Light K E, Hughes M J
Res Commun Chem Pathol Pharmacol. 1979 Mar;23(3):433-51.
The effects of clonidine, a compound used clinically as a centrally active antihypertensive, were studied on isolated, spontaneously beating atria of rabbit and guinea pig. In rabbit atria clonidine induces a marked depression of spontaneous chronotropic activity which was not altered by pretreatment with atropine, and not completely reversed by washing. In guinea pig atria, clonidine alone induces a very small increase in rate. A significant decrease in rate however is unmasked when tissues are pretreated with metiamide (H2-antagonist). Since both rabbit and guinea pig atria contain histamine receptors the lack of a positive chronotropic response in rabbit atria suggests that clonidine has no direct H2-receptor activity. The small positive response obtained with guinea pig atria which can be blocked by metiamide suggests that clonidine can act indirectly by releasing endogenous histamine stores from this tissue. Clonidines direct action appears to be a significant depression of both rabbit and guinea pig atrial chronotropic activity.
可乐定是一种临床上用作中枢性抗高血压的化合物，研究了其对兔和豚鼠离体、自发搏动心房的作用。在兔心房中，可乐定可显著抑制自发变时性活动，这种抑制不受阿托品预处理的影响，且冲洗后也不能完全逆转。在豚鼠心房中，单独使用可乐定可使心率略有增加。然而，当用甲硫咪特（H2拮抗剂）预处理组织时，心率会显著降低。由于兔和豚鼠心房都含有组胺受体，兔心房缺乏正性变时反应表明可乐定没有直接的H2受体活性。豚鼠心房获得的小的正性反应可被甲硫咪特阻断，这表明可乐定可通过释放该组织中的内源性组胺储存而间接发挥作用。可乐定的直接作用似乎是显著抑制兔和豚鼠心房的变时性活动。
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