Heterogeneity of the interaction between alpha 1- and alpha 2-adrenoceptor agonists with their respective receptors in the vascular system of the pithed rat.

The subdivision of alpha 1- and alpha 2-adrenoceptor-mediated pressor responses to different agonists based upon the influence of beta 2-adrenoceptor-mediated vasodilatation was further investigated in the pithed normotensive rat. The effect of salbutamol (4.18 X 10(-6) mol/kg) on the alpha 1-adrenoceptor-mediated increase in diastolic pressure due to dopamine and amidephrine as well as on the alpha 2-adrenoceptor-mediated pressor response to azepexole, DP-6,7-ADTN, M-7, TL-99 and dopamine was assessed. The alpha 1-pressor responses to amidephrine and dopamine were only slightly attenuated by salbutamol. The alpha 2-adrenoceptor-mediated increase in diastolic pressure due to B-HT 933 was strongly antagonized by salbutamol in contrast to the effect of dopamine, DP-6,7-ADTN and M-7. TL-99 occupied in intermediate position. The data do not support the existence of distinctly different subtypes of alpha 1- and alpha 2-adrenoceptors but favor the hypothesis that both alpha 1- and alpha 2-adrenoceptors are activated in a unique way by each of their respective agonists.