Abstract
Abstract: :
Purpose: Intravenous histamine causes an increase in choroidal blood flow and retinal vessel diameters in healthy subjects. The mechanism underlying this effect remains, however, 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. Methods: Fourteen healthy male subjects partizipated in this randomized, placebo–controlled two–way cross–over study. Histamine (0.32 mg/kg/min over 30 minutes) was infused intravenously in the absence (NaCl infusion as placebo) or presence of the H2 receptor blocker cimetidine (2.3 mg/min over 50 minutes). The infusion of cimetidine or placebo was started 20 minutes prior to the histamine infusion. Ocular hemodynamic parameters, blood pressure and intraocular pressure (IOP) were measured before drug administration, after infusion of cimetidine or placebo and after co–infusion of histamine. Subfoveal choroidal blood flow (CHBF) was measured with laser Doppler flowmetry. Retinal arterial and venous diameters (AD, VD) were measured with a Retinal Vessel Analyzer. Retinal blood velocity (VEL) was assessed with bi–directional laser Doppler velocimetry. Results: None of the administered drugs altered blood pressure or IOP. Hisatmine increased CHBF (+15%, p < 0.001), VD (+3.5%, p < 0.001), AD (+2.2%, p = 0.031), but did not change VEL. Cimetidine alone had no significant effect on ocular hemodynamic parameters. In addition, cimetidine did not modify the effects of histamine on CHBF (p = 0.233 versus placebo), VD (p = 0.633 versus placebo) and AD (p = 0.113 versus placebo). Conclusions: The present data confirm that histamine increases choroidal blood flow and retinal vessel diameters in vivo. This ocular vasodilator effect of histamine is, however, not altered by H2 receptor blockade. Whether the increase in blood flow is mediated via H1 receptors or by other hitherto unidentified mechanisms remains to be elucidated.
Keywords: blood supply • neurotransmitters/neurotransmitter systems • choroid