and uvea. The production of ghrelin by the ocular tissue has been detected both in the anterior and posterior segments, as well as the presence of GHSR-1a. This peptide promotes the relaxation of the iris sphincter and dilator muscles, being this effect independent from GHSR-1a and dependent on prostaglandins release in the first case and dependent on GHSR-1a in the second. Regarding ocular pathology, ghrelin levels in the aqueous humor appear to be decreased in individuals with glaucoma. Moreover, ghrelin has been shown to decrease the intraocular pressure in animal models of ocular hypertension through GHSR-1a. In the posterior segment, the ghrelin-GHSR-1a system interferes with the development of oxygen-induced retinopathy, being protective in the vaso-obliterative phase and deleterious in the vaso-proliferative stage of the disease. Thus, the ghrelin-GHSR-1a system presents as a possible local regulatory mechanism in the eye, with pathophysiological implications, constituting a target for future clinical and therapeutic research and interventions.

Stimulation of aquaporin-mediated fluid transport by cyclic GMP in human retinal pigment epithelium in vitro

PURPOSE

The retinal pigment epithelium (RPE) expresses aquaporin-1 (AQP1) and components of the natriuretic peptide signaling pathway. We hypothesized that stimulation of the natriuretic signaling pathway in RPE with atrial natriuretic peptide (ANP) and with membrane-permeable analogs of cGMP would induce a net apical-to-basal transport of fluid.

METHODS

The hypothesis was tested using human RPE cultures that retain properties seen in vivo. Confluent monolayers were treated with ANP or membrane-permeable cGMP analogs in the presence of anantin, H-8, and an AQP1 inhibitor, AqB013. Fluid movement from the apical to basal chambers was measured by weight and used to calculate net fluid transport.

RESULTS

Our results demonstrated a 40% increase in net apical-to-basal fluid transport by ANP (5 μM) that was inhibited completely by the ANP receptor antagonist anantin and a 60% increase in net apical-to-basal fluid transport in response to the extracellularly applied membrane-permeable cGMP analog pCPT-cGMP (50 μM), which was not affected by the protein kinase G inhibitor H-8. The aquaporin antagonist AqB013 (20 μM) inhibited the cGMP-stimulated RPE fluid flux.

CONCLUSIONS

The effect of cGMP is consistent with an enhancement of the net fluid flux in RPE mediated by AQP1 channels. Pharmacologic activation of cGMP signaling and concomitant stimulation of fluid uptake from the subretinal space could offer insights into a new approach to treating or reducing the risk of retinal detachment.

Central and peripheral forms of C-type natriuretic peptide (CNP): evidence for differential regulation in plasma and cerebrospinal fluid

Aminoterminal proCNP (NTproCNP), a stable product of CNP gene expression and readily measured in human plasma, provides a new approach to studies of CNP which is rapidly degraded at source. CNP is detectable in human CSF but the presence and proportions of NTproCNP in CSF are unknown. Since CNP is widely expressed throughout the CNS, we hypothesized that the ratio of NTproCNP to CNP in CSF is greatly increased when compared to plasma and that CSF CNP peptides may contribute to their concentrations in the systemic circulation. Concurrent plasma and CSF concentrations of CNP forms were measured in 51 subjects undergoing spinal anesthesia for arranged orthopedic procedures. Elevated concentrations of NTproCNP (1045 ± 359 pmol/L), characterized by HPLC-RIA, were found in CSF and greatly exceeded those of CNP (7.9 ± 3.2 pmol/L). The ratio of NTproCNP to CNP in CSF (145 ± 55) was much higher than in plasma (31 ± 27). A significant inverse relation was found between plasma and CSF CNP concentrations (r = -0.29, p < 0.05). cGMP and neprilysin were unrelated to CNP levels in CSF. We conclude that CNP is differentially regulated across the brain in normal health. Despite markedly elevated levels of NTproCNP in CSF, it is unlikely that these contribute to systemic levels in healthy adults. Identifying NTproCNP as the dominant CNP form in CSF opens up the possibility of its use in future studies exploring CNP regulation within the CNS and possible applications in the diagnosis and monitoring of subjects with central neural disorders.

The relation between intraocular pressure change and plasma natriuretic peptide under simulated hypobaric conditions

Purpose:

To ascertain whether the changes in intraocular pressure (IOP) that occur during hypobaric hypoxic exposure are related to plasma N-terminal pro-brain natriuretic peptide (BNP) levels.

Materials and Methods:

The study group comprised 26 healthy participants (all male, mean age 23.1 years). IOP was measured at local ground level, (792 m above sea level), then while in a chamber providing hypobaric hypoxic conditions (the subjects were exposed to a pressure equivalent to 9144 m for 1-3 min), and again after exit from the chamber. In each condition, the mean of three consecutive measurements of IOP was calculated for each eye. For BNP measurements, blood samples were drawn before the participants entered the chamber and just after they left the chamber.

