Greater activation of smooth muscle alpha-2 adrenoceptors by epinephrine in distal than in proximal segments of rat tail artery.

The effects of selective blockade of alpha-1 and alpha-2 adrenoceptors on the vasoconstrictor responses to epinephrine (EPI) and norepinephrine (NOR) were compared in perfused/superfused proximal and distal segments of rat tail artery. The influence of neuronal uptake and of activation of beta adrenoceptors was also investigated. EPI was more potent in distal than in proximal segments. The antagonistic effect of idazoxan (100 nM) against EPI was greater in distal than in proximal segments, whereas the opposite result was obtained with prazosin (0.1-10 nM). No such difference were observed when NOR was used as agonist. Reducing the calcium ion concentration had a greater inhibitory effect on EPI in distal than in proximal segments. Cocaine (4 microM) increased responses to EPI and NOR to a greater extent in proximal than in distal segments. In the presence of cocaine, in proximal segments, antagonism of EPI by prazosin was reduced, whereas in distal segments, antagonism by idazoxan and the inhibitory effect of a reduction in calcium ion concentration were reduced. Propranolol (1 microM) increased responses to EPI and NOR to a greater extent in proximal than in distal segments. In the presence of propranolol, antagonism of EPI by both prazosin and idazoxan was reduced in proximal segments, and the inhibitory effect of a reduction in calcium ion concentration was lost. Forskolin (1 microM) inhibited responses to EPI and prevented the antagonistic effect of idazoxan, but not that of prazosin. From the results obtained it is suggested that smooth muscle alpha-2 adrenoceptors are distributed differently in the proximal and distal ends of the rat tail artery.(ABSTRACT TRUNCATED AT 250 WORDS)