THE ACTION OF I-TETRAHYDROPALMATINE ON_(α_1) AND α_2 ADRENOCEPTORS
Yao Bing, et al (Department of Pharmacology, Xuzhou Medical College, Xuzhou, Jiangsu, China )
1-THP selectively inhibted the specific binding of 3H-Prazosine and 3H-clonidine to α1 and α2 adrenoceptors of rat cerebral cortex cell membrane with affinity ( PKi ) of 5.84 and 5.87 respectively. The cumulative dose-effect curves for PhenylePhrine were shifted to the right Parallelly by prazosin (4.3, 8.6, 12.9, 17.2nM)and 1-THP (4.68, 9.36, 14.0, 23.4uM ) in isolated rat aortic strips with no change in the maximal response. The PA2 values of 1-THP and Prazosin were 5.59 and 8.86 respectively. 1-THP 2.5 mg/kg iv decreased the blood Prassure in normal rats, but increased the blood Pressure in reserPinized rats, and this Pressor action was antagonized by yohimbine 3.56 mg/kg iv. 1-THP 20 mg/kg iv inhibiteb the Pressor action of clonidine. These results indicate that 1-THP blocks α1 adrenoceptors, small-dose 1-THP stimulates Presynaptic ct2 adrenoceptors, and large-dose 1 -THP also blocks PostsynaPtic α2 adrenoceptors.