The intraocular pressure response of human atrial natriuretic factor in glaucoma

A common humoral background of intraocular and arterial blood pressure dysregulation

BACKGROUND

It has been postulated that intraocular pressure, an important glaucoma risk factor, correlates positively with arterial blood pressure (blood pressure). However, results of experimental and clinical studies are often contradictory. It is hypothesized that, in some hypertensive patients, disturbances in intraocular pressure regulation may depend on biological effects of blood borne hormones underlying a particular type of hypertension, rather than on blood pressure level itself.

REVIEW

This review compares the effects of hormones on blood pressure and intraocular pressure, in order to identify a hormonal profile of hypertensive patients with an increased risk of intraocular pressure surge. The PUBMED database was searched to identify pre-clinical and clinical studies investigating the role of angiotensin II, vasopressin, adrenaline, noradrenaline, prostaglandins, and gaseous transmitters in the regulation of blood pressure and intraocular pressure.

RESULTS

Studies included in the review suggest that intraocular and blood pressures often follow a different pattern of response to the same hormone. For example, vasopressin increases blood pressure, but decreases intraocular pressure. In contrast, high level of nitric oxide decreases blood pressure, but increases intraocular pressure.

CONCLUSIONS

Arterial hypertension is associated with altered levels of blood borne hormones. Contradicting results of studies on the relationship between arterial hypertension and intraocular pressure might be partially explained by diverse effects of hormones on arterial and intraocular pressures. Further studies are needed to evaluate if hormonal profiling may help to identify glaucoma-prone patients.

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.

Biology of natriuretic peptides and their receptors

Increasing evidence suggests that natriuretic peptides (NPs) play diverse roles in mammals, including renal hemodynamics, neuroendocrine, and cardiovascular functions. Collectively, NPs are classified as hypotensive hormones; the main actions of NPs are implicated in eliciting natriuretic, diuretic, steroidogenic, antiproliferative, and vasorelaxant effects, important factors in the control of body fluid volume and blood pressure homeostasis. One of the principal loci involved in the regulatory actions of NPs is their cognate plasma membrane receptor molecules, which are activated by binding with specific NPs. Interaction of NPs with their receptors plays a central role in physiology and pathophysiology of hypertension and cardiovascular disorders. Gaining insight into the intricacies of NPs-specific receptor signaling pathways is of pivotal importance for understanding both hormone-receptor biology and the disease states arising from abnormal hormone receptor interplay. During the last decade there has been a surge in interest in NP receptors; consequently, a wealth of information has emerged concerning molecular structure and function, signaling mechanisms, and use of transgenics and gene-targeted mouse models. The objective of this present review is to summarize and document the previous findings and recent discoveries in the field of the natriuretic peptide hormone family and receptor systems with emphasis on the structure-function relationship, signaling mechanisms, and the physiological and pathophysiological significance in health and disease.

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).

Elevation of atrial natriuretic peptide levels in aqueous humor of the rabbit by kappa opioid receptor agonists

Following the topical administration of three kappa agonists (bremazocine, spiradoline and ICI 204448) to rabbit eyes, aqueous humor samples were analyzed for levels of atrial natriuretic peptide (ANP). Bremazocine (BRE) and spiradoline (SPR) elevated aqueous ANP levels in a dose-dependent manner. In contrast, ICI had no significant effect on ANP levels in aqueous humor. Nor-binaltorphimine (nor-BNI), a selective kappa opioid receptor antagonist, was used to assess kappa opioid receptor involvement and glibenclamide, an ATP-sensitive K+ channel blocker, was used to test for the role of ATP-sensitive potassium channels in ANP release. Pretreatment with nor-BNI antagonized the increases in ANP levels observed with both BRE and SPR. Likewise, glibenclamide suppressed the stimulation of ANP secretion by bremazocine. In summary, BRE and SPR increased ANP levels in aqueous humor of rabbits, in part, via activation of K+(ATP) channels that are assumed to be associated with kappa opioid receptors.

