Role of nitric oxide in choroidal blood flow regulation during light/dark transitions.

PURPOSE: Several studies have recently shown that a transition from light to dark is associated with a reduction in choroidal blood flow. The mechanism underlying this effect is unclear but may be related to changes in neural input. In the present study, the authors hypothesized that either the alpha-receptor agonist phenylephrine or the nitric oxide synthase (NOS) inhibitor L-NMMA may alter the choroidal blood flow response during a transition from light to dark. METHODS: In 15 healthy male nonsmoking subjects, the response of choroidal perfusion was studied in a randomized placebo-controlled three-way crossover study. Phenylephrine, L-NMMA or placebo was administered on different study days, and the effect of a light/dark transition on choroidal perfusion parameters was studied. Subfoveal choroidal blood flow and fundus pulsation amplitude were assessed with laser Doppler flowmetry and laser interferometry, respectively. RESULTS: Before drug administration, a transition from light to dark reduced both choroidal hemodynamic parameters by 11% to 20%. Neither phenylephrine nor placebo altered basal choroidal blood flow or choroidal blood flow responses to the light/dark transitions. By contrast, the NOS inhibitor L-NMMA significantly reduced basal choroidal blood flow by 20.5% +/- 5.9% (P < 0.001) and basal fundus pulsation amplitude by 21.5% +/- 4.8% (P < 0.001). In addition, the response of subfoveal choroidal blood flow (-6.2% +/- 3.2%; P = 0.008) and fundus pulsation amplitude (-4.2% +/- 2.4%; P < 0.001) to the light/dark transition was significantly diminished. CONCLUSIONS: The present study indicates that NO plays a role in the choroidal blood flow decrease during a transition from light to dark. Given that L-NMMA is a nonspecific inhibitor of NOS, the present study does not clarify whether this NO is from endothelial or neural sources.

Effects of dopamine on retinal and choroidal blood flow parameters in humans.

AIM: To investigate the effect of dopamine on retinal and choroidal blood flow in humans. METHODS: We investigated the effect of two doses of intravenous dopamine (5 and 10 microg/kg/min) via a randomised double-masked crossover study in 12 healthy subjects chosen from a total of 16. Blood flow parameters in retina, optic nerve head and choroid were assessed with bi-directional laser Doppler velocimetry, laser Doppler flowmetry and laser interferometric measurement of fundus pulsation amplitude, respectively. RESULTS: Intravenous dopamine dose-dependently increased retinal blood cell velocity and fundus pulsation amplitude (p<0.001). At the highest administered dose red blood cell velocity in retinal vessels increased by 37% and fundus pulsation amplitude by 24%. By contrast, optic nerve head blood flow did not change with dopamine administration. CONCLUSIONS: Our data indicate that dopamine has a pronounced enhancing effect on the retinal perfusion in humans. Further studies are required to establish the exact role of dopamine in the regulation of choroidal and optic nerve head blood flow.

Effects of adrenomedullin on ocular hemodynamic parameters in the choroid and the ophthalmic artery.

PURPOSE: Adrenomedullin acts as a vasodilator and may play a role in inflammatory processes in the eye. This study was designed to determine whether nitric oxide formation is involved in the response to adrenomedullin in the ocular vasculature in vivo. METHODS: The effects of systemic intravenous adrenomedullin (3.2-16.0 pmol/[kg. min])) on choroidal blood flow were assessed by measurement of fundus pulsation amplitude and laser Doppler flow in the macula, and on blood flow in the ophthalmic artery by ultrasound Doppler flow in pilot studies (n = 7). Subsequently, in a double-blind randomized placebo-controlled crossover study in eight healthy male subjects the effects of 12.8 pmol/(kg. min) adrenomedullin on ocular and systemic hemodynamics were investigated. Adrenomedullin was co-infused with the nitric oxide synthase inhibitor N(G)-monomethyl-L-arginine (3 mg/kg bolus and 30 micro g/[kg. min] continuous intravenous infusion) or vehicle control on separate study days. RESULTS: Adrenomedullin dose dependently increased choroidal blood flow and flow velocity in the ophthalmic artery. N(G)-monomethyl-L-arginine reduced the effect of adrenomedullin on fundus pulsation amplitude, but did not alter the flow response in the ophthalmic artery. Systemic hemodynamics were unaffected by adrenomedullin infusion. CONCLUSIONS: Ocular blood flow is sensitive to changes in adrenomedullin concentrations. The acute vasodilator effects of adrenomedullin are nitric oxide-dependent in the choroid, but not in the ophthalmic artery.

