1The effects of the ax-adrenoceptor antagonist prazosin and the x2-adrenoceptor antagonist yohimbine were examined against stimulation-evoked contractions in human isolated saphenous veins.2 The concentration of yohimbine producing 30% inhibition of stimulation-evoked contractions (IC30) was 13.2 nM, whereas the IC30 of prazosin was greater than 250 nM. 3 The inhibition of stimulation-evoked contractions by yohimbine was not prejunctionally mediated since yohimbine (0.01-0. I IM) significantly potentiated the stimulation-evoked overflow of tritium in tissues pre-incubated with [3H]-noradrenaline. 4 The high potency of yohimbine and the low potency of prazosin indicate that neuro-effector transmission in human saphenous vein is mediated predominantly by postjunctional M2-adrenoceptors.

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We have examined the responsiveness of alpha- and beta-adrenoceptors in aortae from 1.5, 3, 6 and 24 month old rats. The isometric contraction to phenylephrine was antagonised competitively by prazosin with a pA2 value of 9.45, suggesting that the receptor is an alpha 1-adrenoceptor. The potency of phenylephrine was significantly reduced in 24 months old as compared with all younger rats combined. The maximum contraction to phenylephrine was unaltered in 24 month old rats. The maximum contraction to potassium chloride was significantly less than that to phenylephrine only in 1.5 months old rats. In tissues contracted by potassium chloride, isoprenaline produced a marked relaxation in 1.5 months old animals, but there was a progressive loss with increasing age of the beta-adrenoceptor-mediated relaxation which was markedly reduced by 6 months and abolished in 24 months old. It is concluded that, in the rat aorta, there is a decrease in alpha 1-adrenoceptor responsiveness in senescence, and a loss of beta-adrenoceptor-mediated responses in maturation.

1 We have examined the effects of antagonists on the isometric contraction of the human saphenous vein produced by 5-hydroxytryptamine (5-HT).2 The 5-HT2-antagonist ketanserin (1 gM) had little effect on the lower part of the concentrationresponse curve to 5-HT, but markedly shifted the upper part of the curve. 3 Yohimbine caused an approximately parallel shift of the concentration-response curve to 5-HT, with a pA2 of 5.48, much lower than its pA2 against noradrenaline in the absence (6.36) or presence (7.06) of cocaine. 4 It is concluded that there are two components to the contractile response to 5-HT in human saphenous vein: at low concentrations 5-HT activates a yohimbine-sensitive receptor, and at higher concentrations 5-HT activates a 5-HT2-receptor.

1 We have examined the pre-and post-junctional effects of a series of x-adrenoceptor agonists and antagonists at M2-adrenoceptors in the pithed rat preparation and the human isolated saphenous vein. 2 In the pithed rat, there was no difference in relative agonist and antagonist potencies between preand post-junctional CX2-adrenoceptors but the absolute potencies of antagonists differed: antagonists were more potent prejunctionally. 3 In the human saphenous vein, the ot2-adrenoceptor antagonist yohimbine had pre-and postjunctional actions over the same concentration range. 4 We have no evidence to suggest differences between pre-and post-junctional m2-adrenoceptors: differences in absolute antagonist potencies in the pithed rat may be due to non-equilibrium conditions.