The vascular changes after ephedrine tachyphylaxis.

The changes elicited after ephedrine tachyphylaxis in the dog femoral and rabbit aortic strips isolated from the untreated and ephedrine pre-treated animals have been studied. In untreated strips, ephedrine exhibited dose-dependent contractions which were blocked by phenoxybenzamine. These contractile responses to ephedrine were reduced after pre-treatment with ephedrine in vivo. In rabbit aortic strips previously contracted with noradrenaline or KCL, ephedrine induced dose-dependent relaxations at high concentrations, which were not affected by propranolol. These relaxation responses were likewise diminished after ephedrine. Dose-related contractile responses to noradrenaline were potentiated at low concentrations and depressed at high concentrations after ephedrine whereas those to adrenaline were inhibited over the entire agonist range. Responses to KCL were not affected. These reductions in the responses to noradrenaline and adrenaline after treatment with ephedrine in vivo were inhibited by icreased calcium2+ concentration. From the results, it can be presumed that the observed changes in vascular responsiveness may be partially involved in the development of ephedrine tachyphylaxis.