The characteristics of [3H]-clonidine binding to an alpha-adrenoceptor in membranes from guinea-pig kidney

Blinded, Multicenter Evaluation of Drug-induced Changes in Contractility Using Human-induced Pluripotent Stem Cell-derived Cardiomyocytes

Animal models are 78% accurate in determining whether drugs will alter contractility of the human heart. To evaluate the suitability of human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) for predictive safety pharmacology, we quantified changes in contractility, voltage, and/or Ca²⁺ handling in 2D monolayers or 3D engineered heart tissues (EHTs). Protocols were unified via a drug training set, allowing subsequent blinded multicenter evaluation of drugs with known positive, negative, or neutral inotropic effects. Accuracy ranged from 44% to 85% across the platform-cell configurations, indicating the need to refine test conditions. This was achieved by adopting approaches to reduce signal-to-noise ratio, reduce spontaneous beat rate to ≤ 1 Hz or enable chronic testing, improving accuracy to 85% for monolayers and 93% for EHTs. Contraction amplitude was a good predictor of negative inotropes across all the platform-cell configurations and of positive inotropes in the 3D EHTs. Although contraction- and relaxation-time provided confirmatory readouts forpositive inotropes in 3D EHTs, these parameters typically served as the primary source of predictivity in 2D. The reliance of these “secondary” parameters to inotropy in the 2D systems was not automatically intuitive and may be a quirk of hiPSC-CMs, hence require adaptations in interpreting the data from this model system. Of the platform-cell configurations, responses in EHTs aligned most closely to the free therapeutic plasma concentration. This study adds to the notion that hiPSC-CMs could add value to drug safety evaluation.

Corticotropin releasing factor, but not alcohol, modulates norepinephrine release in the rat central nucleus of the amygdala

Alcohol misuse and dependence is a widespread health problem. The central nucleus of the amygdala (CeA) plays important roles in both the anxiety associated with alcohol (ethanol) dependence and the increased alcohol intake that is observed during withdrawal in dependent animals. We and others have shown the essential involvement of the corticotropin releasing factor (CRF) system in alcohol's synaptic effects on the CeA and in the development of ethanol dependence. Another system that has been shown to be critically involved in the molecular underpinnings of alcohol dependence is the norepinephrine (NE) system originating in the locus coeruleus. Both the CRF and NE systems act in concert to facilitate a stress response: central amygdalar afferents release CRF in the locus coeruleus promoting widespread release of NE. In this study, we are the first to use fast-scan cyclic voltammetry to classify local electrically-evoked NE release in the CeA and to determine if acute alcohol and CRF modulate it. Evoked NE release is action potential dependent, is abolished after depletion of monoaminergic vesicles, differs pharmacologically from dopamine release, is insensitive to acute alcohol, and decreases in response to locally applied CRF. Taken together, these results indicate that NE release in the CeA is released canonically in a vesicular-dependent manner, and that while acute alcohol does not directly alter NE release, CRF decreases it. Our results suggest that CRF acts locally on NE terminals as negative feedback and potentially prevents hyperactivation of the CRF-norepinephrine stress pathway.

Noradrenergic Mechanisms in Fentanyl-Mediated Rapid Death Explain Failure of Naloxone in the Opioid Crisis

In December 2018, the Centers for Disease Control declared fentanyl the deadliest drug in America. Opioid overdose is the single greatest cause of death in the United States adult population (ages 18-50), and fentanyl and its analogs [fentanyl/fentanyl analogs (F/FAs)] are currently involved in >50% of these deaths. Anesthesiologists in the United States were introduced to fentanyl in the early 1970s when it revolutionized surgical anesthesia by combining profound analgesia with hemodynamic stability. However, they quickly had to master its unique side effect. F/FAs can produce profound rigidity in the diaphragm, chest wall and upper airway within an extremely narrow dosing range. This clinical effect was called wooden chest syndrome (WCS) by anesthesiologists and is not commonly known outside of anesthesiology or to clinicians or researchers in addiction research/medicine. WCS is almost routinely fatal without expert airway management. This review provides relevant clinical human pharmacology and animal data demonstrating that the significant increase in the number of F/FA-induced deaths may involve α-adrenergic and cholinergic receptor-mediated mechanical failure of the respiratory and cardiovascular systems with rapid development of rigidity and airway closure. Although morphine and its prodrug, heroin, can cause mild rigidity in abdominal muscles at high doses, neither presents with the distinct and rapid respiratory failure seen with F/FA-induced WCS, separating F/FA overdose from the slower onset of respiratory depression caused by morphine-derived alkaloids. This distinction has significant consequences for the design and implementation of new pharmacologic strategies to effectively prevent F/FA-induced death. SIGNIFICANCE STATEMENT: Deaths from fentanyl and F/FAs are increasing in spite of availability and awareness of the opioid reversal drug naloxone. This article reviews literature suggesting that naloxone may be ineffective against centrally mediated noradrenergic and cholinergic effects of F/FAs, which clinically manifest as severe muscle rigidity and airway compromise (e.g., wooden chest syndrome) that is rapid and distinct from respiratory depression seen with morphine-derived alkaloids. A physiologic model is proposed and implications for new drug development and treatment are discussed.

