Two distinct alpha1-adrenergic receptor sites in rat liver: differential binding of (--)-[3H]dihydroergocryptine. Effects of guanine nucleotides and proteolysis; implications for a two-site model of alpha-receptor regulation

Decreased alpha1-adrenergic receptor binding in the cerebral cortex and brain stem during pancreatic regeneration in rats

The purpose of this study was to investigate the role of brain alpha1-adrenergic receptor binding in the rat model of pancreatic regeneration using 60-70% pancreatectomy. The alpha1-adrenergic receptors kinetics was studied in the cerebral cortex and brain stem of sham operated, 72 h pancreatectomised and 7 days pancreatectomised rats. Scatchard analysis with [3H]prazosin in cerebral cortex and brain stem showed a significant decrease (P < 0.01), (P < 0.05) in maximal binding (Bmax) with a significant decrease (P < 0.001), (P < 0.01) in the Kd in 72 h pancreatectomised rats compared with sham respectively. Competition analysis in cerebral cortex and brain stem showed a shift in affinity during pancreatic regeneration. The sympathetic activity was decreased as indicated by the significantly decreased norepinephrine level in the plasma (P < 0.001), cerebral cortex (P < 0.01) and brain stem (P < 0.001) of 72 h pancreatectomised rats compared to sham. Thus, from our results it is suggested that the central alpha1-adrenergic receptors have a functional role in the pancreatic regeneration mediated through the sympathetic pathway.

Determination of alpha 1-adrenoceptor subtype selectivity by [3H]-prazosin displacement studies in guinea-pig cerebral cortex and rat spleen membranes

1. [3H]-prazosin homogeneously labels alpha 1-adrenoceptors in guinea-pig cerebral cortex and rat spleen membranes with dissociation constants of 1.28 and 1.49 x 10(-10) M respectively. 2. Phentolamine and WB 4101 displacement studies show that guinea-pig cerebral cortex contains 30% alpha 1A- and 70% alpha 1B-adrenoceptor subtypes, whereas rat spleen contains a virtually homogeneous alpha 1B-adrenoceptor subtype population. The alpha 1-adrenoceptor population of rat thoracic aorta is predominantly of the alpha 1A-adrenoceptor subtype, and in guinea-pig thoracic aorta it is mainly of the alpha 1B-adrenoceptor subtype. 3. Half of the compounds displacing [3H]-prazosin bound to guinea-pig cerebral cortex membranes display alpha 1A-adrenoceptor selectivity. Among these compounds, WB 4101 and methoxamine are most selective, displaying selectivity ratios of approximately 38 and approximately 26 respectively. 4. The affinity constants of the non-selective compounds for the alpha 1-adrenoceptor in guinea-pig cerebral cortex membranes correlate well with the affinity constants obtained for alpha 1B-adrenoceptors in rat spleen membranes. The affinities of selective compounds for the alpha 1B-adrenoceptor subtype in guinea-pig cerebral cortex correlate very well with their affinity for alpha 1B-adrenoceptor in the rat spleen homogenate. Both regression lines coincide with the line of identity. The affinity constants of selective compounds for the alpha 1A-adrenoceptors in guinea-pig cerebral cortex only apparently correlate with the affinity for either the alpha 1B-adrenoceptors in guinea-pig cerebral cortex or in the rat spleen. Regression analyses indicate a straight line relationship (r2>0.9) between pKEA and Pk1B but the regression lines deviate from the line of identity.

Heparin and other polyanions uncouple alpha 1-adrenoceptors from G-proteins

Several polyanionic compounds antagonize the interaction between receptors and the G-proteins that regulate adenylate cyclase or K+ channels, possibly by binding to a basic stretch of the receptor that is proposed to mediate its interaction with the G-proteins. We have studied the effects of polyanions on the interaction between the liver alpha 1-adrenoceptor and the G-protein through which it stimulates polyphosphoinositide turnover. Heparin [concn. causing 50% of maximal effect (EC50) = 0.5 microM], Trypan Blue (EC50 7.1 microM) or suramin (EC50 2.1 microM) prevented formation of the high-affinity adrenaline-receptor-G-protein complex without affecting antagonist binding. After alkaline treatment of the membranes, previously reported to cause G-protein removal, binding of agonists was insensitive to both guanine nucleotides and heparin. We conclude that these polyanions uncouple the alpha 1-adrenoceptor from its G-protein, suggesting that similar coupling mechanisms may underlie receptor activation of the G-proteins that activate polyphosphoinositide hydrolysis and those that regulate adenylate cyclase. This action of heparin severely limits its utility as a selective antagonist of the Ins(1,4,5)P3 receptor in intact cells.

