Postjunctional α1- and α2-adrenoceptors mediating contraction in isolated human groin arteries and veins

By means of selective agonists and antagonists for α₁- and α₂-receptors, the α-receptor subtypes in human groin arteries and veins were characterized and compared. In the arteries the α₁-receptor blocker prazosin caused a concentration-dependent parallel displacement of the noradrenaline (NA) concentration-response (cr) curve without reduction of maximum (pA₂=9.86); the selective α₂-receptor antagonist rauwolscine in the concentration 10^-8 M caused a right-ward shift of the NA cr-curve without reduction of E_max, but 10^-7 M and 10^-6 M caused little or no further shift. In the veins, the two antagonists had the opposite effects. Rauwolscine caused a concentration-dependent right-ward shift of the NA cr-curve without depression of maximum (pA₂=9.03); prazosin 10^-9 M significantly displaced the NA cr-curve, whereas 10^-8 M and 10^-7 M caused little or no further shift. The responses to the α₂-receptor agonist clonidine in the arteries were too small to allow calculations of pEC50 values; in the veins contractions were elicited in all vessel segments investigated (pEC₅₀=6.24). Phenylephrine, selective for α₁-receptors, was significantly more potent in arteries than in veins. NA was significantly more potent in veins than in arteries. It is concluded that in human groin vessels, there is a functional predominance of arreceptors in the arteries and of a2-receptors in the veins.     

Pharmacological characterization of postjunctional α-adrenoceptors in isolated human omental arteries and veins

The α-adrenoceptors in human omental arteries and veins were characterized and compared. In the arteries both prazosin (pA₂ 9.48) and rauwolscine (pA₂ 7.19) displaced the noradrenaline (NA) concentration-response (cr) curve towards higher concentrations without reduction of maximum. Neither clonidine, nor oxymetazoline had any consistent contractile effects. Phenylephrine had a lower potency than NA, but a similar intrinsic activity. In the veins, both prazosin (pA₂ 9.72) and rauwolscine (pA₂ 8.11) displaced the NA cr-curve towards higher concentrations, but also significantly depressed maximum. Clonidine and oxymetazoline contracted veins from 3 out of 7 and 4 out of 6 patients, respectively. Their pD₂-values were similar to that of NA, but their intrinsic activities were significantly lower. NA was more potent than phenylephrine in these vessels, and there was no significant difference in intrinsic activity. The results suggest that in human omental arteries, the postjunctional a-adrenoceptors are mainly of the α₁-type, even if a small population of α₂-adrenoceptors cannot be excluded. In omental veins, there seems to be a functionally important population of postjunctional α₂-adrenoceptors occurring together with a population of α₁-adrenoceptors.     

A heterogeneous population of alpha 1-adrenoceptors mediates contraction of the isolated follicle wall from the bovine ovary

Strips from Graafian follicles of bovine ovaries were tested for their contractile response in vitro in order to characterize the type of post-junctional alpha-adrenoceptor involved. Electrically induced contractions were inhibited concentration-dependently by the alpha 1-antagonist, prazosin. Besides noradrenaline the alpha 1-selective agonists, methoxamine and phenylephrine, caused the strips to contract, whereas the alpha 2-selective agonists clonidine, oxymetazoline and B-HT920 were without effect. However, the alpha 1-selective antagonist prazosin gave a line with a slope less than unity in the Schild plots with noradrenaline and methoxamine. From results obtained with or without the presence of two classes of neuronal uptake blockers (desipramine and cocaine) it is concluded that the post-junctional alpha 1-receptor population is inhomogeneous. The regular appearance of the Schild plot obtained with phenylephrine may be due to involvement also of a component of noradrenaline release by this agonist. The pA2 value in the test with phenylephrine was 9.27, with a corresponding kB of 3.81 +/- 1.15 X 10(-10) M.     

