THE ACTION OF ANGIOTENSIN ON INTRAOCULAR PRESSURE

Low-dose Bevacizumab Decreases Ocular Hypotensive Effect of Angiotensin II in Sprague Dawley Rats

PURPOSE

Although the effectiveness of anti-VEGF agents in ophthalmology has been thoroughly documented, we do not fully comprehend the epidemiology and mechanistic background of their side effects, including intraocular and systemic hypertension. Here, we investigate the interference of a low-dose bevacizumab with key neuronal and humoral mechanisms maintaining blood and intraocular pressure homeostasis.

MATERIALS AND METHODS

Intraocular pressure (IOP), blood pressure (BP), and heart rate (HR) were measured in SPRD rats pretreated with bevacizumab or 0.9% NaCl at baseline and after infusion of angiotensin II, a humoral mediator involved in BP and IOP regulation. Superior cervical gangliectomy was performed to assess the effect of sympathetic nervous system on the analyzed parameters. Additionally, we studied the expression of a subset of genes related to renin-angiotensin system in the anterior segment of the eye.

RESULTS

At baseline, there was no significant difference in IOP, BP, and HR between rats pretreated with 0.9% NaCl and bevacizumab. Infusion of angiotensin II lowered IOP in rats pretreated with 0.9% NaCl, but not in rats pretreated with bevacizumab (30 min: ∆4.22 ± 1.2 vs. baseline, p > .05; ∆0.83 ± 0.66 vs. baseline, p < .05) This effect was paralleled by an increased expression of angiotensin II type 1b and type 2 receptors in the anterior segment of the eye (AT1b: 1 ± 0.65 vs 7.35 ± 2.84, p < .05; AT2: 1 ± 0.05 vs. 12.8 ± 0.1, p < .05). Angiotensin II infusion increased BP in both groups (10 min: bevacizumab ∆44.6 ± 3.2, p < .05; 0.9%NaCl ∆37.1 ± 5.1, p < .05), whereas did not have any effect on HR. Sympathetic ocular denervation did not affect any of the analyzed parameters.

CONCLUSIONS

We found that low-dose bevacizumab interferes with IOP-lowering properties of angiotensin II. This effect might be related to increased expression of angiotensin II receptors in the anterior segment of the eye.

Many Faces of Renin-angiotensin System - Focus on Eye

The renin-angiotensin system (RAS), that is known for its role in the regulation of blood pressure as well as in fluid and electrolyte homeostasis, comprises dozens of angiotensin peptides and peptidases and at least six receptors. Six central components constitute the two main axes of the RAS cascade. Angiotensin (1-7), an angiotensin converting enzyme 2 and Mas receptor axis (ACE2-Ang(1-7)-MasR) counterbalances the harmful effects of the angiotensin II, angiotensin converting enzyme 1 and angiotensin II type 1 receptor axis (ACE1-AngII-AT1R) Whereas systemic RAS is an important factor in blood pressure regulation, tissue-specific regulatory system, responsible for long term regional changes, that has been found in various organs. In other words, RAS is not only endocrine but also complicated autocrine system. The human eye has its own intraocular RAS that is present e.g. in the structures involved in aqueous humor dynamics. Local RAS may thus be a target in the development of new anti-glaucomatous drugs. In this review, we first describe the systemic RAS cascade and then the local ocular RAS especially in the anterior part of the eye.