Results:

IOP during hypobaric hypoxic exposure (18.00 ± 3.70 mmHg) was significantly greater than that before (15.66 ± 2.10 mmHg, P < 0.001) or after (16.10 ± 2.63 mmHg, P = 0.001) the exposure. IOP levels before and after the exposure were not significantly different (P = 0.136). Plasma BNP levels measured before and after exposure to hypobaric hypoxic conditions were not significantly different (P = 0.462).

Conclusion:

Plasma BNP levels did not change after short-term hypobaric hypoxic exposure, while the IOP increased. This increase may have been caused by some other systemic factors. As the hypobaric hypoxic conditions were reversed, IOP decreased to normal levels.

NPR-C is expressed in the cholinergic and dopaminergic amacrine cells in the rat retina

Natriuretic peptide receptor C (NPR-C) is known to bind all natriuretic peptides with similar affinity. Given their biological role it is interesting that natriuretic peptides and their activated guanylate cyclases (NPR-A and NPR-B) are expressed in retinal amacrine cells. The purpose of this study is to examine the presence of NPR-C in the rat retina and its relationship to cholinergic and dopaminergic amacrine cells using immunofluorescence techniques. NPR-C immunoreactivity was found in several layers of the retina including the ganglion cell layer (GCL), inner nuclear layer (INL), outer plexiform layer (OPL), and inner segments of photoreceptors (IS). Immunofluorescence double-labeling showed the co-localization of NPR-C with tyrosine hydroxylase, a marker of dopaminergic cells, and with choline acetyltransferase (ChAT), a marker of cholinergic cells. These data suggest that natriuretic peptides may play a role in maintaining the retinal functions via interaction with NPR-C.

Skeletal contributions to plasma CNP forms: evidence from regional sampling in growing lambs

Unlike the cardiac circulating hormones, atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP), C-type natriuretic peptide (CNP) appears to be largely tissue-based and circulates at concentrations considered insufficient to affect organ function. Consistent with CNP's crucial role in regulating skeletal growth, serial studies in juveniles show that both plasma CNP and aminoterminal proCNP (NTproCNP) are highly correlated with growth velocity raising the possibility that skeletal tissues contribute to circulating concentrations of CNP forms during the growing period. Hypothesizing that venous blood draining from bone dense regions is relatively enriched in CNP, we have performed trans-organ regional blood sampling for measurement of CNP forms in 4-week-old lambs and compared the findings to simultaneous levels of ANP and BNP. Because bone growth and CNP synthesis are inhibited by glucocorticoids, identical studies were also undertaken in lambs pretreated with dexamethasone. Highly significant positive arterio-venous gradients of CNP were found across the head, heart, leg and foot. Dexamethasone significantly reduced the CNP arterio-venous gradient across the head and leg but not heart, liver or kidney. In contrast, there was no evidence of tissue secretion of ANP or BNP except across the heart, and no effect on these gradients from dexamethasone. These findings of CNP enrichment in samples from bone dense regions in growing lambs, and their selective reduction by dexamethasone, provide in vivo evidence linking plasma and skeletal tissue concentrations of CNP and further support the use of plasma CNP forms as markers of bone growth.

Expression of natriuretic peptide-activated guanylate cyclases by cholinergic and dopaminergic amacrine cells of the rat retina

Recently, the natriuretic peptides were detected in the cholinergic and dopaminergic amacrine cells of the retina. We performed immunofluorescence labeling of rat retinal sections to examine the immunoreactivity of natriuretic peptide-activated guanylate cyclases (NPR-A and NPR-B) in the rat retina, in particular whether they were localized to dopaminergic and cholinergic amacrine cells. NPR-A and NPR-B immunoreactivity was detected in several layers of the retina including amacrine cells. In amacrine cells, both NPR-A and NPR-B were co-localized with tyrosine hydroxylase, a marker of dopaminergic cells. NPR-B, but not NPR-A, was localized to amacrine cells expressing choline acetyltransferase (ChAT), a marker of cholinergic cells. These findings suggest that natriuretic peptides have different regulatory systems in dopaminergic and cholinergic amacrine cells in rat retina.