Pharmacological characterization of postjunctional alpha-adrenoceptors in isolated human omental arteries and veins

The alpha-adrenoceptors in human omental arteries and veins were characterized and compared. In the arteries both prazosin (pA2 9.48) and rauwolscine (pA2 7.19) displaced the noradrenaline (NA) concentration-response (cr) curve towards higher concentrations without reduction of maximum. Neither clonidine, nor oxymetazoline had any consistent contractile effects. Phenylephrine had a lower potency than NA, but a similar intrinsic activity. In the veins, both prazosin (pA2 9.72) and rauwolscine (pA2 8.11) displaced the NA cr-curve towards higher concentrations, but also significantly depressed maximum. Clonidine and oxymetazoline contracted veins from 3 out of 7 and 4 out of 6 patients, respectively. Their pD2-values were similar to that of NA, but their intrinsic activities were significantly lower. NA was more potent than phenylephrine in these vessels, and there was no significant difference in intrinsic activity. The results suggest that in human omental arteries, the postjunctional alpha-adrenoceptors are mainly of the alpha 1-type, even if a small population of alpha 2-adrenoceptors cannot be excluded. In omental veins, there seems to be a functionally important population of postjunctional alpha 2-adrenoceptors occurring together with a population of alpha 1-adrenoceptors.     

Functional beta 1- and beta 2-adrenoceptors in the human myocardium

Functional beta-adrenoceptor populations in the human heart were studied in vitro in electrically-paced strips of the right auricular and ventricular myocardium. The relative potency of selected agonists in producing inotropic responses (Tmax, T'max) in the presence of blockers for neuronal and extraneuronal uptakes was found to be as follows: isoprenaline greater than noradrenaline = adrenaline = salbutamol greater than dobutamine. Prenalterol had a negative inotropic effect in these preparations. The selective beta 1-(practolol) and beta 2-(H 35/25) blockers reduced inotropic responses to adrenaline (Tmax, T'max) and noradrenaline (T'max) in the auricular strips. These results indicate the participation of beta 2-adrenoceptors in inotropic responses in the human auricular and ventricular myocardium. For comparison, inotropic responses of electrically-paced rat myocardium to beta-adrenergic agonists in the presence of blockers for neuronal and extraneuronal uptakes were likewise studied. The relative potencies for Tmax were: noradrenaline = adrenaline greater than prenalterol greater than dobutamine = salbutamol. Given the high relative potency of salbutamol in the human myocardial strips (analogous to that previous shown in the beta 2-dominated atria of the frog and trout) and the low relative potency of salbutamol in the rat tissue, these findings indicate a greater population of functionally active beta 2-adrenoceptors in the human than in the rat myocardium.     

The role of alpha1-adrenoceptors in the clonidine-induced contraction and relaxation of rat aorta

Clonidine causes dilatation of the aorta in the presence of endothelium, while it causes contraction of the aorta in the absence of endothelium. The present study was carried out to clarify the role of alpha-1-adrenoceptors in the vascular action of clonidine. The aortic rings were suspended in Krebs-Henseleit (K-H) medium, and the effects of alpha-1- and alpha-2-adrenoceptor antagonists on the clonidine-induced contractions were measured. Moreover, the role of the phosphatidylinositol (PI) response was examined. The aortic slices were incubated in K-H medium containing, [3H]myo-inositol and clonidine. The formation of [3H]inositol monophosphate (IP1) was measured with a liquid scintillation counter. Clonidine caused contraction of the aorta in the absence of endothelium, in a dose-dependent manner. This contraction was inhibited by antagonists in the following order of the potency: prazosin > phentolamine > spiperone > urapidil = yohimbine > L-659066 > atipamezole. On the other hand, clonidine inhibited norepinephrine (NE)-induced contraction in the aorta in the absence and in the presence of endothelium. Clonidine enhanced IP1 accumulation in the aorta in the absence of endothelium, whereas it inhibited NE-induced IP1 accumulation in the aorta. The present results show that alpha-1-adrenoceptors are probably involved in the clonidine-induced contraction and relaxation of the rat aorta.     