Characteristic localization of alpha 1- and alpha 2-adrenoceptors in the human kidney

1. The localization of alpha-adrenoceptors in the plasma membranes of human kidney were investigated by radioligand binding, using an alpha 1-antagonist, [3H]-bunazosin, and an alpha 2-antagonist, [3H]-rauwolscine. 2. Both the maximum binding (Bmax) and dissociation constant (Kd) of [3H]-bunazosin were greater in the cortex than in the medulla. The Bmax of [3H]-rauwolscine in the medulla was greater than in the cortex. 3. Thus, alpha 1-adrenoceptors appeared to be localized predominantly in the cortex, while the alpha 2-adrenoceptors were mainly present in the medulla of the human kidney.

Clonidine inhibits fluid absorption in the rabbit proximal convoluted renal tubule

Previous studies have shown that norepinephrine (NE) and the beta-adrenoceptor agonist, isoproterenol (I), enhance fluid absorption (JV) in isolated, perfused proximal convoluted tubule segments (PCT). Pretreatment of PCT with the beta-adrenoceptor antagonist, propranolol, inhibited the action of NE and produced a significant decline in JV, suggesting modulation of JV by both alpha- and beta-adrenoceptors. The present studies further characterize the alpha-adrenoceptor control of JV in isolated perfused PCT using specific agonists and antagonists. Basal JV declined significantly with the addition of the alpha 2-adrenoceptor agonist, clonidine (10(-4) M), to the bath; however, it was unchanged with the addition of the alpha 1-adrenoceptor agonist, methoxamine (10(-6) or 10(-4) M). With the addition of 10(-6) M isoproterenol JV increased significantly, and returned to control values with the subsequent addition of clonidine (10(-6) or 10(-4) M). Pretreatment of PCT with the alpha 2-adrenoceptor antagonist, yohimbine (10(-5) M), or with pertussis toxin (100 ng/ml) did not interfere with the stimulation of JV by isoproterenol, but abolished the inhibition of isoproterenol-stimulated JV by clonidine. Thus, clonidine inhibits JV in PCT via an alpha 2-adrenoceptor. This effect is mediated by a pertussis toxin inhibitable GTP-binding protein, but not one that is coupled to adenylyl cyclase.

Amphetamine antagonizes the presynaptic inhibitory effect of clonidine through an interaction at the level of the alpha 2-adrenoceptors

1. In the whole rat vas deferens 20 microM noradrenaline (NA) and 0.011 microM clonidine decreased (36 +/- 7.4% and 80 +/- 6.0% respectively) the motor response induced by hypogastric nerve stimulation. These effects were reverted by 1 microM yohimbine. Amphetamine 5.4 microM failed to antagonize the inhibitory effect of NA and attenuated clonidine effect. 2. The effect of amphetamine was not altered by preincubation with either cocaine 1 microM, (-)-propranolol 0.3 microM or cocaine plus prazosin 0.028 microM. 3. In reserpine pretreated animals amphetamine 5.4 microM shifted to the right the concentration-response curve (CRC) to clonidine 0.62 +/- 0.05 log units with a KB value of 1.83 +/- 0.30 microM. 4. Binding of [3H]clonidine and [3H]prazosin were inhibited by amphetamine. Amphetamine was 90 times more potent to inhibit [3H]clonidine binding. 5. The results obtained suggest a possible direct interaction between clonidine and amphetamine on alpha-adrenoceptor.

