Submitted on May 11, 2005
Accepted on June 21, 2005
Effect of histamine and cimetidine on retinal and choroidal blood flow in humans
Hemma Resch1, Claudia Zawinka1, Solveig Lung1, Gunther Weigert1, Leopold Schmetterer2, and Gerhard Garhofer3*
1 Clinical Pharmacology, Austria Medical University of Vienna, Vienna, Austria
2 Clinical Pharmacology, Austria Medical University of Vienna, Vienna, Austria; Center of Biomedical Engineering and Physics, Austria Medical University of Vienna, Vienna, Austria
3 Clinical Pharmacology, Austria Medical University of Vienna, Vienna, Austria; Department of Ophthalmology, Austria Medical University of Vienna, Vienna, Austria
* To whom correspondence should be addressed. E-mail: gerhard.garhoefer{at}meduniwien.ac.at.
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 the hemodynamic effects of histamine in the choroid and the retina. 18 healthy male non-smoking volunteers were included (age range: 21-35 years, mean: 25.5 years, SD: 3.0) in this randomized, double masked, placebo-controlled two-way cross-over study. Histamine (0.32µg/kg/min over 30 minutes) was infused intravenously in the absence (NaCl as placebo) or presence of the H2 blocker cimetidine (2.3mg/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 co-infusion 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 bi-directional 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 the effects of histamine on choroidal blood flow (p=0.6 versus placebo), fundus pulsation amplitude (p=0.5 versus placebo), retinal venous diameter (p=0.2 versus placebo) and retinal arterial diameter (p=0.5 versus 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 a H2 blocker. Whether the increase in blood flow is mediated via H1 receptors or by other hitherto unidentified mechanisms remains to be elucidated.