Ocular renin-angiotensin system with special reference in the anterior part of the eye

The renin-angiotensin system (RAS) regulates blood pressure (BP) homeostasis, systemic fluid volume and electrolyte balance. The RAS cascade includes over twenty peptidases, close to twenty angiotensin peptides and at least six receptors. Out of these, angiotensin II, angiotensin converting enzyme 1 and angiotensin II type 1 receptor (AngII-ACE1-AT1R) together with angiotensin (1-7), angiotensin converting enzyme 2 and Mas receptor (Ang(1-7)-ACE2-MasR) are regarded as the main components of RAS. In addition to circulating RAS, local RA-system exists in various organs. Local RA-systems are regarded as tissue-specific regulatory systems accounting for local effects and long term changes in different organs. Many of the central components such as the two main axes of RAS: AngII-ACE1-AT1R and Ang(1-7)-ACE2-MasR, have been identified in the human eye. Furthermore, it has been shown that systemic antihypertensive RAS- inhibiting medications lower intraocular pressure (IOP). These findings suggest the crucial role of RAS not only in the regulation of BP but also in the regulation of IOP, and RAS potentially plays a role in the development of glaucoma and antiglaucomatous drugs.

Local ocular renin-angiotensin system - a target for glaucoma therapy?

An active local intraocular renin-angiotensin system (RAS) has recently been shown to exist in the human eye, and evidence is now accumulating that antihypertensive drugs acting on RAS can also lower intraocular pressure. They seem also to work as neuroprotective agents against retinal ganglion cell loss in vivo; though no compounds are in ophthalmological use at present. Classically, the highly vasoconstrictive angiotensin II (Ang II) is the key peptide in the circulatory RAS. However, the final effect of RAS activation at tissue level is more complex, being based not only on the biological activity of Ang II but also on the activities of other products of angiotensinogen metabolism, often exerting opposite effects to Ang II action.

Does the renin-angiotensin system also regulate intra-ocular pressure?

The renin-angiotensin-aldosterone system is known to play an essential role in controlling sodium balance and body fluid volumes, and thus blood pressure. In addition to the circulating system which regulates urgent cardiovascular responses, a tissue-localized renin-angiotensin system (RAS) regulates long-term changes in various organs. Many recognized RAS components have also been identified in the human eye. The highly vasoconstrictive angiotensin II (Ang II) is considered the key peptide in the circulatory RAS. However, the ultimate effect of RAS activation at tissue level is more complex, being based not only on the biological activity of Ang II but also on the activities of other products of angiotensinogen metabolism, often exerting opposite effects to Ang II action. In recent studies, orally administered angiotensin II type 1 receptor blockers and angiotensin-converting enzyme inhibitors lower intra-ocular pressure (IOP), likewise topical application of these compounds, the effect being more prominent in ocular hypertensive eyes. Based on previous findings and our own experimental data, it can strongly be suggested that the RAS not only regulates blood pressure but is also involved in the regulation of IOP.

Effects of mild, moderate and severe exercise on intraocular pressure of sedentary subjects

Previous studies have shown a reduction in intraocular pressure (IOP) from many forms of exertion, ranging from walking to exhausting exercise. The variability in these results may be due to several factors, such as age, diurnal and seasonal variations, drinking water or coffee before the test, acute hyperglycaemia, and physical fitness. The purpose of the study was to investigate the effects of the common means of exertion on IOP in the same subject, after elimination of above-mentioned factors. The effects of sitting, walking, jogging, and running fast till exhaustion were noted on the IOP of 15 healthy sedentary male volunteers at an interval of 4 days. Intraocular pressures were measured with a Goldmann applanation tonometer at the beginning and at 5th, 20th, 40th, and 60th minutes of the first three tests, but in test 4, only at the beginning and end. Post-exercise measurements were taken after every 10 minutes until IOP returned to pre-exercise levels. The effects of all tests were similar on both eyes. During sitting, walking and jogging, the maximum decreases were (mean +/- SD) 1.20 +/- 0.66, 3.20 +/- 1.19, and 5.07 +/- 1.76 mmHg, and occurred at the 20th, 40th and 60th minutes of the tests respectively. During sitting, walking and jogging, 66.7%, 75% and 68.4% of the maximum decreases occurred after 5 min respectively. After running the mean decrease was 5.7 +/- 1.09 mmHg and the average time of running was 10.53 +/- 2.17 min. In the sitting test, recovery occurred at the end of the test, while in walking, jogging and running tests it occurred after 12.67 +/- 44.48, 30.67 +/- 7.99, and 56.00 +/- 11.21 min, respectively. It is concluded that all forms of exertion decrease IOP. It would seem reasonable, at present, not to discourage patients who have glaucoma from walking; perhaps, on the contrary, it should be encouraged.