Immunohistochemical mapping of natriuretic peptide receptor-A in the brainstem of Macaca fascicularis

Natriuretic peptide receptor-A (NPR-A) mediates the biological effects of atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP), and is involved in maintaining cardiovascular homeostasis. In this immunohistochemical study we examined the distribution of NPR-A in the brainstem of the cynomolgus monkey. NPR-A immunoreactivity was localized to neurons in specific brainstem regions. NPR-A-immunoreactive perikarya were found in the red nucleus and the oculomotor nucleus in the midbrain, the parabrachial nucleus and the locus coeruleus in the pons, and the dorsal motor nucleus of the vagus, the hypoglossal nucleus, the cuneate nucleus, the gracile nucleus, the nucleus ambiguus, the lateral reticular nucleus, the reticular formation, and the inferior olivary nucleus in the medulla oblongata. Extensive networks of immunoreactive fibers were apparent in the red nucleus, the oculomotor nucleus, the principal sensory trigeminal nucleus, and the parabrachial nucleus. Double immunostaining revealed NPR-A immunoreactivity in cholinergic neurons of the parabrachial nucleus, the dorsal motor nucleus of vagus, the hypoglossal nucleus, and the nucleus ambiguus. However, there was no colocalization of NPR-A and tyrosine hydroxylase in the locus coeruleus. The wide anatomical distribution of NPR-A-immunoreactive structures suggests that natriuretic peptides, besides having a role in the central regulation of endocrine and cardiovascular homeostasis, may also mediate diverse physiological functions.

Natriuretic peptide receptor-A is functionally expressed on bullfrog retinal Müller cells

By the patch clamp technique, whole-cell currents induced by brain natriuretic peptide (BNP) from isolated bullfrog retinal Müller cells were studied. Application of 100 nM BNP induced a sustained inward current from these cells with a reversal potential of about 0 mV, and the current could be completely blocked by anantin, an antagonist of the A-type NP receptor (NPR-A) and CdCl(2), a blocker of cyclic nucleotide-gated (CNG) non-selective cation channels. Likewise, perfusion with the membrane-permeable cGMP analog 8-bromoguanosine-3',5'-cyclic monophosphate (8Br-cGMP) caused effects that are similar to those of BNP. Moreover, application of BNP failed to induce any current in the presence of 1 mM 8Br-cGMP. By calcium imaging, we further showed a significant increase in intracellular calcium levels ([Ca(2+)](i)) of all parts of Müller cells, including the endfoot, soma and processes following the perfusion of BNP, and the increase could be blocked by anantin. All these results suggest that NPR-A is expressed in bullfrog Müller cells, and activation of the receptor causes an increase of intracellular cGMP levels that activates CNG channels and thereby results in an increased calcium influx.

Ghrelin as a novel locally produced relaxing peptide of the iris sphincter and dilator muscles

Ghrelin is a recently described acylated peptide, which works as a somatosecretagogue and has described effects on the smooth, skeletal and cardiac muscle. We examined the production and effects of ghrelin on relaxation of the iris muscles. Contractile effects of 1-5 human ghrelin (frGhr, 10(-9)-6 x 10(-5)M) and 1-5 human des-octanoyl-ghrelin (d-frGhr; 10(-9)-6 x 10(-5)M) were tested on iris rabbit sphincter (n=11 frGhr; n=7 d-frGhr), dilator (n=6 frGhr; n=6 d-frGhr) and rat sphincter (n=6 frGhr; n=8 d-frGhr) precontracted muscles. On rabbit sphincter the effect of frGhr was also tested in presence of: i) L-NA (10(-5)M; n=7); ii) indomethacin (10(-5)M; n=7); iii) DLys(3)GHRP6 (10(-4)M; n=6); and iv) apamin+carybdotoxin (10(-6)M; n=6). Furthermore, on rabbit dilator the effect of frGhr was tested in presence of DLys(3)GHRP6 (10(-4)M; n=7). Finally, ghrelin mRNA production was assessed by "in situ" hybridization in Wistar rat eyes (n=8). In all muscles, frGhr promoted a concentration-dependent relaxation, maximal at 6 x 10(-5)M, 1.5-3 min after its addition, decreasing tension by 34.1+/-12.1%, 25.8+/-4.8% and 52.1+/-10.3% in the rabbit sphincter, dilator and rat sphincter, respectively. In the rabbit sphincter the relaxing effects of frGhr were: (i) enhanced in presence of DLys(3)GHRP6 (118.1+/-21.1%); (ii) blunted by indomethacin; and (iii) not altered by apamin+carybdotoxin (36.4+/-14.4%) or L-NA (52.4+/-11.4%). Relaxing effects of d-frGhr in rabbit (43.3+/-5.2%) and rat (77.1+/-15.3%) sphincter muscles were similar to those of frGhr. In rabbit dilator muscle, d-frGhr did not significantly alter active tension and the relaxing effect of frGhr was blunted by GHSR-1a blockage. Ghrelin mRNA was identified in iris posterior epithelium. In conclusion, ghrelin is a novel, locally produced, relaxing agent of iris dilator and sphincter muscles, an effect that is mediated by GHSR-1a in the former, but not in the latter. Furthermore, in the sphincter it seems to be mediated by prostaglandins, but not by NO or K(Ca) channels.

