Effect of temperature on alpha 2- and alpha 1-adrenoceptor-mediated responses in the pithed rat and in the rat vena cava.

1. The influence of body temperature on the alpha 1- and alpha 2-adrenoceptor-mediated pressor responses has been investigated in the pithed rat. 2. The pressor responses to noradrenaline, to the full alpha 1-adrenoceptor agonists phenylephrine and cirazoline and to tyramine were not influenced by lowering the temperature from 36-37 degrees C to 27-29 degrees C. In contrast, the dose-response curves for the pressor effects of the partial alpha 1-adrenoceptor agonist ST 587 and of the alpha 2-adrenoceptor agonist B-HT 920 (2-amino-6-allyl-5,6,7,8-tetrahydro-4H-thiazolo-[4,5-d]-azepine), B-HT 933 (2-amino-6-ethyl-5,6,7,8-tetrahydro-4H-[4,5-d]azepine), clonidine, moxonidine and M-7 (2-dimethylamino-5,6-dihydroxy-1,2,3,4-tetrahydronaphthalene hydrobromide) were markedly depressed (without change in the ED50 values), when the body temperature was lowered from 36-37 degrees C to 27-29 degrees C. 3. After i.v. administration of yohimbine, there was a rightward shift of the dose-response curve for B-HT 920, and the degree of this shift was the same at all temperatures investigated. 4. In the rat vena cava preincubated with [3H]-noradrenaline, the B-HT 920-induced alpha 2-adrenoceptor-mediated inhibition of electrically evoked tritium overflow was also reduced at lower temperature. 5. These results are compatible with the suggestion that cooling decreases the alpha 2-adrenoceptor-mediated pre- and postsynaptic responses in the rat vena cava and pithed rat respectively, leaving the pressor effect induced by full, but not partial alpha 1-adrenoceptor agonists in the pithed rat unaffected. 6. These differences may partly be related to differences in receptor reserve for alpha 1- and alpha 2-adrenoceptors in the pithed rat preparation.