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

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Abstract
Postsynaptic α-adrenoceptors in the rat tail artery have been examined by determining the pA2 values for antagonists against several α-adrenoceptor agonists. In this tissue the α-adrenoceptor agonists all produce concentration-dependent mechanical responses with the following rank order of potency: clonidine > norepinephrine > phenylephrine > UK 14304 > B-HT 920. Antagonism by prazosin and yohimbine of phenylephrine, norepinephrine, and clonidine responses does not reveal the anticipated discrimination between α1- and α2-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 α1-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 α-adrenoceptor agonist-induced responses to nifedipine and D 600 is B-HT 920 > clonidine > phenylephrine > 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 α1- and α2-adrenoceptor agonists including methoxamine, St 587, UK 14304, and clonidine. These studies do not provide clear evidence for the existence of discrete postsynaptic α1- and α2-adrenoceptor populations in rat tail artery as indicated by pA2 values or Ca2+ dependence of response.

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