Effects of nimodipine, Bay K 8644 and pinacidil on alpha 1- and alpha 2-adrenoceptor-mediated vasoconstriction in human hand veins

The effects of nimodipine, Bay K 8644 and pinacidil, three drugs interfering with transmembrane Ca2+ fluxes in different ways, were investigated in isolated human hand veins. Their ability to influence the concentration-response relationship for noradrenaline (NA) was assessed in the absence and presence of prazosin or rauwolscine. The contractile response to NA was almost abolished in Ca2+ -free medium. Nimodipine and pinacidil depressed the NA concentration-response curve both in the absence and presence of alpha-adrenoceptor blockers. The NA response was only partially inhibited by nimodipine, indicating that NA may activate nimodipine-insensitive influx pathways, presumably receptor-operated calcium channels. Pinacidil inhibited the contractile response to 124 mM K+ and reduced the NA-induced contraction in the presence of nimodipine, suggesting that pinacidil has actions other than the opening of potassium channels and subsequent membrane hyperpolarization. Bay K 8644 increased the NA potency fourfold in the presence of rauwolscine, whereas it had no effect on the NA response in the presence of prazosin and in the absence of alpha-adrenoceptor blockade. Such an action of Bay K 8644 can be reconciled with alpha 1-adrenoceptor activation causing membrane depolarization and opening of potential-operated calcium channels. It may be concluded that both alpha 1- and alpha 2-adrenoceptor-mediated contractions in human hand veins are highly dependent on Ca2+ influx, although the mechanisms utilized to bring about this influx partly differ between the two receptor subtypes.     

Influence of extracellular calcium and nifedipine on alpha 1- and alpha 2-adrenoceptor-mediated contractile responses in isolated rat and cat cerebral and mesenteric arteries

The influence of extracellular Ca2+ and nifedipine on contractile responses to 10 microM noradrenaline (NA) was investigated in isolated rat and cat middle cerebral (RCA, CCA) and mesenteric (RMA, CMA) arteries. In the CCA (containing predominantly alpha 2-adrenoceptors), the NA-induced contractions developed considerably more slowly than in the RCA, RMA (containing mainly alpha 1-adrenoceptors) and CMA (sensitive to both alpha 1- and alpha 2-adrenoceptor selective antagonists). The tonic component of the NA-induced contraction in the four types of artery was substantially suppressed after only short periods in Ca2+-free solution. In each type of artery, excluding the CCA, the contractile response to 124 mM K+ was more sensitive to Ca2+ deprivation than that to NA. This suggests that NA, besides mobilizing extracellular Ca2+, can also release Ca2+ from an intracellular pool in the RCA, RMA and CMA, but not in the CCA. Thus, alpha 1-adrenoceptor-mediated contractions in the RCA and RMA seem to depend on both Ca2+ influx and intracellular Ca2+ release, whereas alpha 2-adrenoceptor-mediated contractile responses in the CCA appear to rely almost entirely on Ca2+ influx. Both the maximum response and the tonic component of the NA-induced contraction were significantly more sensitive to nifedipine in the CCA than in the RCA. In comparison with the NA-induced contractions in these arteries, those in the RMA and CMA were relatively resistant to nifedipine. In the CCA exposed to NA in Ca2+-free medium, nifedipine almost abolished the contraction induced by re-addition of Ca2+, whereas in the other types of artery, Ca2+ re-application evoked a significant contraction also in the presence of the drug. The differential effects of nifedipine presumably reflect differences between the arteries, not only in the relative contribution of Ca2+ influx and intracellular Ca2+ release to the contractile activation, but also in the nifedipine sensitivity of the Ca2+ entry pathways utilized by NA. It is concluded that the mechanisms through which NA induces contraction seem to be related both to the subtype of alpha-adrenoceptor stimulated by NA and to the type of vessel studied.     

Decreased responsiveness of vascular postjunctional alpha1-, alpha2-adrenoceptors and neuropeptide Y1 receptors in rats with heart failure.

Heart failure is associated with increased sympathetic nerve activity. We hypothesized that chronic sympathetic stimulation in heart failure resulted in decreased vascular sympathetic responsiveness. A pithed rat model was employed to evaluate peripheral vascular alpha-adrenoceptor and neuropeptide Y (NPY) receptor responsiveness. Heart failure was induced in Sprague-Dawley rats by coronary artery ligation. Sham operated rats (Sham) served as controls. Two months after this surgical procedure, both heart failure (n = 30) and Sham (n = 30) rats underwent standard pithing procedure. Pressor responses to preganglionic sympathetic nerve stimulation (PNS) and activation of postjunctional alpha1- and alpha2-adrenoceptors as well as Y1 receptors were studied. In response to PNS, cardiac index was similar between heart failure and sham rats (P = n.s.). Mean arterial pressure (MAP) increased in a frequency-dependent fashion after PNS in heart failure rats as well as in control rats. All the agonists used, i.e. the alpha1-adrenoceptor agonist phenylephrine, the alpha2-adrenoceptor agonists clonidine and BHT933 as well as NPY, induced dose-dependent increases in MAP in heart failure and in sham rats. However, in rats with heart failure, the response to all the agonists studied was significantly decreased and the dose response curves were shifted to the right (P < 0.01). We conclude that in vivo vascular response to postjunctional alpha1- and alpha2-adrenoceptors as well as Y1 receptors are decreased in rats with heart failure.     