The natriuretic peptides: universal volume controllers

Since the discovery of the natriuretic effect of atrial natriuretic peptide (ANP), a family of other natriuretic peptides similar to ANP were isolated, including atriopeptin, vessel dilator, long-acting natriuretic peptide, urodilatin, and brain natriuretic peptide (BNP) to name a few. ANP was noted to possess natriuretic and diuretic properties that controlled increases in intravascular volume. ANP was also found to be elevated in conditions of increased intraocular pressure and biliary obstruction. BNP was found to be elevated in conditions of increased intracranial pressure, pointing towards its role in controlling cerebrospinal fluid volume. While at the cellular level, ANP controlled individual cell size. This makes the natriuretic peptides not only controllers of intravascular volume, but also modulators of a myriad of cavity volumes down to the control of individual cell volume.

Atriopeptin, sodium azide and cyclic GMP reduce secretion of aqueous humour and inhibit intracellular calcium release in bovine cultured ciliary epithelium

This study examined the involvement of cyclic GMP, protein kinase G and intracellular Ca2+ movements in the modulation of aqueous humour formation. Using the bovine arterially-perfused eye preparation, drug effects on intraocular pressure and aqueous humour formation rate were measured by manometry and fluorescein dilution, respectively. Drug effects on intracellular [Ca2+] were determined by fura-2 fluorescence ratio technique in nontransformed, cultured ciliary epithelium. Intra-arterial injection of atriopeptin (50 pmol) or sodium azide (10 nmol) produced significant reduction in aqueous humour formation (>38%). This was blocked by selective inhibition (KT-5823) of protein kinase G, but not by selective inhibition (KT-5720) of protein kinase A. Reductions of intraocular pressure produced by atriopeptin or azide were almost completely blocked by KT-5823. ATP (100 microM) caused rapid, transient increase in intracellular Ca2+ followed by a slow decline and prolonged plateau. This response showed concentration-dependent inhibition by atriopeptin, azide or 8-bromo cyclic GMP, and this inhibition of the rapid (peak) Ca2+ increase was enhanced by zaprinast (100 microM; phosphodiesterase inhibitor). KT-5823 blocked the suppression of the peak Ca2+ response but not suppression of the plateau. Arterial perfusion of ATP (0.1-100 microM) produced a concentration-dependent decrease in aqueous humour formation. Aqueous humour formation in the bovine eye can be manipulated through cyclic GMP, operating via protein kinase G. Close parallels appear when Ca2+ movements are modified by similar manipulations of cyclic GMP, suggesting that Ca2+ transients may play an important role in aqueous humour formation and that interplay occurs between cyclic GMP and Ca2+.

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.

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.

Non-lysosomal cycling pathway for atrial natriuretic peptide activated by protein kinase C in human NPE cells

Atrial natriuretic peptide (ANP) stimulates aqueous humor formation in primates, but the membrane-bound receptors which mediate this effect have not been well studied in the eye. Endocytosis of [125I]ANP bound to natriuretic peptide C receptors was characterized in fetal human nonpigmented ciliary epithelial (NPE) cells. [125I]ANP which bound to cells at 4 degreesC was detected in the cell interior after a temperature shift to 37 degreesC. Appearance of ligand within the cell peaked at 5 min, and then declined towards zero over 20 min. The endocytosis inhibitor phenylarsine oxide blocked the appearance of internalized ligand, whereas the lysosomotropic drug chloroquine had no effect on internalization but blocked subsequent loss of internalized ligand. Chloroquine also blocked the accumulation of degraded ligand in the extracellular medium. Treatment with phorbol 12-myristate, 13-acetate accelerated the loss of internalized ligand from cells and increased the accumulation of ligand in the extracellular medium. Ligand in the medium was also increased by dioctanoylglycerol but not by 4alpha phorbol didecanoate, an isomer which does not activate protein kinase C. The protein kinase inhibitors staurosporine and bisindolylymaleimide blocked the increase in ligand. Phorbol ester-stimulated loss of internalized ligand occurred in the presence of chloroquine. TCA precipitation of ligand in the extracellular medium showed that both degraded and undegraded [125I]ANP were present. However, in the presence of chloroquine only, undegraded ANP was detected in the medium, and phorbol esters stimulated its rate of appearance by approximately 2 fold. A similar stimulation occurred when cells containing internalized ligand, but stripped of membrane-bound ligand, were exposed to phorbol esters. The data suggest that ANP bound to natriuretic peptide C receptors on NPE cells is endocytosed, and that protein kinase C activates a non-lysosomal pathway for ANP retroendocytosis in these cells.

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.

