Postsynaptic alpha-adrenoceptor characterization and Ca2+ channel antagonist and activator actions in rat tail arteries from normotensive and hypertensive animals

Postsynaptic alpha-adrenoceptors in the rat tail artery have been examined by determining the pA2 values for antagonists against several alpha-adrenoceptor agonists. In this tissue the alpha-adrenoceptor agonists all produce concentration-dependent mechanical responses with the following rank order of potency: clonidine greater than norepinephrine greater than phenylephrine greater than UK 14304 greater than B-HT 920. Antagonism by prazosin and yohimbine of phenylephrine, norepinephrine, and clonidine responses does not reveal the anticipated discrimination between alpha 1- and alpha 2-adrenoceptors. Thus, pA2 values for prazosin (9.1-9.5), yohimbine (7.2-7.4), and corynanthine (7.0-7.1) and idazoxan (7.6) do not show large differences between these receptor agonists and suggests the predominance of alpha 1-adrenoceptor mediated contractile responses in this preparation. Significant differences between antagonist activities (pA2 values) in Wistar Kyoto (WKY) and spontaneously hypertensive rats (SHR) artery preparations have not been observed. The sensitivity sequence of alpha-adrenoceptor agonist-induced responses to nifedipine and D 600 is B-HT 920 greater than clonidine greater than phenylephrine greater than norepinephrine. Dependence of agonist response upon extracellular Ca2+ parallels the sensitivity to Ca2+ channel antagonists. Sensitivity to D 600 of phenylephrine responses increased with decreasing concentration of phenylephrine or with receptor blockade by phenoxybenzamine: sensitivity of responses to B-HT 920 was not affected by these procedures. Tail artery strips from WKY and SHR do not exhibit major differences in sensitivity to D 600 or to Ca2+ depletion. Bay k 8644, a Ca2+ channel activator, produces concentration-dependent mechanical responses in the tail artery in the presence of modestly elevated K+ concentrations (10-15 mM): these actions of elevated K+ can be mimicked by both alpha 1- and alpha 2-adrenoceptor agonists including methoxamine, St 587, UK 14304, and clonidine. These studies do not provide clear evidence for the existence of discrete postsynaptic alpha 1- and alpha 2-adrenoceptor populations in rat tail artery as indicated by pA2 values or Ca2+ dependence of response.