Detection by northern analysis of alpha 1-adrenergic receptor gene transcripts in the rat

In Northern blots of total cellular and poly(A+) RNA isolated from rat liver, renal cortex, spleen, and brain probed with a full-length cDNA encoding the hamster alpha 1-adrenergic receptor, hybridization was observed to two distinct mRNAs, at approximately 3.3 kb and approximately 2.7 kb. Only the approximately 2.7 kb mRNA species was visualized in Northern blots of total cellular and poly(A+) RNA isolated from cardiac ventricular muscle. From screening a rat heart cDNA library with the full-length hamster alpha 1-adrenergic receptor cDNA, a 632 base pair cDNA was isolated. Based upon its high degree of identity, 86% at the nucleotide level, with the hamster alpha 1-adrenergic receptor cDNA, this cDNA was considered to include the 3' end of the rat alpha 1-adrenergic receptor. When used as a probe in Northern blots of liver RNA, both the approximately 3.3 kb and approximately 2.7 kb mRNAs were visualized. Both mRNA species were expressed in fetal as well as adult liver, but steady-state levels of each gene transcript were approximately 3-fold higher in adult compared to fetal liver. Finally, results from Southern analysis of restriction enzyme fragments of genomic DNA suggest that the two gene transcripts may be products of a single gene.

Effects of oestrogen on adenylate cyclase system and glucose output in rat liver

Effects of chronic oestrogen treatment on catecholamine- and glucagon-sensitive adenylate cyclase activity and glucose output in hepatocytes of castrated male rats were studied. In hepatocytes from male intact or castrated rats, the beta-adrenergic agonist isoprenaline did not stimulate adenylate cyclase activity and glycogenolysis, but glucagon markedly stimulated all these activities. Treatment of castrated animals with 17 beta-oestradiol for 7 days led to the appearance of beta-adrenergic-stimulated increases in both cyclic AMP generation and glucose output. The basal, glucagon- or fluoride-stimulated activities of adenylate cyclase of hepatic membranes prepared from oestrogen-treated rats were similar to those of control animals. Treatment with oestrogen did not influence the number or affinity of beta-adrenergic receptors. In hepatic plasma membranes from control rats, GTP failed to decrease the affinity of beta-adrenergic receptors for agonists, whereas the GTP-induced shift was apparently observed in those from oestrogen-treated animals. These results suggest that oestrogen is able to facilitate the coupling of hepatic beta-adrenergic receptors to the enzyme by increasing the effectiveness of receptor-guanine nucleotide regulation.

Complex interactions of agonists with alpha 1-adrenoceptors in intact cells

1. The apparent Ki values of (-)-noradrenaline (NA), (+)- and (-)-adrenaline (Ad), phenylephrine and the mono-fluorinated NAs (in position 2, 5 or 6) for alpha 1-adrenoceptors of intact BC3H1 cells labelled with [3H]-prazosin were greatly dependent on the incubation temperature. 2. The EC50 values of these compounds for stimulation of the inositol phosphate (IP) accumulation at 37 degrees C were intermediate between their apparent dissociation constants at 2 degrees C (Ki2 degrees) and at 37 degrees C (Ki37 degrees). 3. The fact that an irreversible blockade of 46% +/- 6% (n = 3) of the [3H]-prazosin binding sites by phenoxybenzamine reduced the maximal IP-formation induced by NA by 57% +/- 5% (n = 3) shows that there is a direct coupling between alpha 1-adrenoceptors and phospholipase C in BC3H1 cells. 4. The Ki37 degrees s of all agonists tested were in the same range (0.1 to 1 mM) and showed no simple correlation with their EC50 values. 5. The Ki2 degrees values for all the agonist correlated linearly with their EC50 values but were about 20-100 times lower than the respective EC50 values (except for the partial agonist methoxamine). In order to explain this difference, we propose that the apparent high affinity in the cold could be due to an [3H]-prazosin-induced alteration of the active site of the alpha 1-adrenoceptor, increasing its apparent affinity for catecholamines.