Atrial natriuretic peptide inhibits osmotical glial cell swelling in the ischemic rat retina: Dependence on glutamatergic-purinergic signaling

Atrial natriuretic peptide (ANP) is a regulator of the water and electrolyte content in the brain which also mediates cell volume homeostasis. Here, we determined whether the expression of ANP in the retina of the rat undergoes changes during ischemia-reperfusion, and whether ANP affects the osmotic swelling of Müller glial cells in postischemic retinas under hypotonic conditions. Transient retinal ischemia was induced by elevation of the intraocular pressure above systolic blood pressure for 1h. At 1 and 3 days after reperfusion, there was an increased content of ANP protein in the retina, as determined by Western blotting. The increase of the retinal ANP content was markedly reduced when triamcinolone acetonide (10 mM in 2 microl vehicle) was intravitreally injected before ischemia. ANP inhibited the osmotic swelling of Müller cell somata in retinal slices. The effect of ANP was mediated by activation of NP receptors expressed by retinal neurons which evoked a release of glutamate. The stimulation of metabotropic glutamate receptors expressed by Müller cells evoked an autocrine purinergic signaling mechanism that resulted in the opening of K(+) and Cl(-) channels; the ion efflux counteracted the osmotic swelling of Müller cells. It is concluded that the expression of ANP is transiently upregulated in the postischemic retina of the rat. The increased expression of ANP may represent a part of the retinal response to transient ischemia and may inhibit cytotoxic glial cell swelling.

Modulation by brain natriuretic peptide of GABA receptors on rat retinal ON-type bipolar cells

Natriuretic peptides (NPs) may work as neuromodulators through their associated receptors [NP receptors (NPRs)]. By immunocytochemistry, we showed that NPR-A and NPR-B were expressed abundantly on both ON-type and OFF-type bipolar cells (BCs) in rat retina, including the dendrites, somata, and axon terminals. Whole-cell recordings made from isolated ON-type BCs further showed that brain natriuretic peptide (BNP) suppressed GABAA receptor-, but not GABAC receptor-, mediated currents of the BCs, which was blocked by the NPR-A antagonist anantin. The NPR-C agonist c-ANF [des(Gln18, Ser19, Gln20, Leu21, Gly22)ANF(4-23)-NH2] did not suppress GABAA currents. The BNP effect on GABAA currents was abolished with preincubation with the pGC-A/B antagonist HS-142-1 but mimicked by application of 8-bromoguanosine-3',5'-cyclomonophosphate. These results suggest that elevated levels of intracellular cGMP caused by activation of NPR-A may mediate the BNP effect. Internal infusion of the cGMP-dependent protein kinase G (PKG) inhibitor KT5823 essentially blocked the BNP-induced reduction of GABAA currents. Moreover, calcium imaging showed that BNP caused a significant elevation of intracellular calcium that could be caused by increased calcium release from intracellular stores by PKG. The BNP effect was blocked by the ryanodine receptor modulators caffeine, ryanodine, and ruthenium red but not by the IP3 receptor antagonists heparin and xestospongin-C. Furthermore, the BNP effect was abolished after application of the blocker of endoplasmic reticulum Ca2+-ATPase thapsigargin and greatly reduced by the calmodulin inhibitors W-7 and calmidazolium. We therefore conclude that the increased calcium release from ryanodine-sensitive calcium stores by BNP may be responsible for the BNP-caused GABAA response suppression in ON-type BCs through stimulating calmodulin.

Neuroprotective effect of atrial natriuretic peptide against NMDA-induced neurotoxicity in the rat retina

Atrial natriuretic peptide (ANP) can regulate aqueous humor production in the eye and has recently been suggested to play some functional roles in the retina. It has also been reported that ANP increases tyrosine hydroxylase (TH) mRNA levels and intracellular dopamine levels in PC12 cells. The effect of ANP on TH levels and the role of ANP in retinal excitotoxicity remain unknown. In this study, we investigated the effects of ANP on TH expression and dopamine levels in rat retina after intravitreal injection of NMDA. Immunohistochemistry localized natriuretic peptide receptor-A (NPRA) in the ganglion cell layer (GCL), the inner nuclear layer (INL) and the outer nuclear layer (ONL) in the rat retina. Quantitative real-time PCR and Western blot analysis showed a dramatic reduction in retinal TH levels 5 days after NMDA injection, while ANP, at a concentration of 10(-4) M, ameliorated this reduction in TH mRNA and TH protein levels. High-performance liquid chromatography (HPLC) analysis showed that NMDA reduced dopamine levels in the retina, and that ANP attenuated this reduction. Moreover, morphological analysis showed that ANP ameliorated NMDA-induced neurotoxicity through NPRA. The ameliorative effect of ANP was inhibited by a dopamine D(1) receptor antagonist. These results suggest that ANP may have a neuroprotective effect through possible involvement of dopamine induction.