Vasomotor nerve control of isolated arteries and veins.

In order to compare neuro-effector function in different blood vessels, frequency-response relationships were determined for the following preparations: 1)Isolated rings of the proximal saphenous, distal saphenous and ear arteries, the parietal branch of the internal iliac vein and the small saphenous vein from the rabbit, 2) spiral strips of the rabbit pulmonary artery and 3) longitudinal preparations of the rat portal vein. In each rabbit tissue only one low (less than or equal to 4 Hz) and one high (larger than or equal to 8 Hz) transmural nerve stimulation frequency was applied until steady state responses were obtained and these were expressed as a percentage of a maximum response to exogenous noradrenaline (NA) applied in each experiment. The general shape of the frequency-response curves was similar, but differences in steepness and amplitude of the maximum neurogenic response relative to exogenous NA were found. The steepness of the frequency-response relationships of the veins tended to be greater than those of the arteries. It appears that factors such as close neuro-muscular contacts, presence of terminal nerve fibres within the media and the operation of mechanisms for myogenic propagation of activity contribute to the effectiveness of neurogenic vascular control as revealed by frequency-response curves. In vivo, geometrical factors can greatly augment the hemodynamic significance of the observed differences.     

Cooling enhances alpha 2-adrenoceptor-mediated vasoconstriction in human hand veins

The contribution of different receptor subtypes in the contractile response during cooling in human hand vessels is of considerable interest in the understanding of cold-induced peripheral vasospasm as it appears in Raynaud's phenomenon. Subcutaneous vein segments from 50 patients undergoing hand operations not related to vascular disorders were examined in vitro. The temperature in the organ bath was initially 37 degrees C and was either continuously lowered to 10 degrees C or kept constant at 37 degrees C, 29 degrees C or 20 degrees C. The characteristics of the alpha-adrenoceptor-mediated motor response were elucidated with the use of the alpha 1-antagonist, prazosin, and the alpha 2-antagonist, yohimbine. A great variability between individuals in the proportions of alpha 1- and alpha 2-adrenoceptors was found. In the majority of the vessels continuous cooling to 25 degrees C augmented a noradrenaline-induced contraction. This augmentation was unaltered in the presence of prazosin but abolished by yohimbine, suggesting that it was mediated via the alpha 2-adrenoceptor. In the remaining vessels with a predominating alpha 1-adrenoceptor-mediated response a cold-induced relaxation was registered. This could be the result of a reduced alpha 1-adrenoceptor-mediated contraction at this low temperature. These varying reactions to cooling were unaffected by the beta-antagonist, propranolol, and by endothelial denudation. The results obtained in corresponding experiments with the alpha 1-agonist methoxamine and alpha 2-agonist, oxymetazoline, were conflicting, probably due to the poor selectivity of these agonists in human tissues.(ABSTRACT TRUNCATED AT 250 WORDS)     

Cooling enhances alpha2-adrenoceptor-mediated vasoconstriction in human hand veins