The impact of hypothyroidism and thyroxine replacement on the expression of hepatic alpha 1-, alpha 2- and beta-adrenergic receptors in rat liver plasma membranes

1. Liver plasma membranes were isolated from control, propylthiouracil-induced hypothyroid and thyroxine-replaced rats; relative specific activities of 5'-nucleotidase were found to be similar, 5.6-6.1, demonstrating that comparable purity levels were achieved. 2. Radioligand binding studies indicated that hepatic alpha 1-, alpha 2- and beta-adrenergic receptor binding to control liver membranes was 1963.23 +/- 59.34, 77.64 +/- 2.20 and 111.18 +/- 11.04 fmol.mg-1, respectively. 3. Hypothyroidism induced a 67% and 54% decrease, respectively, in hepatic alpha 1- and alpha 2-adrenergic receptor binding with no change in beta-adrenergic receptor binding. 4. Thyroxine replacement achieved an 85% and 100% restoration, respectively, in hepatic alpha 1- and alpha 2-adrenergic receptor expression with no effect on the beta-adrenergic receptor.

Subcellular distribution of alpha 1-adrenergic receptors in rat liver

The distribution of alpha 1-adrenergic receptors in rat liver subcellular fractions was studied using the alpha 1-adrenergic receptor ligand [3H]prazosin. The highest number of [3H]prazosin binding sites was found in a plasma membrane fraction followed by 2 Golgi and a residual microsomal fraction, the numbers of binding sites were 1145, 845, 629 and 223 fmol/mg protein, respectively. When the binding in these fractions was compared with the activity of plasma membrane 'marker' enzymes in the same fractions a relative enrichment of [3H]prazosin binding sites was found in the residual microsomes and one of the Golgi fractions. Photoaffinity labelling with 125I-arylazidoprazosin in combination with SDS-polyacrylamide gel electrophoresis revealed the specific binding to 40 and 23 kDa entities in a Golgi fraction, while in plasma membranes the binders had an apparent molecular mass of 36 and 23 kDa. When [3H]prazosin was injected in vivo into rat portal blood followed by subcellular fractionation of liver, a pattern of an initial rapid decline and thereafter a slow decline of radioactivity was noted in all fractions. Additionally, in the two Golgi fractions a transient accumulation of radioactivity occurred between 5 and 10 min after the injection. The ED50 values for displacement of [3H]prazosin with adrenaline was lowest in the plasma membrane fraction, followed by the residual microsomes and Golgi fractions, the values were 10(-6), 10(-5) and 10(-4) mol/l, respectively. On the basis of lack of correlation between distribution of alpha 1-adrenergic antagonist binding and adenylate cyclase activity, differences in the molecular mass of alpha 1-adrenergic antagonist binders, differences in the kinetics of in vivo binding and accumulation of [3H]prazosin and also differences in agonist affinity between plasma membrane and Golgi fractions, it is concluded that alpha 1-adrenergic receptors are localized to low-density intracellular membranes involved in receptor biosynthesis and endocytosis.

A water-soluble derivative of prazosin prazosinamine hydrochloride [1-(4'-amino-6',7'-dimethoxyquinazolin-2'-yl)-4-(6''-aminohexanoyl) piperazine hydrochloride], reversibly inhibits the calcium-mobilizing action of alpha 1-adrenergic agonists in the perfused rat liver

A newly-synthesized derivative of prazosin, prazosinamine hydrochloride, was examined for its ability to antagonize the interaction of the alpha 1-adrenergic agonist phenylephrine with liver cells. Using a Ca2--selective electrode to measure changes in perfusate Ca2+ concentration, prazosinamine was found to be as effective as prazosin in inhibiting the phenylephrine-induced efflux of Ca2+ from the perfused liver. Maximal and half-maximal inhibition occurred at 150 nM and 25 nM prazosinamine, respectively. Prazosinamine appears to share the alpha 1-specificity of prazosin, but has other unique and desirable properties. Its solubility in aqueous media is about three orders of magnitude higher than that of prazosin. Also, its antagonistic effects are rapid in onset, and are reversed within seconds of terminating its infusion into the liver. These attributes seem to make this agent more useful than prazosin for adrenergic receptor studies in perfused tissues. The molecule can also be readily coupled to other ligands.

