Effect of nos inhibition on retinal arterial and capillary circulation in early arterial hypertension.

BACKGROUND: Arterial hypertension is involved in the pathogenesis of end organ damage by influencing the ability of the vascular endothelium to produce nitric oxide (NO). This study analyzes changes of retinal and systemic NO-dependent circulation parameters by inhibiting nitric oxide synthase (NOS) in both hypertensive and normotensive individuals. METHODS: In a double-blind crossover trial, 19 hypertensive patients (H, age 28.2 +/- 0.9 years) and 19 normotensive controls (N, age 26.9 +/- 0.9 years) were randomized treated either with candesartan or placebo. Both retinal capillary flow (RCF) and mean blood flow velocity of the central retinal artery (VCRA) were registered before and after NOS inhibition with N-monomethyl-L-arginine (L-NMMA, 3 mg/kg). In a subpopulation mean arterial pressure (MAP), cardiac output (CO), and the total peripheral resistance (TPR) were determined simultaneously. RESULTS: Changes from baseline: In normotensive and hypertensive subjects infusion of L-NMMA led to an increase of MAP (N, +13.3 +/- 1.8%, P < 0.01; H, +14.3 +/- 2.4%, P < 0.01) and TPR (N, +36.9 +/- 3.8%, P < 0.01; H, +45.0 +/- 4.5%, P < 0.01), and to a decrease of CO (N, -21.1 +/- 1.5%, P < 0.01; H, -24.6 +/- 2.3%, P < 0.01). The L-NMMA effect on VCRA and RCF differed between controls and hypertensives. VCRA changed by + 17.3 +/- 6.2% (P < 0.05) and RCF by -7.3 +/- 3.0% (P < 0.05) in controls. In hypertensive subjects corresponding results were + 9.5 +/- 5.2% (P = NS) and + 2.7 +/- 3.8% (P = NS), respectively. The decrease of RCF due to L-NMMA was reduced in hypertension as compared to controls (P < 0.05). The calculated cross-sectional area of CRA was reduced by -58.7% in controls and increased by + 31.1% in hypertensive subjects. There was no significant correlation between the flow in the systemic and retinal circulation. CONCLUSION: Only normotensives L-NMMA induces an acceleration of VCRA due to a probable vasoconstriction of the central retinal artery and despite of a reduced RCF. Already in early hypertension the NOS-dependent vascular tone in retinal arteries and capillaries is impaired. The regulation of the retinal capillary flow appeared to be independent from systemic circulation.

Flickering light increases retinal blood flow.

PURPOSE: To examine the retinal blood flow in normal eyes before and during retinal stimulation by flickering light. DESIGN: A prospective cross-sectional study. PARTICIPANTS AND TESTING: Twenty-seven eyes of 27 normal subjects with a mean age +/- SD of 38 +/- 15 years (study I) and 21 eyes of 21 normal subjects with a mean age +/- SD of 46 +/- 17 years (study II) were examined with respect to capillary retinal blood flow and central retinal artery and central retinal vein blood flow velocities during flickering light stimulation. A luminance flicker light with a frequency of 8 Hz increased the neuronal activity of retinal ganglion cells. In study I, the retinal capillary blood flow was measured before and during flickering by scanning laser Doppler flowmetry (670 nm, Heidelberg Retina Flowmeter). In study II, the blood flow velocities in the central retinal artery and central retinal vein were examined by pulsed Doppler sonography. MAIN OUTCOME MEASURES: Change in blood flow velocities in the central retinal artery and vein and in retinal capillary blood flow after full-field flicker stimulation. RESULTS: In study I, measurements of blood flow during retinal flicker stimulation showed a significant increase in the mean value of blood flow +/- SD from 317 +/- 72 arbitrary units to 416 +/- 103 arbitrary units. The change was on average 46 +/- 19%. In study II, the systolic and end-diastolic blood flow velocities in the central retinal artery increased significantly (P < 0.0001): systolic, 9 cm/s to 15 cm/s (+62%); end-diastolic, 2.7 cm/s to 5.3 cm/s (+96%). In the central retinal vein, the systolic and end-diastolic blood flow velocities increased significantly (P < 0.0001): systolic, 4.3 cm/s to 6.7 cm/s (+56%); end-diastolic, 1.8 cm/s to 3.6 cm/s (+100%). The authors found no significant change in blood pressure and heartbeat frequency. CONCLUSIONS: Visual stimulation of the retina by flickering light strongly increased the juxtapapillary retinal capillary blood flow and central retinal artery blood flow velocity in normal eyes.