Differential regulation of natriuretic peptide receptors on ciliary body epithelial cells

Atrionatriuretic peptide (ANP) lowers intraocular pressure in the eyes of humans and rabbits. We examined the effects of natriuretic peptides on cGMP formation and 125I-labelled-ANP binding to cultured cells derived from ciliary body epithelium, the site of aqueous humour formation in the eye. ANP, brain natriuretic peptide (BNP) and C-natriuretic peptide (CNP) at 1 microM stimulated cGMP formation 8.2(+/-1.2)-fold, 4.8(+/-0.6)-fold and 87.3(+/-12.1)-fold respectively. 125I-ANP bound to intact cells at a single site, with a dissociation constant KD=0.30+/-0.01 nM. BNP was as effective as ANP in displacing 125I-ANP, whereas CNP displaced label with a slightly higher IC50. 125I-ANP binding was displaced >95% by c-ANP, a specific ligand for natriuretic peptide C receptors (NPR-C). Cross-linking of 125I-ANP to cells labelled predominantly a protein of Mr 62000. These data suggest that 125I-ANP binding was primarily to NPR-C, whereas cGMP stimulation occurred primarily via natriuretic peptide B receptors (NPR-B). Vasopressin and histamine, both activators of the inositol phosphate/diacylglycerol phosphate pathway in non-pigmented ciliary epithelial cells, inhibited CNP stimulation of guanylate cyclase (NPR-B) and 125I-ANP binding (NPR-C) by 30-38%. Inhibition was mimicked by PMA, dioctanoylglycerol and phorbol didecanoate, whereas 4alpha phorbol didecanoate had no effect. Staurosporine and bisindolylmaleimide both blocked inhibition of 125I-ANP binding and cGMP formation by PMA. These results suggest that protein kinase C (PKC) down-regulates both NPR-B and NPR-C. PKC down-regulation of NPR-B varied inversely with CNP concentration. Inhibition by 1 microM PMA was 30.6(+/-4.0)% with 500 nM CNP, but 83.4(+/-8.8)% with 10 nM CNP, indicating that increasing CNP could partially overcome inhibition by PMA. Since extracellular CNP levels were not affected by PKC activation, the effect of PKC on NPR-B is best explained as a reduction in NPR-B affinity for CNP. NPR-C measured as 125I-ANP binding was likewise reduced 36.4(+/-5.1)% by exposure to PMA. In contrast with NPR-B inhibition, however, inhibition of NPR-C was due largely to a reduction in the number of receptor binding sites per cell rather than a reduction in receptor affinity for ligand. The data therefore suggest that both NPR-B and NPR-C are down-regulated by PKC, but that the mechanisms of down-regulation of the two receptors are different.

Atriopeptin lowers aqueous humor formation and intraocular pressure and elevates ciliary cyclic GMP but lacks uveal vascular effects in the bovine perfused eye

The effects of atriopeptin (AP) on intraocular pressure (IOP), aqueous humor formation (AHF) and ciliary cyclic GMP in bovine eyes perfused in vitro through a ciliary artery were investigated. AHF was monitored fluorophotometrically by perfusing the anterior chamber with Bárány's mock aqueous humor containing fluorescein. To study the effect of AP on cyclic GMP synthesis, eyes were injected with an intra-arterial bolus of AP prior to dissection of the ciliary processes. For comparison, individual ciliary processes, or isolated ciliary epithelial cells were incubated with AP. Cyclic GMP was extracted by homogenization, aqueous partition and column chromatography, and measured by radioimmunoassay (RIA). Decreases in IOP or AHF were seen approximately 15 min after the injection of AP and persisted throughout a 60-80 min perfusion. Arterial perfusion pressure was not significantly altered by AP, even when vascular tone was raised by adding noradrenaline to the perfusate. In whole eyes, ciliary cyclic GMP increased in response to AP. Cyclic GMP in isolated ciliary processes, or cultured ciliary epithelial cells incubated with AP also increased. We conclude that the decrease in IOP in the bovine eye by AP is due to a reduction in AHF, which is independent of any vascular effect. Ciliary cyclic GMP may be the intracellular mediator involved.

