In situ effects of histaminergic agents on adrenergic neurotransmission in rat mesenteric microcirculation.

Experiments were performed to study in situ the effects of histamine and some H1- or H2-agonists and antagonists on the contractions of the rat mesenteric methaarterioles (RMMA) induced by 0.6 nM noradrenaline (NA) applied locally or by electrical stimulation (ES). All drugs tested (cimetidine--300 nM, the others --30 nM) were also applied locally 30 s before the contractile stimulus. NA and ES strongly contracted the blood vessels. Of all drugs studied only diphenhydramine (DPH) evoked a slight contraction of RMMA. Histamine markedly inhibited the contractile responses to NA and ES. Similar effects were exerted by 2-(2-aminoethyl)thiazole (2-AET) its inhibitory action on the ES-induced contractions being weaker than that of histamine. Dimaprit (DMP) only tended to inhibit the RMMA contractions. DPH abolished the NA-induced contractions while cimetidine (CIM) did not change the effects of NA and ES. The results suggest that H1-receptors are mainly involved in the modifying action of histaminergic drugs on the adrenergic neurotransmission in RMMA.