Hepatic adrenergic receptors in the genetically diabetic C57 BL/KsJ (db/db) mouse

1. At 30 weeks of age, homozygote diabetic C57 BL KsJ (db/db) mice were grossly obese, lethargic and displayed moderate hair loss relative to heterozygote control C 57 BL KsJ (db/+) mice. 2. In diabetic mice, compared to control, the total body weights, liver weight: body weight ratios, and blood glucose levels were increased 2.3 fold, 20% and 3.1 fold, respectively. 3. Analysis of plasma membranes isolated from control and diabetic mouse liver established that comparable purity levels were achieved since relative specific activities of the plasma membrane markers 5'-nucleotidase and gamma-glutamyltranspeptidase were similar: 10.2 and 11.4 fold with respect to 5'-nucleotidase in control and diabetic states respectively; and 8.0 and 8.3 fold with respect to gamma-glutamyltranspeptidase in control and diabetic states respectively. 4. A select effect of diabetes on gamma-glutamyltranspepetidase, however, was observed. The activity of this enzyme was found to be reduced 16% in diabetic liver compared to control liver. 5. Assessment of [3H]prazosin and [3H]dihydrolalprenolol binding to mouse liver plasma membranes indicated that although there was no difference in beta-adrenergic receptor binding in control and diabetic states, alpha 1-adrenergic receptor binding was found to be reduced 43% in diabetic mouse liver plasma membranes. 6. Scatchard analyses of kinetic studies indicate that the reduction is a reflection of decreases in alpha 1-adrenergic receptor numbers with no change in alpha 1 receptor affinity in the diabetic state: since for diabetic and control liver plasma membranes, Kd values were 3.41 +/- 0.02 nM and 3.40 +/- 0.01 nM respectively; and Bmax were 650.12 +/- 16.44 fmol mg-1 and 380.76 +/- 12.92 fmol mg-1, respectively.

Effects of adrenalectomy on binding to and actions of adrenergic receptors

Adrenalectomy results in significant changes in the mechanism of adrenergic activation of hepatic glycogenolysis. In adrenalectomized rats a greater role for the beta-adrenergic receptor is observed, whereas the alpha 1-adrenergic-mediated phosphorylase activation declines. Our present findings document that adrenalectomy causes a significant decrease in the high-affinity population of the alpha 1-adrenergic receptor labelled with [3H]adrenaline. Our data indicate a large increase in the number of beta-adrenergic binding sites after adrenalectomy. This increase was not consistent with the observed modest increase in the beta-adrenergic-mediated activation of cyclic AMP accumulation and glycogen phosphorylase. When alpha-adrenergic antagonists are present along with the catecholamine, a 100% increase in the adrenaline-mediated accumulation of cyclic AMP in hepatocytes from adrenalectomized rats was observed. Adrenalectomy was also shown to cause a significant increase in the hepatic alpha 2-adrenergic binding sites. These data are consistent with an inhibitory role on the beta-adrenergic-mediated activation of glycogenolysis by the hepatic alpha 2-adrenergic receptor in adrenalectomy.

Effects of guanine nucleotides and cations on agonist affinity of alpha 1-adrenoceptors in brown adipose tissue

In order to investigate the coupling mechanism of alpha 1-adrenoceptors in brown adipose tissue, the effects of guanine nucleotides and cations on agonist binding were studied with a membrane fraction obtained from hamsters. The affinity of the alpha 1-receptor for the adrenergic agent was followed in competition experiments with [3H]prazosin. It was found that the addition of GTP diminished the affinity of the alpha 1-receptor for norepinephrine but not for the alpha-antagonist phentolamine. This effect seemed to be dependent upon the presence of Mg2+ and could not be observed in isolated cells, indicating an intracellular site of action. A reduction of the Mg2+ concentration from the conventional 10 mM to the more physiological level of 1 mM markedly increased the affinity of the receptor for norepinephrine; this effect again was agonist-specific. The addition of Na+ (150 mM) decreased the agonist affinity of the receptor. It is suggested that the (not adenylate cyclase-coupled) alpha 1-adrenergic pathway in brown adipose tissue also is regulated by guanine nucleotides and modulated by the cations Mg2+ and Na+, indicating an involvement of a guanine nucleotide binding protein. Such a system may therefore be a general mechanism for the transduction of hormone stimulation over the cell membrane.