Effects of angiotensin, vasopressin and atrial natriuretic peptide on intraocular pressure in anesthetized rats

The effects of atrial natriuretic peptide (ANP), vasopressin (AVP) and angiotensin (ANG) on blood and intraocular pressures of pentobarbital anesthetized rats were evaluated following intravenous, intracerebroventricular or anterior chamber routes of administration. Central injections did not affect intraocular pressure. Equipressor intravenous infusions of ANG raised, whereas AVP decreased, intraocular pressure. Direct infusions of a balanced salt solution (0.175 microliter/min) raised intraocular pressure between 30 and 60 min. Adding ANG or ANP slightly reduced this solvent effect but AVP was markedly inhibitory. An AVP-V1 receptor antagonist reversed the blunting of the solvent-induced rise by the peptide, indicating receptor specificity. Acetazolamide pretreatment lowered intraocular pressure, but the solvent-induced rise in intraocular pressure and inhibition by AVP still occurred without altering the temporal pattern. Thus, these effects appear unrelated to aqueous humor synthesis rate. The data support the possibility of intraocular pressure regulation by peptides acting from the blood and aqueous humor.

Local generation and action of angiotensin II in dog iris sphincter muscle

Existence of the renin-angiotensin system was pharmacologically investigated in the dog isolated iris sphincter muscle. The sphincter muscle contracted in response to tetradecapeptide, a synthetic renin substrate, angiotensin (ANG) I and ANG II dose-dependently. The contractions induced by these peptides were suppressed by treatment with saralasin, indomethacin and aspirin. Contractile responses to tetradecapeptide and ANG I were also reduced by KRI-1314, a renin inhibitor, and captopril, respectively. ANG II stimulated the release of prostaglandin (PG) F2 alpha from the sphincter muscle. Angiotensin-converting enzyme activity was measurable in the sphincter muscle. Miosis was observed by intracameral injection of ANG I and ANG II into the anterior chamber. These results strongly suggest that angiotensin generating enzymes function in the sphincter muscle and ANG II produced by these enzymes contracts the sphincter muscle via the formation of PG (s), possibly PG F2 alpha.

Potentiation of norepinephrine secretion by angiotensin II in the isolate rabbit iris-ciliary body

The prejunctional effects of angiotensin II (AII) on stimulation-evoked secretion of 3H-norepinephrine (3H-NE) were investigated by in vitro methods in isolated, superfused rabbit iris-ciliary body preparations. AII (0.1-10 nM) concentration-dependently enhanced the field- stimulated release of 3H-NE (EC50 = 0.1 nM), nearly doubling evoked neurotransmitter release with no apparent effect on spontaneous 3H-NE efflux. The response to 1 nM AII was abolished by the selective AII receptor antagonist saralasin [( Sar1,Val5, Ala8]-angiotensin II; 500 nM), which alone did not modify 3H-NE overflow. AII-mediated effects on neurosecretion were partially additive to those of forskolin and were not potentiated by phosphodiesterase inhibition, suggesting that AII utilizes a mechanism other than increased cAMP synthesis to facilitate neurotransmitter release. AII also strongly enhanced calcium ionophore (A23187)-induced 3H-NE release in iris-ciliary body segments, indicating that AII can modulate calcium-dependent exocytosis at step(s) distal to calcium influx. These results demonstrate that sympathetic nerves in the rabbit eye contain prejunctional, facilitatory AII receptors, and support the possible involvement of the renin-angiotensin system in regulation of ocular sympathetic neurotransmission in vivo.