Characterization of alpha 1- and alpha 2-adrenoceptors directly associated with basolateral membranes from rat kidney proximal tubules

We have used 2-(beta-(3-125iodo-4-hydroxyphenyl)-ethylaminoethyl)-tetr alo ne ([125I]HEAT or BE2254), an alpha 1-selective antagonist, and [3H]yohimbine, an alpha 2-selective antagonist, to demonstrate and characterize binding sites in basolateral membranes from rat kidney cortex. Parathyroid hormone (PTH) stimulated the adenylate cyclase activity of the basolateral membranes, whereas thyrocalcitonin, arginine vasopressin (AVP) and isoproterenol did not. Therefore, the basolateral membranes were probably derived from the proximal tubules. The specific binding of [125I]HEAT and [3H]yohimbine to basolateral membranes was rapid, reversible, saturable and of high affinity. The maximum densities of alpha 1- and alpha 2-receptors were 364 and 1130 fmoles/mg protein, indicating that the ratio of alpha 1- to alpha 2-adrenoceptors was about 1:3. The specific binding of [125I]HEAT and [3H]yohimbine to the basolateral membranes was displaced by various adrenergic agents in a manner that suggests that the labeled sites probably represent alpha 1- and alpha 2-adrenoceptors respectively. These results suggest that the binding sites of [125I]HEAT and [3H]yohimbine, which appear to be alpha 1- and alpha 2-adrenoceptors, exist in the basolateral membranes of the proximal tubules.

Effects of chemical sympathectomy with 6-hydroxydopamine on alpha- and beta-adrenoceptors and muscarinic cholinoceptors in rat kidney

The autonomic receptors in the rat kidney were characterized using the radioligands [3H]prazosin, [3H]clonidine, [3H]dihydroalprenolol (DHA) and [3H]quinuclidinyl benzilate (QNB). The specific binding of [3H]prazosin, [3H]clonidine, [3H]DHA and [3H]QNB to rat kidney membranes was saturable and of high affinity, and showed a pharmacological specificity as well as stereospecificity which characterized renal alpha 1-, alpha 2- and beta-adrenoceptors and muscarinic cholinoceptors, respectively. There was a relatively greater density of alpha-adrenoceptors than beta-adrenoceptors or muscarinic cholinoceptors in the rat kidney. Chemical sympathectomy of rats with 6-hydroxydopamine X HBr (6-OHDA, 50 X 2 mg/kg i.v., 24 h interval) caused a significant increase (21-56%) in the Bmax values for renal [3H]prazosin, [3H]clonidine and [3H]DHA binding at 1 and 2 weeks following the treatment, without a change in the Kd values. 6-OHDA treatment had no significant effect on the Kd and Bmax values for [3H]QNB binding at 1-3 weeks after the treatment. The norepinephrine (NE) concentration was reduced (68-76%) in the 6-OHDA-treated rat kidney. In conclusion, the present study provides biochemical evidence for the possible localization of postsynaptic alpha 1-, alpha 2- and beta-adrenoceptors and muscarinic cholinoceptors in the rat kidney and also for the regulation of these adrenoceptors by the sympathetic nervous system.

The effect of aging on alpha-adrenoceptors and their responses in rabbits

The effects of age on alpha-adrenoceptor responses, sensitivity and number were studied in rabbits aged from 1 to 36 months. Three types of investigation were carried out: conscious animal studies, isolated tissue studies and radioligand binding studies. Specific [3H]-prazosin binding decreased with age in both spleen and heart suggesting that the number of alpha 1-receptors declined at least in the tissues studied. The specific binding of [3H]-clonidine to spleen membranes and [3H]-yohimbine to platelets was not affected by age. In vitro responsiveness to alpha-adrenoceptor agonists decreased with age in abdominal aorta and renal artery, while the affinity of adrenoceptors for prazosin (pA2) was not altered. The decrease may be non-specific as responses to potassium were also altered. No change in alpha 2-adrenoceptor mediated platelet aggregation was observed. No change in pressor or depressor responses to full adrenoceptor agonists or to antagonists was observed in vivo. However, responses to clonidine, which is a partial agonist at alpha 1-adrenoceptors, were decreased. While aging influenced alpha-adrenoceptor subtypes differently, there was no direct relation between functional changes and number of receptors.

Light microscopic autoradiography of the distribution of [3H]rauwolscine binding to alpha 2-adrenoceptors in rat kidney

The localization of [3H]rauwolscine binding to microscope slide mounted sections of rat kidney has been examined using the technique of in vitro labelling and autoradiography. Binding to sections equilibrated within 60 min and was reversible following the addition of 10 microM phentolamine. Saturation studies revealed a single population of high affinity (KD 4.27 nM) non-interacting sites (nH 0.97) with a density of 11.1 fmol/section. Stereoselectivity was observed with respect to the isomers of noradrenaline and the relative affinity of a series of alpha-adrenoceptor antagonists suggested binding to alpha 2-adrenoceptors. Autoradiographic studies using 3H-Ultrofilm showed that the binding is largely confined to the renal cortex. More detailed studies using emulsion coated coverslips indicates that the major concentration of binding sites is over the proximal tubules. This study provides evidence that alpha 2-adrenoceptors, known to be coupled in an inhibitory fashion to renal adenylate cyclase, are highly localized to particular structures in the kidney.