Natriuretic peptide receptors on human trabecular meshwork cells

The effects of natriuretic peptides on cGMP formation and [125I]ANP binding in human trabecular meshwork cells were investigated. CNP at 1 microM stimulated cGMP formation approximately 18-25 fold, with a half maximal effective concentration approximately 20-30nM. BNP at 1 microM stimulated approximately 7 fold, while ANP stimulated cGMP formation 2-fold at 1 microM but had little or no effect at concentrations below 1 microM. Displacement binding of [125I]ANP to intact TM cells in the presence of unlabeled ANP indicated a single binding site with a dissociation constant approximately 0.15nM.c-ANP, which binds specifically to natriuretic peptide C receptors, displaced > 95% [125I]ANP binding to surface receptor sites with a half-maximal effective concentration comparable to that of ANP or BNP. c-ANP had no inhibitory effect on CNP stimulation of cGMP formation. The data suggest that human TM cells possess natriuretic peptide B receptors as the primary guanylyl cyclase-containing subtype and C receptors as the numerically predominant subtype of natriuretic peptide receptors.

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

The rates of secretion and removal of aqueous humour are major determinants of intraocular pressure (IOP). The natriuretic peptides, atrial natriuretic peptide (ANP), brain natriuretic peptide (BNP) and C-type natriuretic peptide (CNP) are vasodilators with variable effects on electrolyte and water transport at sites such as the nephron. There is some evidence that they may also affect fluid balance in the eye. As a first step in understanding the function of these peptides in the eye, we have used the technique of cDNA amplification with the polymerase chain reaction to demonstrate the presence of mRNA transcripts encoding the three natriuretic peptide receptors (NPR-A, NPR-B and NPR-C) in the retina, choroid and ciliary process of the rat and rabbit eye. In addition we have observed a differential distribution of ANP, BNP and CNP mRNAs in ocular tissues suggesting that at least part of the natriuretic peptide immunoreactivity detected in the eye arises from local synthesis of peptide. Thus, the eye appears to be able to synthesize all the components of the natriuretic peptide system necessary to modulate IOP independently of changes in the plasma concentrations of these peptides.

Topically applied hydralazine: effects on systemic cardiovascular parameters, blood-aqueous barrier, and aqueous humor dynamics in normotensive humans

Local application of hydralazine has been found to alter intraocular pressure in animal eyes. This study was undertaken to determine, in normotensive humans, the effects of topically-applied hydralazine on systemic cardiovascular parameters, blood-aqueous barrier, and aqueous humor flow. Two different concentrations of hydralazine were used: 0.03%, and 0.1%. Twenty healthy normotensive subjects were studied. Blood pressure, pulse rate, and intraocular pressure were measured every hour for six hours after hydralazine or placebo was instilled into the conjunctival sac in a double-masked, randomized fashion. With either hydralazine dose, there was no significant change in systemic blood pressure or pulse. In addition, spectrophotometrically-measured polarization of fluorescence and flare failed to show any significant breakdown of the blood-aqueous barrier. In most individuals, application of hydralazine was followed by a brief, mild to moderate, conjunctival hyperemic response. Compared to placebo at the same time of day, a small increase in intraocular pressure was observed with 0.03% hydralazine (p < .05). At 0.1%, this increase tended to be less. There was no statistically significant difference in aqueous humor flow between hydralazine-treated and placebo-treated eyes at the lower concentration (3.20 +/- 0.63 vs. 3.05 +/- 0.61 microL/min (mean +/- S.D.) or at the higher concentration (3.37 +/- 0.53 vs. 3.28 +/- 0.60 microL/min) of hydralazine. These results suggest that acute topical application of 0.1% hydralazine to the eyes of normal humans does not cause clinically significant cardiovascular effects or significant ocular toxicity.

Evidence for a new role of natriuretic peptides: control of intraocular pressure

To study the possible physiological role of atrial natriuretic peptide (ANP) in the regulation of intraocular pressure (IOP) the effects of an increase of endogenous ANP within the physiological range induced by the neutral endopeptidase 24.11 (NEP) inhibitor candoxatril were examined. In a single masked placebo controlled trial, seven patients were studied with normal IOP (six male, one female; average age 50 (range 37-62 years). Intraocular pressure in each eye was measured after 2 weeks of placebo, after 4 weeks of candoxatril 200 mg twice daily, and during the first 3 days of placebo washout. With 4 weeks of candoxatril, endogenous plasma ANP levels increased from 4.2 (SEM 1.5) to 6.0 (1.5) pmol/l (p < 0.04) and there was a significant decrease in mean arterial pressure from 119 (4) to 110 (3) mm Hg (p < 0.02; 12 hours after treatment). There was a significant reduction in IOP after 4 weeks' treatment with candoxatril (right eye 2.1 (0.8) mm Hg, p < 0.05 paired t test, left eye 2.8 (0.8) mm Hg, p < 0.02). The mean fall in IOP was 11% (4%) in the right eye and 16% (3%) in the left eye and the fall in IOP was greater the higher the initial IOP. The reduction in IOP with chronic NEP inhibition was positively correlated with the increase in ANP levels but not with changes in blood pressure. These findings suggest that ANP may play a physiological role in the regulation of IOP. As the fall in IOP was greater in subjects with higher initial IOP, NEP inhibitors may be of therapeutic value in the management of glaucoma.