Flicker light-induced vasodilatation in the human retina: effect of lactate and changes in mean arterial pressure.

PURPOSE: Diffuse luminance flicker light increases retinal and optic nerve head blood flow in animals and humans, but the exact mechanisms that mediate increased flow have yet to be identified. In the current study, the effect of increased plasma lactate levels on flicker-induced vasodilatation in the retina was investigated in three independent studies in healthy humans. METHODS: In the first study, plasma lactate concentrations were increased by bicycle exercise in 12 volunteers, and the change in retinal vessel diameter to 8-Hz square-wave flicker stimulation was measured with the Zeiss Retinal Vessel Analyzer (Carl Zeiss Meditec, Oberkochen, Germany). In a different study, sodium lactate was administered intravenously, and flicker responses were measured in 12 subjects. As a control experiment accounting for pressure increases induced by exercise, the effect of elevated ocular perfusion pressure on the flicker response was investigated during tyramine infusion (n = 12). RESULTS: The increase in plasma lactate concentration during intravenous infusion from 1.3 +/- 0.4 to 6.3 mmol/L and during dynamic exercise from 1.2 +/- 0.3 to 9.4 mmol/L decreased flicker responses in retinal arteries from 5.3% +/- 0.9% to 1.7% +/- 0.6% (P < 0.001) and from 3.6% +/- 0.6% to 2.0% +/- 0.8% (P = 0.03), respectively. In contrast, an increase of mean blood pressure from 81 +/- 3 to 92 +/- 3 mm Hg after tyramine infusion had no significant effect on flicker-induced vasodilatation in retinal arteries and veins. CONCLUSIONS: The signaling between neuronal activity and flow response in the human retina is sensitive to changes in blood lactate levels, whereas changes in systemic blood pressure have no major effect. Whether an increased cytosolic redox impairment contributes to flicker-induced vasodilatation has yet to be clarified.

Typical investigational medicinal products follow relatively uniform regulations in 10 European Clinical Research Infrastructures Network (ECRIN) countries.

BACKGROUND: In order to facilitate multinational clinical research, regulatory requirements need to become international and harmonised. The EU introduced the Directive 2001/20/EC in 2004, regulating investigational medicinal products in Europe. METHODS: We conducted a survey in order to identify the national regulatory requirements for major categories of clinical research in ten European Clinical Research Infrastructures Network (ECRIN) countries-Austria, Denmark, France, Germany, Hungary, Ireland, Italy, Spain, Sweden, and United Kingdom-covering approximately 70% of the EU population. Here we describe the results for regulatory requirements for typical investigational medicinal products, in the ten countries. RESULTS: Our results show that the ten countries have fairly harmonised definitions of typical investigational medicinal products. Clinical trials assessing typical investigational medicinal products require authorisation from a national competent authority in each of the countries surveyed. The opinion of the competent authorities is communicated to the trial sponsor within the same timelines, i.e., no more than 60 days, in all ten countries. The authority to which the application has to be sent to in the different countries is not fully harmonised. CONCLUSION: The Directive 2001/20/EC defined the term 'investigational medicinal product' and all regulatory requirements described therein are applicable to investigational medicinal products. Our survey showed, however, that those requirements had been adopted in ten European countries, not for investigational medicinal products overall, but rather a narrower category which we term 'typical' investigational medicinal products. The result is partial EU harmonisation of requirements and a relatively navigable landscape for the sponsor regarding typical investigational medicinal products.

Compassionate use of interventions: results of a European Clinical Research Infrastructures Network (ECRIN) survey of ten European countries.