[3H]-rauwolscine binding to alpha 2-adrenoceptors in the mammalian kidney: apparent receptor heterogeneity between species

Binding of the alpha 2-adrenoceptor antagonist [3H]-rauwolscine was characterized in membrane preparations from the kidneys of mouse, rat, rabbit, dog, and man. In all species, binding reached equilibrium within 45 min and dissociated at a single exponential rate after addition of phentolamine 10 microM. Saturation studies showed that the affinity of [3H]-rauwolscine was similar in all species (2.33-3.03 nM) except man where it was significantly higher (0.98 nM). Marked differences were seen in the density of binding sites, increasing in the order: man less than dog less than rabbit less than rat less than mouse. In all cases, Hill coefficients were not significantly different from unity. [3H]-rauwolscine binds with low affinity (KD greater than 15 nM) to membranes prepared from guinea-pig kidney. The low affinity binding is not due to the absence of particular ions in the incubation medium or to receptor occupation by endogenous agonist. The binding in all species was found to be stereoselective with respect to the isomers of noradrenaline. However, differences were seen in the characteristics of agonist interactions with the binding site both between isomers and between species. Marked differences in affinity of particular alpha-adrenoceptor antagonists were observed for alpha 2-adrenoceptors labelled by [3H]-rauwolscine. These differences were most evident with the alpha 1-adrenoceptor selective antagonist prazosin which displayed inhibition constants (Ki values) of 33.2, 39.5, 261, 570 and 595 nM in rat, mouse, dog, man and rabbit, respectively. Differences are apparent in the characteristics of alpha 2-adrenoceptors labelled by [3H]-rauwolscine between species and it is suggested that the differences observed for alpha 1-selective antagonists such as prazosin may be related to binding to additional sites in the vicinity of the alpha 2-adrenoceptor.

Distribution and function of peripheral alpha-adrenoceptors in the cardiovascular system

alpha-Adrenoceptors may be subdivided based on their anatomical distribution within the synapse. Presynaptic alpha-adrenoceptors are generally of the alpha 2-subtype and modulate neurotransmitter liberation via a negative feedback mechanism. Postsynaptic alpha-adrenoceptors are usually of the alpha 1-subtype and mediate the response of the effector organ. Although this "anatomical" subclassification is generally applicable, many exceptions exist. A more useful classification of alpha-adrenoceptor subtypes is based on a pharmacological characterization in which selective agonists and antagonists are used. Peripheral alpha-adrenoceptors are critical in the regulation of the cardiovascular system. Postsynaptic alpha-adrenoceptors in arteries and veins represent a mixed population of alpha 1/alpha 2-adrenoceptors, with both subtypes mediating vasoconstriction. In the peripheral arterial circulation, postsynaptic vascular alpha 1-adrenoceptors are found in the adrenergic neuroeffector junction, whereas postsynaptic vascular alpha 2-adrenoceptors are located extrajunctionally. In the venous circulation, it appears that alpha 2-adrenoceptors may be predominantly junctional, whereas alpha 1-adrenoceptors may be predominantly extrajunctional. It has been proposed that junctional alpha-adrenoceptors will respond predominantly to norepinephrine liberated from sympathetic neurons, whereas extrajunctional alpha-adrenoceptors likely respond to circulating catecholamines. The functional role of extrajunctional alpha-adrenoceptors may be more important in disease states such as hypertension and congestive heart failure where circulating levels of catecholamines may be high and contribute to the maintenance of elevated vascular resistance. alpha 2-Adrenoceptors are also associated with the intima and may play a role in the release of an endogenous relaxing factor from the endothelium.(ABSTRACT TRUNCATED AT 250 WORDS)