Identification of subtypes of [3H]prazosin-labelled alpha 1 receptor binding sites in rat brain

Detailed antagonist competition curves for [3H]prazosin-labelled binding sites in rat cerebral cortex membranes reveal anomalous binding characteristics. Dihydroergocryptine and indoramine compete in a steep, monophasic manner while WB4101 and phentolamine exhibit shallow competition curves. Computer-assisted analysis of binding data indicate that both WB4101 and phentolamine discriminate identical subpopulations of [3H]prazosin binding sites, which each comprise approximately 50% of specific [3H]prazosin binding. These data suggest the presence of subtypes of [3H]prazosin-labelled alpha 1 adrenergic receptors in rat cerebral cortex.

Characterization of the alpha adrenergic receptor population in hippocampus up regulated by serotonergic raphe deafferentiation

Serotonergic raphe deafferentiation elicits an up regulation of a nM (3H)WB-4101 binding site in rat hippocampus for which norepinephrine displays high affinity and prazosin displays low affinity. Guanine nucleotide affects the nM binding to hippocampal alpha-1 adrenergic receptors. Firstly, Gpp(NH)p, a nonhydrolyzable analog of GTP, inhibits (3H)WB-4101 binding at 3 nM concentration of the radioligand, the ligand concentration labelling the lower affinity, nM, binding site. Secondly, the addition of Gpp(NH)p causes recovery of the heterogeneity of binding sites lost upon preincubation of the membranes with 100 microM epinephrine, apparently by decreasing the affinity of the nM (3H)WB-4101 binding site for the adrenergic receptors. The phenomenon was still observed in the presence of saturating concentrations of the alpha-2 antagonist, yohimbine, and the beta antagonist, propranolol. The results imply that Gpp(NH)p regulates ligand binding to hippocampal alpha-1 agonist sites. It is likely that agonist and antagonist binding sites for the alpha-1 receptor exist in hippocampus with the agonist site being modulated by serotonin.

A comparison of the binding characteristics of the beta-adrenoceptor antagonists 3H-dihydroalprenolol and 125I-iodocyanopindolol in rat liver

The binding characteristics of 3H-dihydroalprenolol and 125I-iodocyanopindolol have been compared in a particulate fraction from regenerating rat liver. When total 3H-dihydroalprenolol binding and inhibition of total 3H-dihydroalprenolol binding by (-)isoprenaline, (-)alprenolol and (+/-)cyanopindolol was investigated, it was found that all agents were bound to two classes of saturable binding sites. In the inhibition studies, the presence of two binding components was not obvious until the data were transformed into Hofstee plots and these were decomposed, except in the case of (+/-)cyanopindolol. Only (+/-)cyanopindolol was found to distinguish clearly between the two saturable binding sites identified by 3H-dihydroalprenolol, as indicated by a broad plateau in the inhibition curve. When 125I-iodocyanopindolol was used as radioligand, only one saturable binding site was identified, even in the presence of less selective inhibiting ligands. The lower affinity component of 3H-dihydroalprenolol binding could be inhibited by 10 microM phentolamine. However, binding experiments with 3H-prazosin indicated that the lower affinity component was not identical with the alpha-adrenoceptor. Phentolamine did not influence 125I-iodocyanopindolol binding. Thus, due to its higher specific activity and a high degree of selectivity, 125I-iodocyanopindolol appears to be the ligand of choice.

Evidence for heterogeneous distribution of alpha 1, alpha 2- and beta-adrenergic binding sites on rat-liver cell surface

Fractionation of preparations of rat-liver membranes on linear sucrose gradients revealed different profiles for the binding of alpha 1-, alpha 2- and beta-adrenergic radioligands. The peaks of binding activities of [3H]prazosin and [3H]epinephrine were clearly separated from those of [3H]yohimbine and [125I]iodocyanopindolol which appeared at lower sucrose densities. Enzyme marker activities in the sucrose subfractions indicated the presence of plasma membranes in all of the subfractions. Furthermore, the binding peaks of the various adrenergic radioligands cannot be correlated with the presence of membranes derived from microsomes, lysosomes or Golgi apparatus. Pretreatment of rat livers with concanavalin A, in order to prevent the fragmentation of the plasma membranes during isolation, resulted in the shift of the binding of [3H]yohimbine and [125I]iodocyanopindolol to sucrose-gradient subfractions of higher densities, clearly separate from fractions containing microsomes and Golgi apparatus. There was no distinct separation of the binding peaks of prazosin, yohimbine, and cyanopindolol in sucrose-gradient subfractions from concanavalin A-pretreated livers. These results are consistent with the hypothesis that alpha 1-, alpha 2-, and beta-adrenergic binding sites are associated with plasma membranes, and are heterogeneously distributed on the rat-liver cell surface.

