Gibiszer K P, Pórszász J
Acta Physiol Acad Sci Hung. 1977;49(2):167-74.
In cats under chloralose-urethan anaesthesia, tachyphylaxis developed to the hypotensive effect of clonidine. Development of this tachyphylaxis was slow in the case of intravenous low doses (10--20 micrograms/kg) and rapid after high doses (50 micrograms/kg). No tachyphylaxis developed to contractions of the nictitating membrane. The phenomenon resembles the tachyphyaxis seen after the application of indirect sympathomimetics. A tachyphylaxis developed to the latter agents inhibited the hypotensive action of clonidine which, after such an intervention, invariably turned into a vasopressor effect. Clonidine exhibited the same behaviour after pretreatment with cocaine, bretylium or reserpine. Infusion of noradrenaline (10 micrograms/kg/min) counteracted the tachyphylaxis. Thus, clonidine is assumed to exert its hypotensive effect in an indirect way, i.e. by interference with the metabolism of noradrenaline.
在使用氯醛糖-乌拉坦麻醉的猫中，可乐定的降压作用出现了快速耐受性。静脉注射低剂量（10 - 20微克/千克）时，这种快速耐受性的发展较为缓慢，而高剂量（50微克/千克）后则迅速出现。瞬膜收缩未出现快速耐受性。该现象类似于应用间接拟交感神经药后出现的快速耐受性。对后者药物产生的快速耐受性抑制了可乐定的降压作用，在此类干预后，可乐定的作用总是转变为升压效应。在用可卡因、溴苄铵或利血平预处理后，可乐定表现出相同的行为。输注去甲肾上腺素（10微克/千克/分钟）可抵消快速耐受性。因此，推测可乐定以间接方式发挥其降压作用，即通过干扰去甲肾上腺素的代谢。