Heterogeneity of alpha-2 adrenergic receptors

Alpha adrenergic receptors are subdivided into alpha-1 and alpha-2 subtypes on the basis of their pharmacologic properties. An evaluation of data from radioligand binding and functional experiments indicates that the receptors classified as alpha-2 are not a homogeneous group. The best example of this heterogeneity is the differences in the pharmacologic properties of alpha-2 receptors in rodent and non-rodent mammalian species. Prazosin generally has a high affinity for rodent alpha-2 receptors, but a low affinity for non-rodent alpha-2 receptors, while oxymetazoline is more potent at non-rodent alpha-2 receptors. A definition of alpha-2 adrenergic receptor subtypes is proposed with prazosin having a lower affinity (200-300 nM) at alpha-2A receptors and a higher affinity (5-10 nM) at alpha-2B receptors. Using this definition, the human platelet appears to have only the alpha-2A subtype, while all the receptors in the neonatal rat lung are of the alpha-2B subtype. The rat brain has roughly equal amounts of alpha-2A and -2B receptors while in the rat submandibular gland, about 85% of the receptors are alpha-2A. The ability to pharmacologically define putative alpha-2 adrenergic receptor subtypes should promote the development of additional subtype selective drugs which will increase our understanding of adrenergic pharmacology and may provide new therapeutic approaches.

Functional characterization of the alpha adrenergic receptor modulating the hydroosmotic effect of vasopressin on the rabbit cortical collecting tubule

To characterize the type of alpha adrenergic receptor, the effects of specific alpha adrenergic agonists and antagonists on antidiuretic hormone [( Arg8]-vasopressin [AVP])-induced water absorption were evaluated in cortical collecting tubules isolated from the rabbit kidney and perfused in vitro. In the presence of AVP (100 microU/ml), net fluid volume absorption (Jv, nanoliters per minute per millimeter) was 1.39 +/- 0.09 and osmotic water permeability coefficient (Pf, X 10(-4) centimeters per second) was 150.2 +/- 15.0. The addition of 10(-6) M phenylephrine (PE), an alpha adrenergic agonist, resulted in a significant decrease in Jv and Pf to 0.72 +/- 0.11 (P less than 0.005) and 69.9 +/- 10.9 (P less than 0.005). The addition of 10(-4) M prazosin (PZ), an alpha adrenergic antagonist, did not cause any significant change in Jv and Pf, which were 0.71 +/- 0.09 (P = NS vs. AVP + PE) and 67.8 +/- 9.5 (P = NS vs. AVP + PE), respectively. In a separate group of tubules, in the presence of AVP (100 microU/ml) and PE (10(-6) M), Jv and Pf were 0.78 +/- 0.17 and 76.1 +/- 18.0, respectively. The addition of 10(-6) M yohimbine (Y), an alpha 2 adrenergic antagonist, resulted in a significant increase in Jv to 1.46 +/- 0.14 (P less than 0.01) and Pf to 157.5 +/- 22.3 (P less than 0.005). Y (10(-4) M) or PZ (10(-4) M) alone did not significantly affect Jv and Pf in the presence of AVP )100 microU/ml). The effect of the natural endogenous catecholamine norepinephrine (NE) on Jv and Pf in the presence of AVP and propranolol (PR) was next examined. Jv and Pf were 1.53 +/- 0.07 and 176.3 +/- 5.2, respectively, in the presence of AVP (100 microU/ml) and PR (10(-4) M). The addition of NE (10(-8) M) resulted in a significant decrease in Jv to 1.19 +/- 0.11 (P less than 0.05) and Pf to 127.0 +/- 11.3 (P less than 0.02). Increasing the concentration of NE to 10(-6) M resulted in a further decrease in Jv and Pf to 0.70 +/- 0.10 (P less than 0.01 vs. NE 10(-8) M) and 68.5 +/- 10.6 (P less than 0.01 vs. NE 10(-8) M), respectively. The inhibitory effect of NE on AVP-induced water absorption was blocked by Y, but not by PZ. The effect of the alpha 2 adrenergic agonist clonidine (CD) on Jv and Pf was also examined. In the presence of AVP (10 microU/ml) Jv and Pf were 1.65 +/- 0.04 and 175.1 +/- 13.1, respectively. The addition of CD (10(-6) M) resulted in a significant decrease in Jv to 1.08 +/- 0.12 (P < 0.01) and Pf to 108.1 +/- 15.4 (P < 0.01). Increasing the concentration of CD to 10(-4) M resulted in a further significant decrease in Jv and Pf to 0.57 +/- 0.13 (P < 0.02 vs. CD 10(-6) M) and 54.7 +/- 13.8 (P < 0.01 vs. CD 10(-6) M), respectively. Similar results were obtained in the presence of AVP (100 microU/ml). The inhibitory effect of CD on AVP-induced water absorption was blocked by Y. CD did not significantly affect Jv and Pf in the presence of 8-bromo adenosine 3',5'-cyclic monophosphate. These studies indicate that alpha adrenergic agonists directly inhibit AVP-mediated water absorption at the level of renal tubule, an effect that can be blocked by specific alpha2 adrenergic antagonists, but not by specific alpha1 adrenergic antagonists. Alpha2 adrenergic stimulation directly inhibits AVP-mediate water absorption at the level of the tubule, an effect that can be blocked by a specific alpha2 adrenergic antagonist. This effect appears to be exerted at the level of activation of adenylate cyclase, since it is absent in the present of cyclic AMP.