Specific binding of [3H]prazosin to myocardial cells isolated from adult rats

The characteristics of alpha-adrenoceptors in rat myocardium were investigated by specific binding of [3H]prazosin to cells isolated from adult rat heart by perfusion with collagenase and hyaluronidase. The cells were incubated in Krebs-Ringer bicarbonate buffer gassed with 95% O2 and 5% CO2 at 31 degrees with the appropriate concentrations of the different ligands. Non-specific binding was defined by the addition of 10(-5) mole/l. phentolamine. The binding of [3H]prazosin was saturable and reached equilibrium within 15 min. Scatchard analysis showed a straight line giving an apparent dissociation constant, Kd, equal to 155.9 +/- 8.0 pmole/l. and a maximal number of binding sites equal to 76.7 +/- 11.1 fmole/mg protein. Inhibition of specific [3H]prazosin binding by different adrenergic blockers showed the order of potency characteristic of alpha 1-adrenoceptors: prazosin much greater than phentolamine greater than yohimbine much greater than propranolol. Inhibition by adrenergic agonists showed the order of potency: adrenaline greater than noradrenaline = phenylephrine greater than isoprenaline. The same orders of potency were observed in the presence of propranolol. However, propranolol slightly decreased the affinity for noradrenaline and phenylephrine. Hofstee analyses of the inhibition curves showed two binding components for all ordinary alpha-adrenoceptor blockers and agonists including unlabelled prazosin. In contrast, [3H]prazosin showed only one binding component. Both binding components were of the alpha 1-adrenoceptor subtype according to the order of potency of blockers. The different ligands had different affinity ratios for the two binding components giving them different profiles. Trifluoperazine, a phenothiazine compound, also had high affinity for the [3H]prazosin binding sites. This drug, however, apparently detected one class of binding sites only, as interpreted from the Hofstee analysis. Hill analyses of the inhibition data consistently yielded Hill constants, nH, in the range 0.75-0.85 except for [3H]prazosin, where nH = 1.02 and for trifluoperazine, where nH = 1.07. Although the two binding components may serve different functions, it seems impossible at present to relate the negative and the positive inotropic components, respectively, of the alpha-adrenergic inotropic response observed in functional studies only to one or the other binding component.

The hepatic alpha 1-adrenergic receptor

Since the relatively recent advent of radioligand binding techniques, it has been possible to directly identify and characterize hepatic adrenergic receptors as well as study their physiological regulation. While it is now clear that alpha 1-adrenergic receptors constitute the major population of hepatic adrenergic receptors and are primarily responsible for the actions of catecholamines in liver, relatively little is known about the molecular mechanisms underlying alpha 1-responses. Recent results suggest that guanine nucleotides may be implicated in the transmission of the hormonal signal from the hepatic alpha 1-receptor to its effectors in a manner analogous to that described for adenylate cyclase-linked receptors. The lack of an easily measurable proximal membrane response for the alpha 1-receptor has been a severe handicap in our understanding of the mechanism of transmission of the hormonal signal. It is likely that until such a response is defined, alpha 1-adrenergic research will continue to lag behind research on the beta-adrenergic receptor.

Binding and uptake of [3H]adrenaline by perfused rat liver

The binding and uptake of [3H]adrenaline by the intact perfused rat liver was investigated. We showed that the administration of [3H]adrenaline to liver resulted in the rapid uptake of the radioligand, and that such uptake was independent of any Ca2+ redistributions induced by the hormone. At low adrenaline concentrations (less than 50 nM) uptake was inhibited by prazosin, whereas at higher hormone concentrations a significant proportion of total [3H]adrenaline uptake could not be inhibited by this antagonist. [3H]Adrenaline uptake could be directly correlated with adrenaline-induced responses such as an increased rate of respiration and glycogenolysis. The partial inhibition (approx. 25%) of [3H]adrenaline uptake by antagonists was sufficient for the total inhibition of hormone-induced responses. The effect of various pharmacological agents on [3H]adrenaline uptake was investigated, and the contribution of tissue-related factors to alpha-adrenergic agonist-antagonist interactions in vivo is discussed.

