No evidence for differences between pre‐ and postjunctional α₂‐adrenoceptors in the periphery

Abstract

1 We have compared prejunctional x2-adrenoceptors in rat and guinea-pig vas deferens and rat and guinea-pig atria with postjunctional a2-adrenoceptors in human saphenous vein and human platelets employing the antagonists yohimbine and SK&F 104078 and other a2-adrenoceptor antagonists. 2 Yohimbine was approximately 10 times more potent prejuunctionally than SK&F 104078 at antagonizing the inhibition by the x2-adrenoceptor agonist xylazine of stimulation-evoked contractions in rat and guinea-pig vas deferens, and at increasing stimulation-evoked release of tritium in rat and guinea-pig atria pre-incubated with [3H]-noradrenaline. 3 Yohimbine was approximately 10 times more potent postjunctionally than SK&F 104078 at antagonizing contractions to noradrenaline in human saphenous vein and at displacing [3H]-yohimbine binding in human platelet membranes. 4 For the antagonists yohimbine, SK&F 104078, prazosin, phentolamine, CH 38083 and urapidil, there was a significant correlation between prejunctional potency in rat vas deferens atrium and postjunctional potency in human platelet, although the correlation was improved by the omission of prazosin. 5 We have no evidence for differences between functional pre-and postjunctional ac2-adrenoceptors in the periphery, although these functional receptors may differ from the ligand binding site in the human platelet.

Epididymal contraction and sperm parameters are affected by clonidine

“…We found high pA 2 values to prazosin against contractions induced by the selective agonist of a 1 -adrenoceptors phenylephrine which is compatible to those reported for a 1 -adrenoceptors involved in the contractions of other smooth muscles (Adenekan & Tayo, 1982;Stam et al, 1999). In addition, the pA 2 values obtained in this study for idazoxan against contractions induced by UK 14 304 were comparable to those reported for a 2 -adrenoceptors (Doxey et al, 1984;Connaughton & Docherty, 1990). Thereby, the profile of antagonists tested in this study against contractions of distal cauda epididymis induced by selective agonists of a-adrenoceptors suggests the participation of both post-synaptic a 1 -and a 2 -adrenoceptors.…”

Investigation of postjunctional α1‐ and α2‐adrenoceptor subtypes in vas deferens from wild‐type and α2A/D‐adrenoceptor knockout mice

“…Preparation of rat vas deferens membranes was carried out largely as described for rat kidney membranes in Connaughton & Docherty (1990). The resultant pellets were used immediately or stored at −20°C for later use.…”

Investigations of the subtype of α2‐adrenoceptor mediating contractions of the human saphenous vein

“…Due to the difficulties inherent in amassing data from the human saphenous vein, it was not possible to obtain pA2 values from Schild plots for the 12 antagonists, so that the less reliable KB value was used. Admittedly, we have previously obtained pA2 values for three of the antagonists employed in this study (yohimbine, prazosin, SKF 104078: Connaughton & Docherty, 1990b), so that these three at least can be regarded as competitive antagonists in the human saphenous vein. The postjunctional a-adrenoceptors of the human saphenous vein appear to be a homogeneous population of C2-adrenoceptors with no evidence for aladrenoceptors since: yohimbine was approximately 10 times more potent than prazosin in antagonizing contractions to noradrenaline; both yohimbine and prazosin produced parallel shifts in noradrenaline concentration-response curves (see Figure 1); both yohimbine and prazosin behaved competitively against noradrenaline with slopes of Schild plots not significantly different from negative unity (Connaughton & Docherty, 1990a); prazosin inhibited contractions to the a,-adrenoceptor selective agonist phenylephrine with a KB similar to that obtained against noradrenaline; the potent al-adrenoceptor antagonists HV 723 (see Muramatsu et al, 1990) had relatively low potency against noradrenaline.…”