The involvement of central alpha adrenoceptors in the antihypertensive actions of methyldopa and clonidine in the rat

The antihypertensive effects of methyldopa and clonidine result from agonist activity at alpha adrenoceptor sites within the brain. Methyldopa is converted enzymatically to alpha-methylnoradrenaline in noradrenergic neurones in rat brain and replaces the natural transmitter, noradrenaline. Radioligand receptor studies show that alpha-methylnoradrenaline differs from noradrenaline in being much more selective (70 times) for the alpha 2 subclass of adrenoceptors than noradrenaline and it is likely that the antihypertensive action of methyldopa results from selective activation of alpha 2 adrenoceptors by alpha-methylnoradrenaline in the nucleus tractus solitarius and the anterior hypothalamus. Radioligand studies also show that clonidine is a selective alpha 2 adrenoceptor agonist although it probably interacts with alpha 1 adrenoceptors at higher concentrations. With regard to a withdrawal syndrome after cessation of clonidine treatment, the cardiovascular and behavioural components can now be characterised in a rat model. The components include increases in basal blood pressure and heart rate, as well as increases in cardiovascular reactivity and also increases in rapid eye movement (REM) sleep, body shakes and tremor which is reminiscent of an opiate withdrawal syndrome. Increased central noradrenergic activity is involved in this syndrome and alpha 1 and alpha 2 adrenoceptors mediate opposing effects on the REM sleep rebound component.

Autoradiographic localization of prazosin binding sites in rat kidney

[3H]Prazosin is a selective ligand for alpha 1-adrenoceptors. Using an in vitro autoradiographic technique specific binding of [3H]prazosin has been demonstrated, mainly to the renal cortex of the rat. These results provide further evidence that, in the rat, there is a high density of alpha 1-adrenoceptors within the renal cortex.

Evidence from binding studies for beta 1-adrenoceptors associated with glomeruli isolated from rat kidney

The beta adrenoceptor antagonist radioligand [3H] dihydroalprenolol (DHA) has been used to characterise beta adrenoceptors in membranes prepared from rat renal glomeruli. Association of the ligand was rapid and had reached equilibrium within 10 mins at 37 degrees C. Dissociation occurred in two distinct phases, a rapidly dissociating phase (low affinity site) and a slowly dissociating phase (high affinity site). The KD value for the high affinity site calculated from the kinetic experiments was 0.8 nM. Saturation analysis of binding gave comparable values for KD (1.77 nM) and demonstrated that membranes from glomeruli had four times the density of binding sites measured in renal cortex. In all saturation studies Hill coefficients were not significantly different from unity. Binding was stereoselective with respect to the (-) isomers of isoprenaline and propranolol and the potency of the selective displacing agents betaxolol (beta 1 adrenoceptors) and ICI 118,551 (beta 2 adrenoceptors) indicated that the receptors are of the beta 1 subtype.

The sympathetic nervous system in the pathogenesis of acute renal failure

The purpose of this review is to highlight the participation and the possible role of the renal sympathetic nervous system in the development of renal cell damage and the consequent dysfunctions and filtration filtration failure.

Gluconeogenesis in guinea pig renal tubule fragments--effects of noradrenaline, 3':5' cyclic AMP and angiotensin II

1. Tubule fragments were isolated by collagenase treatment of guinea pig kidney cortex. 2. 3':5'-Cyclic AMP increased gluconeogenesis from lactate, pyruvate, malate and fructose. 3. Noradrenaline decreased gluconeogenesis from lactate, pyruvate, 2-oxoglutarate and fructose. 4. Angiotensin II slightly, but significantly, increased gluconeogenesis from lactate. 5. Gluconeogenesis from glycerol as sole substrate was negligible. Gluconeogenesis from combinations of glycerol together with either lactate, pyruvate, 2-oxoglutarate or malate was appreciably greater than the sum of the glucose output observed when these substrates were added singly.