Genetic linkage of familial open angle glaucoma to chromosome 1q21-q31

Glaucoma is a significant cause of blindness world wide. There is evidence to suggest that at least a subset of the disease is determined genetically. We studied 37 members of a family affected with an autosomal dominant form of juvenile open angle glaucoma and 22 were found to be affected. Linkage analysis using short tandem repeat markers mapped the disease-causing gene to chromosome 1q21-q31. Eight markers were significantly linked (Zmax > 3.0) to the disease, with the highest lod score 6.5 (theta = 0), provided by D1S212. The atrial natriuretic peptide (ANP)/receptor system has been proposed to have a role in glaucoma and one of the ANP receptor genes maps to chromosome 1q.

Experimental glaucoma significantly decreases atrial natriuretic factor (ANF) receptors in the ciliary processes of the rabbit eye

ANF binding sites were analysed in the ciliary processes of rabbits with unilateral experimental glaucoma which had been induced by injecting alpha-chymotrypsin into the posterior chamber of the right eyes. The intraocular pressure (IOP) of glaucomatous eyes was significantly greater (28.4 +/- 4 mmHg) than that of normotensive control eyes (13.1 +/- 1.4 mmHg, mean +/- S.E.M., n = 23, P less than 0.05). ANF concentrations in aqueous humour and the ciliary processes were significantly higher in glaucomatous eyes (91 +/- 2 pg ml-1 and 30.4 +/- 4.2 pg g-1 wet weight) than in normal eyes (3.1 +/- 2.2 pg ml-1 and 10.2 +/- 2.7 pg g-1 wet weight, respectively, n = 6, P less than 0.01). The number of ANF-binding sites (Bmax) in the ciliary processes of glaucomatous rabbit eyes was significantly decreased in comparison to the controls (24 +/- 4 vs. 13 +/- 3 fmol mg-1 protein, n = 10, P less than 0.05). These data suggest that ANF receptors in the ciliary processes are down-regulated and that ANF may play an important role in the pathophysiology of experimental glaucoma.

Effects of atrial natriuretic factor (ANF) on intraocular pressure and aqueous humor flow in the cynomolgus monkey

Atrial natriuretic factor (ANF: human sequence) was examined for its effects on basal and terbutaline-stimulated aqueous humor flow, intraocular pressure (IOP) and uveoscleral outflow in cynomolgus monkeys under pentobarbital anesthesia. A dilution method with radioactively labeled albumin was used for the determination of aqueous humor flow. ANF was given by i.v. infusion or intracamerally. Intracameral administration of terbutaline increased the aqueous humor flow significantly; 1.10 +/- 0.05 microliter min-1 in the control eye and 1.69 +/- 0.06 microliter min-1 in the treated eye. I.v. infusion of ANF, 97 fmol kg-1 min-1, increased the aqueous humor flow by about 44% from basal values in the control eye. There was a small but not statistically significant increase on the terbutaline-treated side. The IOP was not changed by ANF at this dose. An ANF dose of 97 pmol kg-1 min-1 increased the aqueous humor flow by 51% in the control eye and by 19% in the terbutaline-treated eye. A further rise of about 8% in aqueous humor flow was registered on the control side when the infused ANF-dose was doubled. Doubling the dose also resulted in a decrease of the IOP by 1.3 +/- 0.3 mmHg on the control side and 2.2 +/- 0.4 mmHg on the terbutaline-stimulated side. Intracameral administration of ANF (81-162 pmol ml-1 perfusion fluid) increased the aqueous humor flow transiently by approximately 50% with a maximum after about 2 hr. The uveoscleral outflow tended to increase and IOP tended to decrease in the ANF-treated eye compared with the control. However, these effects were not statistically significant. These results suggest that ANF may be involved in the control of aqueous humor formation.