Effect of noradrenaline on retinal blood flow in healthy subjects

PURPOSE: To gain insight into the role of circulating catecholamines on retinal blood flow in vivo. DESIGN: Nonrandomized, open, crossover design. PARTICIPANTS: In 10 healthy male subjects, tyramine and noradrenaline were administered in stepwise increasing doses. These doses were selected to induce comparable changes in systemic blood pressure. METHODS: During each infusion step, retinal vessel diameter and retinal venous blood speed were measured with the Zeiss retinal vessel analyzer (Zeiss, Jena, Germany) and laser Doppler velocimetry, respectively. MAIN OUTCOME MEASURES: Retinal blood flow through a major temporal vein was calculated. RESULTS: As expected, tyramine and noradrenaline induced a systemic hypertensive response. Tyramine caused a moderate increase in noradrenaline plasma levels, whereas exogenous noradrenaline increased noradrenaline plasma levels more than 10-fold. Nevertheless, neither tyramine nor noradrenaline induced any effect on retinal hemodynamic parameters. CONCLUSIONS: These data indicate that even high levels of circulating noradrenaline have little impact on retinal vascular tone and retinal blood flow. Hence, the adrenergic system appears not to play a major role in retinal blood flow regulation.