Localisation of [3H]clonidine binding to membranes from guinea pig renal tubules

The selective radioligand [3H]clonidine has been used to localise alhpa 2 adrenoceptors in guinea pig kidney. Chemical sympathectomy with 6-hydroxydopamine produced no significant change in the number of sites labeled by [3H]clonidine indicating that the majority of binding sites were not located on sympathetic nerve terminals. Binding was enhanced in membranes prepared from renal tubules and considerably reduced in preparations from glomeruli. Subcellular fractions of renal cortex revealed that binding was to plasma membranes and that the greatest binding capacity was present in the fraction rich in basal lateral membranes. It is concluded that the major concentration of renal alpha 2 adrenoceptors are present on renal tubules and that they may be localised to a particular pole of the renal tubule cell.

Characterization and localization of [3H]-clonidine binding in membranes prepared from guinea-pig spleen

1. [3H]-Clonidine binds to membranes prepared from guinea-pig spleen with high affinity. 2. Kinetic experiments indicated that [3H]-clonidine associates rapidly to the binding site and that the binding is reversible. A study of the dissociation of [3H]-clonidine from splenic membranes revealed two components. The slowly dissociating component corresponded to a high affinity process (Kd = 2.1 nmol/l) in good agreement to that obtained by saturation analysis. 3. Over the concentration range used, saturation experiments revealed only a single population of sites with a dissociation constant (Kd) of 2.4 nmol/l and a density of 5.1 pmol/g wet weight tissue. 4. Examination of the relatively potency of a series of alpha-adrenoceptor agonists and antagonists indicates that [3H]-clonidine binding is to alpha 2-adrenoceptors. 5. High levels of binding were obtained to lymphocytes prepared from guinea-pig spleen and to membranes from the splenic capsule. Pretreatment of animals with 6-hydroxydopamine produced changes in apparent affinity of binding with little change in the number of receptor sites. 6. It is concluded that [3H]-clonidine labels is a site resembling the alpha 2-adrenoceptor in guinea-pig spleen. Few if any of these sites are located prejunctionally and a significant fraction are associated with lymphocytes.

Adrenergic receptors in human and experimental pheochromocytoma

Radioligand binding to adrenergic receptors has become an important technique for identification and quantitation of these receptors. In this article we review our recent findings using subtype-selective radioligands for studying alpha 1 (e.g., [3H]prazosin) and alpha 2 (e.g. [3H]yohimbine) receptors in human platelets and rat renal cortical membranes, and we report new results on changes in adrenergic receptors of patients and animals with pheochromocytoma. Five patients with pheochromocytoma were tested prior to surgery. The number of alpha 2-adrenergic receptors on platelets from these patients and the affinity of these receptors for [3H]yohimbine were similar to values for control subjects. In rats with a transplantable pheochromocytoma we found a 50-fold increase in plasma norepinephrine levels. The number of renal cortical beta-adrenergic receptors (identified using [125I]iodohydroxybenzylpindolol) and alpha 1-adrenergic receptors were decreased while the number of alpha 2-adrenergic receptors did not change. Receptor affinities for radioligands were unaltered in animals with pheochromocytoma. These findings indicate that pheochromocytoma is associated with selective decreases in the number of renal beta- and alpha 1-adrenergic receptors without changing alpha 2-adrenergic receptor number or affinity. Furthermore, the number and affinity of alpha 2-adrenergic receptors are not altered in platelets of patients with pheochromocytoma. We conclude that down regulation (an agonist-mediated decrease in receptor number) occurs in vivo for some, but not all, types of adrenergic receptors.

Comparison of [3H]clonidine and [3H]guanfacine binding to alpha 2 adrenoceptors in membranes from rat cerebral cortex

[3H]Clonidine and a new phenylacylguanidine antihypertensive drug, [3H]guanfacine, bind with a high affinity to alpha 2 adrenoceptors in membranes from rat cerebral cortex. Dissociation curves for [3H]clonidine binding indicated the presence of high and low affinity binding sites, whereas with [3H]guanfacine, two components could be distinguished only with difficulty; the major part of the binding being to a high affinity component. Saturation experiments revealed that both ligands bind with similar high affinity but that [3H]guanfacine labels twice the number of sites labeled by [3H]clonidine.