Inhibition of NHE-1 Na+/H+exchanger by natriuretic peptides in ocular nonpigmented ciliary epithelium

The natriuretic peptides (NPs) atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP), and C-type natriuretic peptide (CNP) display hypotensive effects in the mammalian eye by lowering the intraocular pressure (IOP), a function that is mediated by the bilayer ocular ciliary epithelium (CE), in conjunction with the trabecular meshwork. ANP regulates Na+/H+exchanger (NHE) activity, and inhibitors of NHE have been shown to lower IOP. We examined whether NPs influence the NHE activity of the CE, which is comprised of pigmented (PE) and nonpigmented (NPE) epithelial cells, by directly recording the rate of intracellular pH (pHi) recovery from its inner NPE cell layer. NPs inhibited, in a dose-dependent manner (1–100 nM), the rate of pHirecovery with the order of potency CNP > ANP > BNP, indicative that this inhibition is mediated by the presence of NPR type B receptors. 8-Bromo-cGMP (8-BrcGMP), a nonhydrolyzable analog of cGMP, mimicked NPs in inhibiting the rate of Na+-dependent pHirecovery. In contrast, ethylisopropyl amiloride (EIPA, 100 nM) or amiloride (10 μM) completely abolished the pHirecovery by NHE. 18α-Glycyrrhetinic acid (18α-GA), a gap junction blocker, attenuated the inhibitory effect of CNP on the rate of pHirecovery, suggesting that NHE activity in both cell layers of the CE is coregulated. This interpretation was supported, in part, by the coexpression of NHE-1 isoform mRNA in both NPE and PE cells. The mechanism by which the inhibitory effect of NPs on NHE-1 activity might influence the net solute movement or fluid transport by the bilayer CE remains to be determined.

Bremazocine increases C-type natriuretic peptide levels in aqueous humor and enhances outflow facility

A relatively selective agonist of kappa opioid receptors (KOR), bremazocine (BRE), lowers intraocular pressure in rabbits, in part, by increasing natriuretic peptide levels in aqueous humor and by enhancing total outflow facility (TOF). Natriuretic peptide (NP) levels [atrial NP (ANP), brain NP (BNP), and C-type NP (CNP)] were measured in aqueous humor of rabbits either by radioimmunoassay or enzyme immunoassay. TOF was determined in rabbits by two-level constant pressure perfusion of the anterior chamber. Experimental regimens included topical treatment with BRE in the presence or absence of KOR antagonist (norbinaltorphimine), protein kinase C inhibitor (chelerythrine), and natriuretic peptide receptor antagonist (isatin). The rank order of basal NP levels in aqueous humor of rabbits was BNP CNP > ANP. Topical administration of BRE (1-100 microg) caused dose-related elevations of CNP levels in aqueous humor that were inhibited by topical pretreatment with either norbinaltorphimine (100 microg, bilaterally) or chelerythrine (10 microg, bilaterally). Topically administered BRE (100 microg) also elevated levels of ANP and BNP in aqueous humor and evoked an 80% increase in TOF. The increase in TOF was antagonized by topical pretreatment with either norbinaltorphimine (100 microg, bilaterally) or isatin (100 microg, bilaterally). Bremazocine induced an increase in NP (ANP, BNP, and CNP) levels and TOF in rabbits by activating KOR. The increase in CNP levels elicited by BRE was inhibited by norbinaltorphimine and chelerythrine; therefore, this event is most likely mediated by a KOR-linked activation of protein kinase C. These data provide evidence that the increase in TOF elicited by BRE was mediated by a KOR-activated paracrine effect of NPs on tissues within ocular outflow tract(s).

Localisation of endothelin-1 mRNA expression and immunoreactivity in the retina and optic nerve from human and porcine eye. Evidence for endothelin-1 expression in astrocytes

We have investigated the localisation of endothelin-1 (ET-1) mRNA and ET-1-like immunoreactivity in retina and anterior portion of optic nerve from human and porcine eyes. In situ hybridisation method revealed expression of ET-1 mRNA mainly in the innermost layers of the retinas, in the retinal pigment epithelium cells as well as in the astrocytes of the optic nerve. Immunohistochemical studies showed that ET-1-like immunoreactivity appeared in the same regions where ET-1 mRNA was expressed as well as in the inner nuclear layer and in the inner segments of photoreceptors. In the nerve fibre and ganglion cell layers, astrocytes expressed both glial fibrillary acidic protein and ET-1 proteins suggesting that these cells may secrete ET-1. Expression of ETA and ETB receptors in human retina were demonstrated by reverse transcription-polymerase chain reaction. Our results demonstrated expression of ET-1 in glial, neural and vascular components of retina and optic nerve from human and porcine eyes.