Adrenergic regulation of ureogenesis in hepatocytes from adrenalectomized rats. Possible involvement of two pathways of signal transduction in alpha 1-adrenergic action

In hepatocytes from control rats, the ureogenic action of epinephrine is mainly mediated through alpha 1-adrenoceptors and the effect is independent of the presence of extracellular calcium. In hepatocytes from adrenalectomized rats, both alpha 1- and beta-adrenoceptors are involved in the action of epinephrine. Furthermore, the alpha 1-adrenergic-mediated stimulation of ureogenesis in these cells is dependent on the presence of extracellular calcium. Our results indicate that glucocorticoids modulate the calcium dependency of alpha 1-adrenergic effects and are consistent with our suggestion that two pathways are involved in the transduction of the alpha 1-adrenergic signal.

Septal deafferentation increases hippocampal adrenergic receptors: correlation with sympathetic axon sprouting

Denervation of the hippocampal formation in adult rats through lesion of the medial septum and diagonal band or by transection of the fimbria/fornix elicits an increase in the number of putative alpha-adrenergic receptor binding sites labeled by the antagonist ligand [3H]WB4101 [2-(2,6-dimethoxyphenoxyethyl)-aminomethyl-1,4-benzodioxane]. This increase in [3H]WB4101 binding is observable at 6 days postlesion, preceding the ingrowth of sympathetic axons into the partially denervated regions of the hippocampus. The receptor up-regulation is specific for lesions of the septal (primarily cholinergic) innervation of the hippocampus. Damage to noradrenergic, dopaminergic, or serotonergic afferents as well as kainate injections in the lateral septum had no effect on [3H]WB4101 binding levels. In vivo muscarinic/cholinergic-receptor blockade does not mimic the effects of the lesion on receptor binding levels or upon axonal sprouting of the sympathetic neurons. Although [3H]WB4101 binding consistently increased after septal deafferentation, there was no clear-cut effect upon the other adrenergic ligands, including [3H]prazosin, [3H]yohimbine, P-[3H]aminoclonidine, or [3H]dihydroalprenolol. These observations can be interpreted as demonstrating a unique and selective adrenergic receptor increase after a nonadrenergic denervation but accompanying the ingrowth of anomalous adrenergic fibers. We suggest several possible relationships between the new binding sites and the ingrowing axons.

Identification of [3H]prazosin binding sites in crude membranes and isolated cells of brown adipose tissue as alpha 1-adrenergic receptors

The alpha 1-receptor selective adrenergic antagonist [3H]prazosin was used to study adrenergic binding sites in crude membranes and isolated cells from hamster brown adipose tissue. The antagonist labelled a site which fulfilled the criteria for being the alpha 1-receptor which participates in mediation of a part of the norepinephrine-induced respiration (thermogenesis) in intact cells. The similarity between the characteristics of the binding site in crude membrane fractions and in isolated brown fat cells suggested that the site is of postsynaptic origin. In equilibrium binding studies [3H]prazosin bound with very high affinity (Kd = 0.4 nM), and the maximal binding capacity was 72 fmol/mg protein and 207 fmol/10(6) cells, equal to 120 000 receptors per cell. The kinetically calculated Kd had a value of 0.17 nM, in good agreement with that determined in the equilibrium binding experiments. The relative potencies of adrenergic agents to displace [3H]prazosin revealed a typical alpha 1-specificity: WB-4101 = prazosin greater than phentolamine greater than dihydroergocryptine much greater than yohimbine greater than propranolol for antagonists, and L-phenylephrine = L-norepinephrine = L-epinephrine greater than L-isoprenaline greater than D-norepinephrine for agonists. Thus stereoselectivity was also shown. The actual Ki values for antagonists were closely similar in crude membranes and isolated cells whereas the alpha 1-receptor showed a 10-20 times higher affinity for agonists in the cellular preparation than in the crude membranes. The Ki values for the different antagonists and agonists derived from binding studies in isolated cells were compared with the IC50 and EC50 values for these agents obtained from studies on alpha 1-mediated cellular effects. It is suggested that tight coupling exists between alpha 1-receptor occupancy and alpha 1-mediated effects.

