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NEUROHUMORAL CONTROL OF CARDIOVASCULAR FUNCTION

Effect of histamine and cimetidine on retinal and choroidal blood flow in humans

¹Department of Clinical Pharmacology, ²Center of Biomedical Engineering and Physics, and ³Department of Ophthalmology, Medical University of Vienna, Vienna, Austria

Submitted 11 May 2005 ; accepted in final form 21 June 2005

Intravenous administration of histamine causes an increase in choroidal blood flow and retinal vessel diameter in healthy subjects. The mechanism underlying this effect remains to be elucidated. In the present study, we hypothesized that H₂ receptor blockade alters hemodynamic effects of histamine in the choroid and retina. Eighteen healthy male nonsmoking volunteers were included in this randomized, double-masked, placebo-controlled two-way crossover study. Histamine (0.32 µg·kg^–1·min^–1 over 30 min) was infused intravenously in the absence (NaCl as placebo) or presence of the H₂ blocker cimetidine (2.3 mg/min over 50 min). Ocular hemodynamic parameters, blood pressure, and intraocular pressure were measured before drug administration, after infusion of cimetidine or placebo, and after coinfusion of histamine. Subfoveal choroidal blood flow and fundus pulsation amplitude were measured with laser-Doppler flowmetry and laser interferometry, respectively. Retinal arterial and venous diameters were measured with a retinal vessel analyzer. Retinal blood velocity was assessed with bidirectional laser-Doppler velocimetry. Histamine increased subfoveal choroidal blood flow (+14 ± 15%, P < 0.001), fundus pulsation amplitude (+11 ± 5%, P < 0.001), retinal venous diameter (+3.0 ± 3.6%, P = 0.002), and retinal arterial diameter (+2.8 ± 4.2%, P < 0.01) but did not change retinal blood velocity. The H₂ antagonist cimetidine had no significant effect on ocular hemodynamic parameters. In addition, cimetidine did not modify effects of histamine on choroidal blood flow, fundus pulsation amplitude, retinal venous diameter, and retinal arterial diameter compared with placebo. The present data confirm that histamine increases choroidal blood flow and retinal vessel diameters in healthy subjects. This ocular vasodilator effect of histamine is, however, not altered by administration of an H₂ blocker. Whether the increase in blood flow is mediated via H₁ receptors or other hitherto unidentified mechanisms remains to be elucidated.

H₂ receptors; retinal blood velocity; laser-Doppler flowmetry