Natriuretic peptide signalling: molecular and cellular pathways to growth regulation

The natriuretic peptides (NPs) constitute a family of polypeptide hormones that regulate mammalian blood volume and blood pressure. The ability of the NPs to modulate cardiac hypertrophy and cell proliferation as well is now beginning to be recognized. The NPs interact with three membrane-bound receptors, all of which contain a well-characterized extracellular ligand-binding domain. The R1 subclass of NP receptors (NPR-A and NPR-B) contains a C-terminal guanylyl cyclase domain and is responsible for most of the NPs downstream actions through their ability to generate cGMP. The R2 subclass lacks an obvious catalytic domain and functions primarily as a clearance receptor. This review focuses on the signal transduction pathways initiated by ligand binding and other factors that help to determine signalling specificities, including allosteric factors modulating cGMP generation, receptor desensitization, the activation and function of cGMP-dependent protein kinase (PKG), and identification of potential nuclear or cytoplasmic targets such as the mitogen-activated protein kinase signalling (MAPK) cascade. The inhibition of cardiac growth and hypertrophy may be an important but underappreciated action of the NP signalling system.

Natriuretic peptide system in the rat lacrimal gland

The presence and characteristics of the natriuretic peptides and their receptors in the rat exorbital lacrimal gland were investigated. Serial dilution curves of the gland extracts were parallel to the standard curves of synthetic atrial natriuretic peptide (ANP) or C-type natriuretic peptide (CNP). Immunoreactive ANP or CNP in the gland extracts co-eluted with authentic ANP or CNP, and their contents were 4.95 +/- 0.60 and 2.87 +/- 0.53 pg mg(-1)protein (quadruplicate), respectively. By immunohistochemistry, strong immunoreactivities of ANP and CNP were co-localized in the tubules and excretory ducts of the gland, and moderate immunoreactivities were found in the myoepithelial cells and acini. Productions of guanosine 3',5'-cyclic monophosphate by particulate guanylyl cyclase in the gland membranes were stimulated by natriuretic peptides in a dose-dependent manner, and that by CNP was larger than by ANP. Messenger RNAs for ANP, CNP and their receptors were detected by reverse transcription-polymerase chain reaction. These results indicate that natriuretic peptides and their specific receptors are found in the rat lacrimal gland. Therefore, it is suggested that natriuretic peptide system may play physiological roles in the rat lacrimal gland.

Immunohistochemical localization of C-type natriuretic peptide in the rat submaxillary salivary gland

To define the localization and characteristics of C-type natriuretic peptide (CNP) in the rat submaxillary gland, immunohistochemistry and gel permeation-high-performance liquid chromatography were used. Immunoreactive (IR)-CNP was localized in cells of the granular convoluted tubule, striated duct and endothelial cells of the capillary, where atrial natriuretic peptide (ANP) was colocalized in consecutive sections, but not in acini. Gland extracts co-eluted with synthetic CNP and its content was 60.3+/-4.9 pg/mg protein (n=4). Molecular profiles of immunoreactive material showed two peaks corresponding to synthetic CNP((1-53)) and CNP((1-22)). These results indicate that CNP is colocalized with ANP in the duct and endothelial cells of the rat submaxillary gland. Therefore, CNP may have a physiological role in the submaxillary gland by interacting with ANP and/or other biologically active substances in the ducts and granular convoluted tubule cells.