Simultaneous loss and reappearance of alpha 1-adrenergic responses and [3H]prazosin binding sites in rat liver after irreversible blockade by phenoxybenzamine

The relative influences of the in vivo administration of phenoxybenzamine on in vitro binding to alpha 1-adrenergic receptors and alpha 1-receptor-mediated responses were studied. Phenoxybenzamine treatment reduced maximal specific binding of the alpha 1-selective antagonist [3H]prazosin to liver cell membranes. This response was rapid (less than 90 min) and half-maximal following a phenoxybenzamine dose of approx. 10 mg/kg. A similar decrease in the ability of phenylephrine to stimulate glucose release and 45Ca2+ efflux from liver slices was also noted after phenoxybenzamine treatment. During the recovery period following administration of 30 mg/kg phenoxybenzamine, [3H]prazosin specific binding and phenylephrine-stimulated glucose release and 45Ca2+ efflux returned to their respective control levels with t 1/2 values of 42, 49 and 38 h, respectively. At all times studied during the recovery period, alpha 1-binding and both of the alpha 1-responses were similar fractions of their respective control values. These observations indicate that a close relationship exists between the density of [3H]prazosin binding sites and the ability of rat liver to respond to alpha 1-stimulation. We suggest that the binding sites identified in studies using the antagonist [3H]prazosin and those through which the agonist phenylephrine stimulates glucose release and 45Ca2+ efflux are either identical or in equilibrium with each other.

Binding of yohimbine stereoisomers to alpha-adrenoceptors in rat liver and human platelets

1--Displacement of tritiated prazosin binding to rat liver plasma membranes and tritiated yohimbine human platelet membranes shows that (+)-yohimbine, alloyohimbine and alpha-yohimbine (rauwolscine) are selective alpha 2-adrenoceptor antagonists (KD alpha 1/KD alpha 2:635, 46.6 and 112 respectively) whereas corynanthine is more alpha 1-selective (KD alpha 1/KD alpha 2:0.036). 2--11-Methoxy derivatives of alpha-yohimbine and epi-alpha-yohimbine are very weak alpha-adrenoceptor blockers. 3--It is concluded that the aromatic A ring, the Nb atom, and the carboxymethyl moiety are important for the binding of yohimbine to the alpha-adrenoceptor, the carboxymethyl group being important for the alpha 1/alpha 2 specificity of the molecule.

[3H]Dihydroergocryptine binding to alpha-adrenergic receptors of human platelets. A reassessment using the selective radioligands [3H]prazosin, [3H]yohimbine, and [3H]rauwolscine

Which subtype(s) of the alpha-adrenergic receptor occurs on human platelets? Studies of platelet responsiveness to adrenergic compounds and indirect radioligand binding studies addressing this question have yielded contradictory conclusions. These bindings studies employed the ligand [3H]dihydroergocryptine ( [3H]DHE), an alpha-adrenergic antagonist that does not select between alpha 1- and alpha 2-adrenergic receptors and that also binds to other receptor types in some tissues. To determine the subtype of the platelet alpha-adrenergic receptor, we have examined the binding to intact human platelets of [3H]prazosin (alpha 1-selective), [3H]yohimbine (alpha 2-selective), and [3H]rauwolscine (alpha 2-selective), and we have compared the binding of these selective radioligands with that of [3H]DHE. [3H]Yohimbine and [3H]rauwolscine both bound with high affinity (Kd = 2.7 and 4.6 nM, respectively) to an equal number and a single class (Hill coefficient approximately 1.0) of sites ( approximately 300 per platelet), but [3H]yohimbine yielded lower nonspecific binding than did [3H]rauwolscine. In paired experiments, [3H]DHE bound to 1.5 times as many (phentolamine-displaceable) sites as did [3H]yohimbine or [3H]rauwolscine. Unlabeled yohimbine and epinephrine competed for fewer [3H]DHE binding sites than did phentolamine. Thus, in addition to binding to the alpha 2-adrenergic receptors identified by [3H]yohimbine and [3H]rauwolscine, [3H]DHE seems to bind to other sites on human platelets. The nature of these sites is not clear. We found that [3H]prazosin did not identify alpha 1-adrenergic receptors on platelets, and that phenoxybenzamine only inhibited [3H]yohimbine and [3H]DHE binding at higher concentrations than usually observed for alpha 1-adrenergic receptors. We conclude that (1) all alpha-adrenergic sites on human platelets are of the alpha 2 subtype, (2) [3H]DHE may bind to additional, as yet ill-defined, sites in addition to those sites identified by [3H]yohimbine and [3H]rauwolscine, and (3) [3H]yohimbine is the preferred antagonist radioligand for studying the alpha 2-adrenergic receptors on human platelets.