Effects of inhibition of angiotensin converting enzyme and carbonic anhydrase on fluid production by ciliary process, choroid plexus, and pancreas

Inhibitors of angiotensin converting enzyme (ACE) (1) and of carbonic anhydrase (CA) (2,3) decrease intraocular pressure (IOP) in conscious rabbits. We asked whether ACE inhibition decreases IOP through effects on CA-dependent flow of aqueous humor (AH) and whether ACE inhibitors decrease other CA-dependent secretions. We show in anesthetized rabbits (a) that topical inhibitors of ACE decrease both IOP and AH flow as much as systemic inhibitors of CA; (b) that the maximal effects of ACE and CA inhibition are not additive, therefore these treatments may affect one or more components of a single system for fluid production; and that (c) ACE inhibitors do not work through inhibition of CA. Looking at other fluid production systems, we find (d) that cerebrospinal fluid (CSF) production is increased after ventriculocisternal perfusion with a potent ACE inhibitor and (e) that flow of pancreatic juice (PJ) is increased after systemic ACE inhibition.

Ocular and systemic responses to water soluble material derived from Cannabis sativa (marihuana)

A water soluble material, isolated from Cannabis sativa, has been tested in albino and pigmented rabbits and rhesus monkeys for both ocular and systemic effects. Intravenous administration produced a dose-related fall in intraocular pressure in both albino and pigmented rabbits with concentrations as low as 0.005 mg/animal being effective, but no response was found in monkeys. High concentrations (0.2 to 1 mg/animal) induced a hypertensive phase in intraocular pressure prior to the ocular hypotension; higher concentrations (2 or 5 mg/animal) also induced antidiuresis and general relaxation. Tachyphylaxis was found to repeated daily injections. Alpha and beta-adrenergic antagonists caused some reduction of the hypertensive phase but had no effect on the hypotensive phase. Superior cervical ganglionectomy did not influence the time course of the intraocular pressure response. Indomethacin inhibited the hypertensive intraocular pressure phase but was ineffective against the hypotensive phase. Systemic blood pressure was unchanged following intravenous administration of 0.2 mg material/animal. Aqueous tumor protein concentration was increased at both 1 and 6 hours after intravenous administration, becoming greater at the later time. Aqueous humor turnover rate was substantially reduced reaching a minimum 8.75 hours after administration. Topical administration was ineffective in eyes when the epithelium was removed in rabbits with and without pretreatment with aspirin. Neither gastric nor suppository administration of large quantities (10 mg or greater) of material had any influence on intraocular pressure.

Hormonal regulation of intraocular pressure

A detailed review of the hormonal effects on intraocular pressure is presented. There is evidence that corticotropin, vasopressin, thyroxin, insulin, glucocorticoids and mineralocorticoids may play a role in the physiologic regulation of intraocular pressure. Growth hormone, melanocyte stimulating hormone, progesterone, estrogen, chorionic gonadotropin and relaxin may influence intraocular pressure when administered in pharmacologic doses. Whether the key to understanding primary open-angle glaucoma lies in recognizing abnormal endocrine mechanisms, especially involving glucocorticoids, remains unclear at the present time.

Angiotensin tachyphylaxis and its reversal

Tachyphylaxis to angiotensin and some analogues has been demonstrated on spirally cut arterial strips from cat, dog, sheep and rat, and on venous strips from rabbit and cat. Rabbit and guinea pig arteries do not appear to become tachyphylactic. A free C-terminal carboxyl group of angiotensin is necessary for binding with receptor sites and development of tachyphylaxis. Tachyphylaxis seems to represent saturation of receptor sites. It can be reversed by plasma fractions rich in angiotensinase A, possibly by metabolizing the N-terminal part of angiotensin directly from the bound state. Dowex 50 can also reverse it, probably by physical adsorption and stronger binding of angiotensin. Angiotensinase A does not metabolize βaspartyl 1 -angiotensin and does not reverse tachyphylaxis to this peptide. A possible scheme of interaction between peptide and receptor site is presented.