Differential gene expression in the human ciliary epithelium

The generation of expression and subtractive libraries from the ocular ciliary body and cultured ciliary epithelial cells has been instrumental in the cloning, identification and characterization of many genes which, overall reflect a representative profile of transcripts expressed in ciliary nonpigmented, ciliary pigmented and ciliary muscle cells. The cell-specific expression of some of these genes (i.e. a neurotrophic factor, a gene associated with juvenile open glaucoma, and a visual component) reveal a degree of cell differentiation with a diversity of functions and properties higher than previously thought. The protection from light-induced oxidative reactions, free radicals and detoxification, may be partially attributed to the high level of expression in the ciliary epithelium of antioxidative enzymes (i.e., glutathione S-transferase, glutathione peroxidases, selenoprotein-P). The expression of genes encoding plasma proteins (i.e., complement component C4, alpha2-macroglobulin, apolipoprotein D) is in contrast with the view that plasma proteins in aqueous humor are synthesized outside the eye (i.e., liver). The identification of neuropeptide-processing enzymes (i.e., prohormone convertases, carboxypeptidase E, peptidyl-glycine-alpha-amidating monoxigenase), neuropeptides (i.e., secretogranin II, neurotensin) and regulatory peptides (i.e., atrial natriuretic peptide and angiotensinogen) with hypertensive and hypotensive activities provide the molecular basis to support the view that the ciliary epithelium is a neuroepithelium with neuroendocrine functions. We propose a working model to demonstrate that aqueous humor and intraocular pressure are under neuroendocrine control through regulatory peptides synthesized and released by the ciliary epithelium and targeting the peptide producing cells at the inflow system by an autocrine mechanism and/or cells at the outflow system (i.e., trabecular meshwork cells) by a paracrine mechanism. Finally, we hypothesize that these mechanisms could be entrained in the light-dark cycle following the circadian rhythm of aqueous humor and intraocular pressure.

Functional expression of components of the natriuretic peptide system in human ocular nonpigmented ciliary epithelial cells

The expression of the natriuretic peptide system in the human ocular ciliary epithelium (CE) and in cultured nonpigmented (NPE) ciliary epithelial cells was examined. By RT-PCR and DNA sequencing, we demonstrated that the CE and NPE cells express mRNA for (i) ANP; (ii) BNP; (iii) NPR-A, NPR-B, and NPR-C receptors; and (iv) the neutral endopeptidase 24.11. Radioimmunoassay results indicate that BNP is secreted by cultured NPE cells at much higher levels than ANP. NPR-A and NPR-B receptors elicited a cGMP response to ANP, BNP, and CNP, in a rank order of potency (CNP >> ANP >/= BNP), indicative that the NPR-B receptor is predominant in NPE cells. A71915, an inhibitor of NPR-A activity, attenuated (65-75%) cGMP response to ANP and BNP, but not to CNP. C-ANP4-23 elicited an inhibitory effect (30-37%) on basal levels of cAMP in NPE cells and on forskolin NPE-treated cells, indicative that the NPR-C receptor is functional in these cells. PMA induced, in NPE cells, a long-term downregulation (75-85%) of NPR-C receptor mRNA, but not of NPR-A or NPR-B receptor mRNA, suggesting a differential regulation of NPR-C receptor mRNA via activation of PKC. Collectively, our data provide molecular evidence that all the components of the natriuretic peptide system with the exception of CNP are coexpressed in the ocular NPE ciliary epithelial cells, where they may function as local autocrine/paracrine modulators to influence eye pressure.

Natriuretic peptide receptors, NPR-A and NPR-B, in cultured rabbit retinal pigment epithelium cells

We tried to detect natriuretic peptide (NP) receptor (NPR-A and NPR-B) mRNAs in cultured rabbit retinal pigment epithelium (RPE) cells and examined the regulation of their expression in relation to subretinal fluid absorption or RPE cell proliferation. RPE cells from 2-4 passages were grown to confluence on microporous membranes and analyzed for levels of expression of receptor mRNAs by quantitative RT-PCR and Northern blotting. The expression of NPR-B mRNA was approximately tenfold higher than that of NPR-A mRNA. The expression of NPR-A mRNA was not affected by treatments that may change subretinal fluid transport, while that of NPR-B mRNA was inhibited by transmitters involved in light- and dark-adaptation such as dopamine and melatonin. Expression of NPR-B mRNA was also suppressed by platelet-derived growth factor and transforming growth factor-beta. Furthermore, atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP), ligands for NPR-A and B, respectively, inhibited the proliferation of RPE cells, as analyzed by incorporation of [3H]thymidine. These findings suggest that ANP may be involved in constitutive absorption of subretinal fluid and that NPs form an important regulatory system of proliferation in RPE cells.

Type B and type C natriuretic peptide receptors modulate intraocular pressure in the rabbit eye

We investigated (1) the in vivo functional significance of the type B (ANP(B)) and type C (ANP(C)) natriuretic peptide receptors in the rabbit eye by evaluating the effect of intracameral administration of C-type natriuretic peptide (CNP) and C-ANP-(4-23) on intraocular pressure, and (2) the action of CNP on guanylate cyclase activity in the rabbit ciliary process membranes. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) were also studied for comparison. We demonstrated that the natriuretic peptides decrease intraocular pressure and stimulate guanylate cyclase activity, CNP being the most potent. The duration of the effect of C-ANP-(4-23) on intraocular pressure reduction was almost 9-fold that of the BNP and 20-fold that of ANP and CNP effect. This ligand increased threefold the immunoreactive natriuretic peptides levels in aqueous humour. Our data demonstrate the presence of functional ANP(A) and ANP(B) receptors in the rabbit eye and that the ANP(C) receptor modulates the concentration of the natriuretic peptides in the aqueous humour.

