Identification of histamine H-3 receptors in the tail artery from normotensive and spontaneously hypertensive rats

We examined the possible existence of prejunctional histamine Hg receptors on sympathetic nerve fibers innervating rat tail artery. The stimulation-evoked tritium outflow from isolated vessels preincubated with [H-3]-noradrenaline and perfused/superfused in the presence of the alpha(2)-adrenoceptor antagonist rauwolscine, 3 mu M, was inhibited by histamine 10 mu M (by 8%) and the H-3 agonists R-(-)-alpha-methylhistamine, 10 mu M (by 18%), and imetit, 0.1-10 mu M (by less than or equal to 20%). The inhibitory effect of imetit, which did not occur in the absence of ranwolscine, was counteracted by thioperamide, 1 mu M. In the presence of rauwolscine, 3 mu M, the inhibitory effect of imetit also occurred when the current strength or the Ca2+ concentration in the medium was reduced to compensate for the increase in tritium overflow elicited by rauwolscine, indicating that the inhibitory action of imetit is not associated with the increase in noradrenaline release produced by rauwolscine. In spontaneously hypertensive rats (SHRs), imetit also inhibited the overflow of tritium. This inhibitory effect was comparable to that observed in Wistar-Kyoto (WKY) rats and indicates that the sympathetic nerves of the rat tail artery in SHRs, like those in normotensive rats, are endowed with prejunctional histamine H-3 receptors.