CHARACTERISTICS OF HISTAMINE-INDUCED LEUKOCYTE ROLLING IN THE UNDISTURBED MICROCIRCULATION OF THE RAT MESENTERY

1 The main objective of this study was to analyse the role and mode of action of the mast cell mediator histamine in leukocyte-endothelium i nteractions in small venules in vivo. For this put-pose. we used a his tological approach (combined with intravital microscopy) that allows s tudies of rapid mediator-induced venular leukocyte accumulation, refle cting leukocyte rolling, in the undisturbed microcirculation of the ra t mesentery where rolling is normally absent. 2 We first examined the relative importance of histamine and 5-hydroxytryptamine (5-HT) in acu te mast cell-dependent leukocyte recruitment. The mast cell secretagog ue compound 48/80 (i.p. for 15 min) induced a marked venular accumulat ion of polymorphonuclear leukocytes (PMNL) which was almost abolished by combined histamine(1) (H-1)- and histamine(2) (H-2)-receptor blocka de. In contrast, the 5-HT- receptor antagonist methysergide was inacti ve in this regard. Moreover, exogenous 5-HT was less active than exoge nous histamine in evoking venular PMNL accumulation (histamine respons e dose-dependent; 5-HT response bell shaped). Prostaglandin D-2 did no t cause PMNL accumulation. 3 The venular PMNL response to exogenous hi stamine peaked between 15 min and 1 h. was still significantly elevate d at 2 hi and then returned to prechallenge values after 3 h. At all t ime points, the histamine-induced PMNL accumulation was nearly abolish ed by i.v, treatment with the polysaccharide fucoidin (which blocks ro lling but not firm adhesion per se), suggesting that the PMNL response to histamine was due to rolling rather than firm adhesion over the en tire 3 h period. At no time Feint did histamine trigger accumulation o f mononuclear leukocytes (MNL). 4 To examine the role of histamine-rec eptors in the histamine-induced PMNL accumulation (i.e. rolling). the animals were pretreated with diphenhydramine (Hi-receptor antagonist), cimetidine. or ranitidine (H-2-receptor antagonists). Diphenhydramine alone inhibited the venular PMNL response to histamine by 52%, while both H-2-receptor antagonists were completely inactive. However, the c ombination of cimetidine and diphenhydramine reduced the histamine-ind uced PMNL rolling by 82%. Furthermore? in contrast to an H-3-receptor agonist challenge with either the H-1-receptor agonist 2-thiazolylethy lamine or two different H-2-receptor agonists (impromidine, dimaprit) was sufficient to provoke significant venular PMNL accumulation. 5 Tre atment with the nitric oxide-synthase inhibitor L-NAME did not affect the histamine-induced PMNL rolling. On the other hand, 3 h pretreatmen t with dexamethasone reduced the PMNL response to histamine by 73%, an d flow cytometric analysis showed that the dexamethasone treatment alm ost completely inhibited binding of soluble P-selectin to rat isolated PMNLs. 6 We conclude that initial leukocyte recruitment after mast ce ll activation in the rat mesentery is critically dependent on histamin e release. The cellular response to histamine was specifically due to PMNL rolling, involved activation of both H-1- and H-2-receptors, and lasted for 2-3 h. Moreover? the histamine-induced PMNL rolling was not dependent on nitric oxide synthesis, but was sensitive to glucocortic oid treatment, possibly via inhibition of expression or function of le ukocytic P-selectin ligand(s).