Higher proportions of type C than of types A and B natriuretic peptide receptors exist in the rat ciliary body

We investigate the interaction of atrial natriuretic peptide (ANP) brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) with their receptors (NPRA, NPRB and NPRC), as well as the proportion and localization of those receptors in the rat ciliary body. Binding assays and affinity cross-linking experiments demonstrated the presence of the NPRC receptor type. However, the three natriuretic peptides stimulate the guanylate cyclase activity in the ciliary body membranes suggesting the presence of the NPRA and NPRB receptor type. Microautoradiographic data show that the NPRs are localized in the whole ciliary body. Our results indicated that NPRC is the most prominent receptor type in this tissue.

Brain natriuretic peptide: identification of a second natriuretic peptide in human aqueous humour

AIMS/BACKGROUND

To measure aqueous humour levels of brain natriuretic peptide (BNP) and atrial natriuretic peptide (ANP) in humans. To compare peptide levels in glaucomatous and control eyes to test the hypothesis that these peptides are increased in glaucoma. BNP and ANP are cyclic endopeptides whose principal biological effects are natriuresis and vasodilatation. Experimental glaucoma in animal models results in elevated aqueous ANP. Intravenous ANP administration in both animals and humans causes lowering of intraocular pressure (IOP). There are equivocal data to support a role for ANP in IOP regulation in human eyes. There are as yet no published data on BNP in human aqueous humour.

METHOD

This was a case-control study. Cases were primary open angle, pseudoexfoliation, and mixed mechanism glaucoma eyes undergoing trabeculectomy. Controls were cataract extraction eyes. There were 47 trabeculectomy eyes (44 patients) and 47 cataract extraction eyes (46 patients) matched for age, sex, race, systemic medications, and type of anaesthetic. 100-200 microliters of aqueous humour were aspirated by paracentesis as the first step in the surgical procedure. Peptide levels were later measured by radioimmunoassay.

RESULTS

The presence of BNP and ANP in human aqueous humour was confirmed. BNP was present in higher concentrations than ANP. BNP levels tended to be greater in control eyes--glaucoma median 56.5 (range 0-3526.5) pg/ml versus control median 65.16 (range 0-1788) pg/ml (Wilcoxon signed rank test p = 0.78). ANP levels tended to be greater in glaucoma eyes than in controls: glaucoma median 3 (range 0-68.5) pg/ml versus control median 0 (range 0-60) pg/ml (Wilcoxon signed rank test p = 0.82). ANP and BNP were log linearly related in both groups (r glaucoma group = 0.961, r control group = 0.894).

CONCLUSION

This is the first report of BNP and ANP in human aqueous humour. Peptide levels did not differ significantly between glaucoma and cataract extraction eyes. A linear relation between log BNP and ANP was found. Further studies are required to clarify the role of these peptides in aqueous humour production and IOP regulation.

Ocular hypotension induced by intravitreally injected C-type natriuretic peptide

The purpose of the study is to determine if intravitreal injection of c-type natriuretic peptide (CNP) affects intraocular pressure (IOP), aqueous humor dynamics and guanosine 3',5'-cyclic monophosphate (cGMP) concentration in the aqueous humor of the rabbit eye. Also we investigated whether CNP-like immunoreactivities (CNP-LI) were present in porcine aqueous humor and whether CNP-LI were detected in rabbit and porcine ciliary body. The IOP was measured after intravitreal injection of 2 pmol approximately 20 nmol CNP into rabbit eyes. Aqueous humor dynamics (aqueous humor flow, outflow facility, and uveoscleral outflow) and cGMP concentration in the aqueous humor were determined at approximately 6 hr after CNP injection. The CNP concentration in aqueous was measured by radioimmunoassay in porcine eye, and CNP-LI were detected with a monoclonal antibody in porcine and rabbit eyes. Intravitreally injected CNP caused IOP reduction in a dose-dependent manner (P<0.0001) and the maximum effect was observed at 4 approximately 6 hr. CNP increased total outflow facility by approximately 35%, but did not affect aqueous humor flow or uveoscleral outflow. The cGMP concentration in the aqueous of CNP-treated eyes was about 4- to 14-fold higher than that in the contralateral untreated eyes. CNP concentration in aqueous was about 2-fold higher than that in plasma, and CNP-LI were found in non-pigmented epithelium of the ciliary body of both rabbit and porcine eyes. CNP may play an important role in the regulation of IOP.