Mast cell degranulation in rat mesenteric venule: Effects of L-name, methylene blue and ketotifen

Mast cells are present in proximity to the microvessels, and on stimulation with inhibition of NO synthesis, are a rich source of numerous inflammator y mediators. A microcirculatory study was undertaken to clarify whether nit ric oxide (NO) and activation of guanylate cyclase is involved in degranula tion of perivascular mast cells in the rat mesenteric venule, and whether o ral administration of ketotifen suppress the degranulation. Intravital micr oscopy was used to monitor the rates of adherence and extravasation of leuk ocytes in single unbranched venules with diameters between 25 and 35 mu m o f rat mesentery. Leukocyte rolling velocity, red blood cell velocity, vesse l diameter and blood pressure were also measured. Mast cell degranulation w as quantified within 30 mu m from the venule. N-G-nitro-L-arginine methyl e ster (L-NAME) at an intravenous dose of 30 mg kg(-1) increased the number o f degranulated cells, while its enantiomer, D-NAME at the same dose had no effect. Superfusion with methylene blue (MB), an inhibitor of soluble guany late cydase, at 50 mu M elicited similar degranulation of the mast cells. T he degranulation was associated with increased adhesion of leukocytes to th e endothelium and the slowed rolling. Pretreatment with ketotifen at an ora l dose of 1 mg kg(-1) inhibited mast cell degranulation in responses to eit her L-NAME or MB. It is conceivable that guanylate cyclase for NO productio n pathway in endothelial and/or mast cells is involved in the mast cell deg ranulation process, and the process or subsequent action of NO may be prese rved by ketotifen, eliciting down-modulation of mast cell activation. (C) 1 999 Academic Press.