The contribution of different receptor subtypes in the contractile response during cooling in human hand vessels is of considerable interest in the understanding of cold-induced peripheral vasospasm as it appears in Raynaud's phenomenon. Subcutaneous vein segments from 50 patients undergoing hand operations not related to vascular disorders were examined in vitro. The temperature in the organ bath was initially 37 °C and was either continuously lowered to 10 °C or kept constant at 37 °C, 29 °C or 20 °C. The characteristics of the α-adrenoceptor-mediated motor response were elucidated with the use of the α₁-antagonist, prazosin, and the α₂-antagonist, yohimbine. A great variability between individuals in the proportions of α₁- and α₂-adrenoceptors was found. In the majority of the vessels continuous cooling to 25 °C augmented a noradrenaline-induced contraction. This augmentation was unaltered in the presence of prazosin but abolished by yohimbine, suggesting that it was mediated via the α₂-adrenoceptor. In the remaining vessels with a predominating α₁-adrenoceptor-mediated response a cold-induced relaxation was registered. This could be the result of a reduced α₁-adrenoceptor-mediated contraction at this low temperature. These varying reactions to cooling were unaffected by the β-antagonist, propranolol, and by endothelial denudation. The results obtained in corresponding experiments with the α₁-agonist methoxamine and α₂-agonist, oxymetazoline, were conflicting, probably due to the poor selectivity of these agonists in human tissues. The results suggest that in patients with peripheral cold-induced vasospasm administration of an α₂-adrenoceptor antagonist might be of therapeutic interest in order to relieve vasospasm in the hand vessels.     

Pharmacological characterization of postjunctional α-adrenoceptors in isolated feline cerebral and peripheral arteries

The vascular α-adrenoceptors in isolated feline cerebral, lingual and mesenteric arteries were characterized and compared. In the middle cerebral artery the relative order of potency for agonists was: clonidine>oxymetazoline>noradrenaline>phenylephrine which indicates that the postjunctional α-receptor in this vessel is of α₂-type. This view is further supported by the finding that yohimbine, but not prazosin, had a potent, mainly competitive blocking action. In peripheral arteries, clonidine was without effect. In these vessels, the potency difference between phenylephrine and oxymetazoline was more than 40 times less than in cerebral vessels. The pA₂-values for prazosin correlated well with pA₂-values found for the interaction of this drug with α₁-receptors in a variety of other tissues, thus suggesting the occurrence of an α₁-receptor in these arteries. However, the pA₂-values for yohimbine and rauwolscine correlated well with an α₂-receptor, suggesting also the presence of α₂-receptors. Schild plots for prazosin and rauwolscine in lingual arteries displayed slopes significantly lower than unity, which also supports the view of a mixture of α₁-and α₂-receptors in these vessels. However, the Schild plots for the antagonists in mesenteric arteries did not differ significantly from unity, a finding possibly indicating the presence of an α-receptor unable to differentiate between substances that in other tissues act preferentially on α₁- or α₂-receptors.     

Adrenoceptors mediating contraction in the human uterine artery

Pharmacological characterization of adrenoceptors mediating smooth muscle contraction was performed in isolated preparations from the human uterine artery. The mixed alpha 1- and alpha 2-adrenergic agonist, noradrenaline (NA) and the selective alpha 1-agonists, phenylephrine and methoxamine, all contracted the smooth muscle preparations in a concentration-dependent manner. The responses were antagonized competitively by the selective alpha 1-antagonist, prazosin, yielding pA2 values for the three agonists (8.33-9.08) typical for an interaction with alpha 1-receptors. The alpha 2-selective receptor agonists, clonidine and BHT 920, did not exert any contractile effects in the isolated uterine arteries, and the alpha 2-adrenergic antagonist, yohimbine, counteracted the contractile effect of NA only at high concentrations. The concentration-response curve for NA was unaffected by the alpha 2-selective antagonists, rauwolscine and idazoxan. The results suggest that the postjunctional contractile receptors in the human uterine artery primarily are of the alpha 1 type, and give no evidence for any substantial involvement of alpha 2-receptors in this important tributary vessel of the human female reproductive tract.     

Enhancement of insulin secretion during selective blockade of alpha 1- and alpha 2-adrenoceptors in the rat: effects of somatostatin

The effects of somatostatin on plasma concentrations of insulin and glucose in the presence of the selective alpha 1-adrenoceptor blocking agent prazosin or the selective alpha 2-adrenoceptor blocking drug yohimbine were studied in vivo in anesthetized rats. Infusion of both prazosin (0.080 mg/min) and yohimbine (0.018 mg/min) increased plasma insulin levels within 10 min. Prazosin, but not yohimbine, caused a significant increment in plasma glucose concentrations during the infusion of both prazosin and yohimbine, suggesting that the inhibitory effect of somatostatin is not mediated via a direct action on alpha 1- or alpha 2-adrenoceptors. Plasma glucose concentration fell slightly during somatostatin administration. A marked increment in insulin release occurred in response to cessation of the somatostatin infusion, both during prazosin- and yohimbine-treatment. We conclude that somatostatin efficiently inhibits insulin secretion during selective alpha 1- and alpha 2-adrenoceptor blockade and, further, that the insulin off-response after somatostatin treatment is potentiated by alpha-adrenoceptor blockade. This study also indicates that blockade of alpha 1- as well as alpha 2-adrenoceptors leads to an increased insulin secretion.     