BACKGROUND: 'Compassionate use' programmes allow medicinal products that are not authorised, but are in the development process, to be made available to patients with a severe disease who have no other satisfactory treatment available to them. We sought to understand how such programmes are regulated in ten European Union countries. METHODS: The European Clinical Research Infrastructures Network (ECRIN) conducted a comprehensive survey on clinical research regulatory requirements, including questions on regulations of 'compassionate use' programmes. Ten European countries, covering approximately 70% of the EU population, were included in the survey (Austria, Denmark, France, Germany, Hungary, Ireland, Italy, Spain, Sweden, and the UK). RESULTS: European Regulation 726/2004/EC is clear on the intentions of 'compassionate use' programmes and aimed to harmonise them in the European Union. The survey reveals that different countries have adopted different requirements and that 'compassionate use' is not interpreted in the same way across Europe. Four of the ten countries surveyed have no formal regulatory system for the programmes. We discuss the need for 'compassionate use' programmes and their regulation where protection of patients is paramount. CONCLUSIONS: 'Compassionate use' is a misleading term and should be replaced with 'expanded access'. There is a need for expanded access programmes in order to serve the interests of seriously ill patients who have no other treatment options. To protect these patients, European legislation needs to be more explicit and informative with regard to the regulatory requirements, restrictions, and responsibilities in expanded access programmes.

Common definition for categories of clinical research: a prerequisite for a survey on regulatory requirements by the European Clinical Research Infrastructures Network (ECRIN).

BACKGROUND: Thorough knowledge of the regulatory requirements is a challenging prerequisite for conducting multinational clinical studies in Europe given their complexity and heterogeneity in regulation and perception across the EU member states. METHODS: In order to summarise the current situation in relation to the wide spectrum of clinical research, the European Clinical Research Infrastructures Network (ECRIN) developed a multinational survey in ten European countries. However a lack of common classification framework for major categories of clinical research was identified, and therefore reaching an agreement on a common classification was the initial step in the development of the survey. RESULTS: The ECRIN transnational working group on regulation, composed of experts in the field of clinical research from ten European countries, defined seven major categories of clinical research that seem relevant from both the regulatory and the scientific points of view, and correspond to congruent definitions in all countries: clinical trials on medicinal products; clinical trials on medical devices; other therapeutic trials (including surgery trials, transplantation trials, transfusion trials, trials with cell therapy, etc.); diagnostic studies; clinical research on nutrition; other interventional clinical research (including trials in complementary and alternative medicine, trials with collection of blood or tissue samples, physiology studies, etc.); and epidemiology studies. Our classification was essential to develop a survey focused on protocol submission to ethics committees and competent authorities, procedures for amendments, requirements for sponsor and insurance, and adverse event reporting following five main phases: drafting, consensus, data collection, validation, and finalising. CONCLUSION: The list of clinical research categories as used for the survey could serve as a contribution to the, much needed, task of harmonisation and simplification of the regulatory requirements for clinical research in Europe.

Effects of dopamine on human retinal vessel diameter and its modulation during flicker stimulation.

We performed a randomized, subject-blinded, placebo and time-controlled, two-way crossover study in 12 healthy male subjects. Placebo or dopamine was administered on two separate study days. After saline infusion, dopamine hydrochloride was infused in three consecutive doses (5, 10, and 15 microg x kg(-1) x min(-1)). Plasma levels of dopamine were determined at each perfusion step. Arterial and venous retinal vessel diameters were measured with the use of a Zeiss retinal vessel analyzer. Diffuse luminance flicker stimuli of 8 Hz were applied for 60 s. Blood pressure and pulse rate were monitored continuously. Flicker stimulation (8 Hz) increased retinal vessel diameters under basal conditions. The response to 8-Hz flicker light was significantly reduced by dopamine administration. In addition, dopamine slightly but significantly increased retinal vessel diameters. Dopamine hydrochloride significantly increased systolic but not diastolic or mean arterial pressure. The present study indicates that dopamine has a distinct effect on retinal vessel diameters also attenuating the flicker-induced response reactivity of retinal vessels. This implies a role of dopamine in retinal blood flow hemodynamics.