Alpha-adrenergic inhibition of cyclic AMP accumulation in hamster adipocytes. Similarity of receptor with alpha-2 adrenergic receptors

The present communication shows the effects of several alpha-adrenergic agonists and antagonists on cyclic AMP levels in hamster epididymal adipocytes. In response to ACTH (30 mU/ml) in combination with 1-methyl-3-isobutylxanthine (0.10 mM) or adenosine deaminase (1.0 micrograms/ml), cyclic AMP levels increased to a maximum by 10 min and this level was maintained for another 20 min. Elevated cyclic AMP levels were partially suppressed by the alpha-adrenergic agents clonidine, methoxamine, methyl norepinephrine and phenylephrine. The lowest effective concentration of each of these agonists required to suppress cyclic AMP levels was 10 nM clonidine; 3 microM methoxamine; 10 microM methyl norepinephrine; 10 microM phenylephrine. Clonidine and methoxamine suppressed cyclic AMP levels by nearly 65% while phenylephrine and methyl norepinephrine caused only a 30% decline. Studies of the relative potencies of alpha-adrenergic blocking drugs on prevention of the inhibitor effect of clonidine on cyclic AMP levels disclosed that phentolamine and yohimbine were more potent blockers of clonidine action than phenoxybenzamine and prazosin. The rank order of potencies of agonists at causing suppression of cyclic AMP levels and the rank order of potencies of antagonists of clonidine action suggest similarity of the alpha-adrenergic receptors present on hamster adipocytes, which affect cyclic AMP accumulation to alpha-2 adrenergic receptors.

alpha 2-Adrenergic receptors are associated with renal proximal tubules

The location of alpha 2-adrenergic receptors has been investigated in the guinea pig kidney. We used an in vitro labeling autoradiographic technique to examine the distribution of specific [3H]clonidine binding sites with the light microscope. alpha 2-Adrenergic receptors appeared to be located predominantly on the proximal tubules. The implication of this finding for renal physiology is discussed.

Evidence that catecholamines stimulate renal gluconeogenesis through an alpha 1-type of adrenoceptor

1. Noradrenaline stimulates gluconeogenesis through an alpha-adrenoceptor in renal cortical tubule fragments from fed rats incubated with 5 mM-lactate. 2. The selective alpha 1-adrenoreceptor agonist methoxamine stimulated gluconeogenesis, but the selective alpha 2-adrenoceptor agonist clonidine was ineffective. 3. The selective alpha 1-adrenoceptor antagonist thymoxamine blocked the stimulatory effects on gluconeogenesis of noradrenaline and of oxymetazoline (a synthetic alpha-agonist). The selective alpha 2-adrenoceptor antagonist yohimbine was ineffective in this respect. 4. It is concluded that noradrenaline and oxymetazoline stimulate gluconeogenesis in rat kidney via an alpha 1-rather than an alpha 2-type of adrenoceptor.

[3H]-Clonidine binding to alpha-adrenoceptors in membranes prepared from regions of guinea-pig kidney: alteration by monovalent and divalent cations

1 [(3)H]-clonidine binds reversibly to membranes prepared from regions of guinea-pig kidney.2 Higher levels of binding were obtained in the membranes prepared from renal cortex (2.15 +/- 0.27 pmol/g wet wt.) than renal medulla (0.53 +/- 0.07 pmol/g wet wt.) or papilla (0.14 +/- 0.06 pmol/g wet wt.; n = 4).3 Scatchard analysis performed by addition of unlabelled clonidine (1 to 30 pmol) gave figures for the dissociation constant (K(d)) for the binding of [(3)H]-clonidine to renal cortical membranes of 9.0 +/- 0.8 nM and B(max) of 21.6 +/- 1.7 pmol/g wet wt. (n = 4). Hill plots of these data gave gradients close to unity, indicating a lack of co-operative site interactions.4 The monovalent cations, sodium and potassium, and the divalent cation, calcium, produced concentration-dependent decreases in [(3)H]-clonidine binding to membranes prepared from renal cortex, the EC(50)s being respectively 25 mM, 37 mM and 23 mM.5 At low concentrations the divalent cations, magnesium (1 mM) and manganese (0.1 mM), produced enhancement of [(3)H]-clonidine binding. At higher concentrations (>10 mM) both divalent cations inhibited binding.6 Scatchard analysis of [(3)H]-clonidine binding performed in the presence of sodium (100 mM), magnesium (1 mM) or manganese (0.1 mM) revealed that the alterations in binding are primarily due to changes in apparent affinity rather than a change in the number of binding sites. Sodium (100 mM) produced a change in the K(d) from 7.0 +/- 0.4 nM (n = 8) to 42.3 +/- 27.5 nM (n = 3), whereas magnesium (1 mM) decreased the K(d) to 6.0 +/- 0.9 nM and manganese (0.1 mM) to 4.0 +/- 1.0 nM (n = 3).7 The results indicate that [(3)H]-clonidine labels a binding site that has properties resembling an alpha(2)-adrenoceptor, located in the renal cortex. The changes produced by the addition of monovalent and divalent cations are entirely due to changes in the apparent affinity of [(3)H]-clonidine binding.