Intersection patterns of human coronary veins and arteries

Intersections between the coronary veins (CV) and arteries (CA) of 103 adult human hearts were mapped on the heart surface. Then the correlations of these intersection patterns to their localization were studied. Eight spots were selected where one of four major CV (anterior cardiac vein, middle cardiac vein, left posterior ventricular vein, and great cardiac vein) intersected with one of CA and their branches (right coronary artery, posterior interventricular branch, left posterior ventricular branch, circumflex branch, diagonal branch, and anterior interventricular branch). The great cardiac vein (GCV) ran beneath the anterior interventricular branch in 56 specimens out of 103, beneath the diagonal branch in 75 specimens out of 103, and beneath the circumflex branch in 36 specimens out of 103, while the other CV mostly ran over CA. The present observations suggest that the CV on the right side may be formed prior to CA, while the CV on the left side may be formed simultaneously with CA.     

脾淋巴细胞α2—肾上腺素能受体的特征和功能的实验研究

用放射配基结合法研究大鼠脾淋巴细胞α_2-肾上腺素能受体的特征。~3H-可乐定(~3H-CLO)与脾淋巴细胞的结合呈现快速(t1/2:2min)、可逆(t1/2:3—4min)、高亲和力(K_D:6.57±SD1.63nM)、可饱和性(B_(?a):72.4±SD13.4fmo1/10~?细胞)(n=6)和立体结构特异性等特征,表明脾淋巴细胞存在α_2-肾上腺素能受体。肾上腺素能受体激动剂抑制~3H-CLO结合的效能顺序是:肾上腺素>去甲肾上腺素>异丙肾上腺素,表明为α_2亚型。竞争实验资料的计算机分析提示,脾淋巴细胞α_2-肾上腺素能受体存在高、中、低三种亲和力状态,彼此相差2～3个数量级。在体外诱生抗体中,10umo1.L~(-1)可乐定显著抑制诱生的IgM量,可被10umol:L~(-1)酚妥拉明阻断,表明脾淋巴细胞α_2肾上腺素能受体激活抑制抗体应答。     

Alterations in platelet alpha2-adrenoceptors by aspirin

Epinephrine-induced platelet aggregation (mediated through interaction with alpha₂-adrenoceptors) is inhibited by aspirin. To determine if aspirin modulates alpha₂-adrenoceptors, we quantitated dissociation constant (K_D) and maximum number of binding sites (B_max) on isolated platelet membranes using alpha₂-antagonist³H-yohimbine in normal subjects given 650 mg of aspirin orally. Alpha₂-receptor K_D increased from 3.20±1.80 to 7.32±3.32 nM (p<0.02) and B_max from 115±77 to 190±140 fmol/mg protein. To determine if these alterations in alpha₂-receptors by aspirin were mediated through circulatory or intracellular effects, intact platelets or isolated platelet membranes were incubated with aspirin for 30 minutesin vitro. In thesein vitro experiments, alpha₂-receptor K_D increased from 2.92±1.76 to 9.83±8.55 nM and B_max from 140±81 to 191±129 fmol/mg protein (p<0.05). Oral ingestion of aspirin or incubation of aspirin with intact platelets or lysates increased (3 to 10 fold) the concentration of 1-epinephrine required for inhibition, of³H-yohimbine binding by 50% (p<0.05). Basal platelet cyclic AMP as well as its elevation with PGE₁ or PGI₂ and decrease with catecholamines were not influenced by aspirin treatment of platelets. These data indicate that aspirin decreases platelet alpha₂-receptor affinity for agonist as well as antagonist. These effects of aspirin are independent of circulatory or dynamic intraplatelet changes.