Platelet-Activating Factor Modulates Microvascular Dynamics through Phospholipase C in the Hamster Cheek Pouch

We studied the interactions between platelet-activating factor (PAF) and phospholipase C (PLC) in the modulation of microvascular responses in the hamster cheek pouch using intravital microscopy and computer-assisted image analysis. Changes in arteriolar diameter and in integrated optical intensity (IOI, an index of vascular permeability) were measured. Fluorescein-isothiocyanate-labeled dextran 150 (FITC-Dx 150) served as a tracer for macromolecular transport. 2-Nitro-4-carboxyphenyl N,N-diphenylcarbamate (NCDC) and 1-(6-((17β-3-methoxyestra-1,3,5(10)-trien-17-yl)amino)hexyl)-1H-pyrrole-2,5,-dione (U-73122), two PLC inhibitors, were applied topically in separate experiments. PAF at 10−7 M elevated IOI from baseline to a mean ± SEM value of 70.7 ± 8.9 units. Pretreatment with 10−4 and 10−5 M NCDC and with U-73122 at 10−5 and 10−6 M attenuated the maximal increment in mean IOI (±SEM) induced by PAF at 10−7 M to mean ± SEM values of 30.6 ± 6.5, 39.3 ± 6.0, 12.1 ± 4.8, and 41.5 ± 6.0, respectively. The simultaneous vasoconstrictor action of 10−7 M PAF was expressed as the experimental-to-baseline ratio, with the baseline diameter adjusted to a value of 1. PAF constricted the arterioles to a mean ± SEM ratio of 0.30 ± 0.07. Pretreatment with the PLC inhibitors NCDC at 10−4 and 10−5 M NCDC and with U-73122 at 10−5 and 10−6 M attenuated 10−7 M PAF-induced vasoconstriction to mean ± SEM diameter ratios of 0.55 ± 0.05, 0.48 ± 0.06, 0.55 ± 0.08, and 0.58 ± 0.06, respectively. Our results demonstrate that PLC is an element of the biochemical pathway involved in PAF modulation of microvascular permeability and in PAF modulation of arteriolar diameter.