Priming interactions between platelet activating factor and histamine in the in vivo microcirculation

Area of Science:

• Pharmacology
• Microcirculation Physiology
• Immunology

Background:

• Platelet-activating factor (PAF) and histamine are key mediators in inflammatory responses.
• Their interactions in the microcirculation are not fully understood.
• Investigating priming effects can reveal synergistic mechanisms in vascular regulation.

Purpose of the Study:

• To determine if platelet-activating factor (PAF) primes the microcirculation for histamine.
• To characterize the concentration- and time-dependency of PAF priming.
• To elucidate the receptor mediation and cellular involvement in this priming effect.

Main Methods:

• Topical application of substimulatory concentrations of PAF and histamine.
• Measurement of FITC-dextran 150 clearance in vivo.
• Administration of PAF receptor antagonist (WEB 2086), H1 blocker (pyrilamine), and 5-lipoxygenase inhibitor (RG 5901).
• Leukocyte depletion using cyclophosphamide.

Main Results:

• Maximal priming occurred with a 5-min interval between PAF and histamine, increasing FITC-dextran clearance 4.5-fold.
• Reversed agonist order or lower PAF concentrations diminished the priming effect.
• PAF priming was abolished by PAF receptor antagonist, H1 blocker, and leukocyte depletion.
• Leukotriene inhibition partially attenuated the priming effect.

Conclusions:

• PAF primes microvascular histamine actions in a dose- and time-dependent manner.
• This priming is receptor-mediated and involves leukocytes and leukotriene release.
• Histamine can also prime the microcirculation for enhanced PAF responses.