α‐Adrenoceptor subtypes in dog saphenous vein that mediate contraction and inositol phosphate production

1 Studies have been made of the contractile responses to the α‐adrenoceptor agonists phenylephrine (Phen), cirazoline (Cir) or BHT‐920 (BHT) in dog isolated saphenous vein (DSV) rings, using the antagonists yohimbine (Yoh), idazoxan (Idaz), prazosin (Praz), WB‐4101 (WB) and nitrendipine or zero Ca2+ medium. 2 Contractile concentration‐response curves to Phen or BHT were displaced to the right of controls by Yoh (0.01–3 μm) with mean apparent antagonist dissociation constants (pKBs) of 7.9 and 8.6 respectively. Yoh did not show simple competitive antagonism against either agonist, since the Schild plot slopes were significantly less than unity. Neither the antagonist affinity of Yoh against Phen, nor the slope of the Schild plot was modified in the presence of catecholamine uptake inhibitors, nor in the presence of α,β‐methylene ATP, which desensitizes P2‐purinoceptors, suggesting that Phen does not release ATP, or noradrenaline to cause contraction in DSV. In the presence of Praz (0.3 μm) the antagonist potency of Yoh (mean pKB 7.4) against Phen was slightly decreased. Yoh had low potency against responses induced by Cir(pKB6.3). 3 WB (0.001–1.0 μm) was a very potent antagonist of Phen‐induced contractions, however, the biphasic Schild plot against Phen could be separated into two affinity sites, a high pKB of 9.3 (equivalent to that obtained using Cir as the agonist; pKB 9.6) and a lower affinity (pKB 8.6). WB showed an even lower antagonist affinity (pKB 7.4) against BHT‐induced contractions, suggesting that these effects might be mediated by α2A‐adrenoceptors. Praz also appeared to identify two sites using Phen‐induced contractions, a high pKB of 8.4 was equivalent to that obtained with Cir (pKB 8.2) and a lower affinity site (pKB 7.7; pA2 7.6; slope 1.1) at which Praz showed competitive antagonism. Higher concentrations of Praz were required to antagonize contractions to BHT (pKB 5.9). 4 Idaz was a weak partial agonist in this tissue with threshold contractile effects at concentrations in excess of 3 μm. Idaz (0.1–1 μm) competitively antagonized the contractile effects of BHT, but showed low antagonist affinity against Phen at these concentrations. 5 Contractions to Phen were slightly antagonized by nitrendipine (1μm), with a 36% decrease in Emax. Contractions to Phen and Cir were also markedly attenuated in zero calcium medium (with EGTA), but maximum responses of 4.2 ±0.1 and 3.6 ± 0.1 g, could be obtained with these agonists respectively. Only part of the contractile effects to Phen or Cir are therefore due to calcium influx (but L‐type channels are not totally implicated), while the contractile effects of BHT were abolished in zero Ca2+ medium. Yoh (0.1 μm) retained its antagonist effects on Phen‐induced responses in zero Ca2+ medium. 6 The formation of inositol phosphates (InsPs) in the presence of lithium (10 mm) was measured after incubation of intact DSV strips with myo‐2‐[3H]‐inositol. Phen (1–1000μm) and Cir (0.01–10 μm) induced concentration‐dependent increases in total labelled InsP1–3, but BHT showed minimal InsP stimulation. InsPs were recovered after Phen (100 μm) stimulation (10 min) as labelled InsP1 (71%), InsP2 (25%) and InsP3 (4%). Phen (100 μm)‐stimulated InsP1–3 formation was significantly antagonized by Praz (10 nm), but was not fully inhibited even after Praz 1 μm. Yoh and Praz (0.1 and 1.0 μm) were equipotent inhibitors of this response, while Idaz (0.3 μm) showed no effects. 7 The receptors in DSV which are stimulated by Phen to cause contraction show characteristics of the α1A‐adrenoceptor (high pm antagonist affinity for WB‐4101 and extracellular calcium sensitivity) and the α1B‐adrenoceptor (contraction in calcium‐free medium, increase in InsP and low nm antagonist affinity of WB). The paradoxical results obtained with Yoh (potent antagonist effects on Phen‐stimulated PI and pKB 7.9 on contraction) and Praz (low affinity competitive antagonist of Phen‐induced contraction, pKB 7.7 and failure to inhibit completely the PI response at 1 μm), cannot fully exclude an α2B‐subtype characterization of these responses. These pharmacological differences suggest that the adrenoceptor involved in the contractile and in particular the second messenger effects of Phen in DSV is not typically an α1B‐adrenoceptor.