Pharmacology and molecular biology of adrenergic receptors

How to fail with paired VNS therapy

Vagus nerve stimulation (VNS) has gained enormous traction as a promising bioelectronic therapy. In particular, the delivery of VNS paired with training to promote neural changes has demonstrated clinical success for stroke recovery and found far-reaching application in other domains, from autism to psychiatric disorders to normal learning. The success of paired VNS has been extensively documented. Here, we consider a more unusual question: why does VNS have such broad utility, and perhaps more importantly, when does VNS not work? We present a discussion of the concepts that underlie VNS therapy and an anthology of studies that describe conditions in which these concepts are violated and VNS fails. We focus specifically on the mechanisms engaged by implanted VNS, and how the parameters of stimulation, stimulation method, pharmacological manipulations, accompanying comorbidities, and specifics of concurrent training interact with these mechanisms to impact the efficacy of VNS therapy. As paired VNS therapy is increasing translated to clinical implementation, a clear understanding of the conditions in which it does, and critically, does not work is fundamental to the success of this approach.

Polymorphism profiling of nine high altitude relevant candidate gene loci in acclimatized sojourners and adapted natives

Background

Sea level sojourners, on ascent to high altitude, undergo acclimatization through integrated physiological processes for defending the body against oxygen deprivation while the high altitude natives (resident population) are adapted to the prevailing hypobaric hypoxic condition through natural selection. Separating the acclimatization processes from adaptive changes and identifying genetic markers in lowlanders that may be beneficial for offsetting the high altitude hypoxic stress, although challenging, is worth investigating. We genotyped nine candidate gene polymorphisms, suggested to be relevant in high altitude environment, in sea level acclimatized sojourners and adapted natives for understanding differences/commonality between the acclimatized and the adapted cohorts at the genetic level.

Results

Statistically similar genotypic and allelic frequencies were observed between the sea level sojourners (acclimatized) and the high altitude natives (adapted) in six loci viz., EDN1 (endothelin 1) -3A/-4A VNTR, ADRB2 (beta-2 adrenergic receptor, surface) Arg16Gly (rs1042713:A > G), ADRB3 (beta-3 adrenergic receptor) Trp64Arg (rs4994:T > C), eNOS (nitric oxide synthase, endothelial) Glu298Asp (rs1799983:T > G), TH (tyrosine hydroxylase) Val81Met (rs6356:G > A) and VEGF (vascular endothelial growth factor) 963C > T (rs3025039:C > T) while SCNN1B (amiloride-sensitive sodium channel, subunit beta) Thr594Met (rs1799979:C > T) was monomorphic. Genotypic and allelic frequencies in EDN1 9465G > A (rs2071942:G > A) and ADRB2 Gln27Glu (rs1042714:G > C) were significantly different between the acclimatized sojourners and the high altitude natives with higher frequency of GG and GA genotypes of EDN1 rs2071942 and CC genotype of ADRB2 rs1042714 being observed in Ladakh natives. Mutated A allele (AA genotype) of rs2071942 and carriers of G allele (GG + GC genotypes) of rs1042714 were less favorable during acclimatization under recessive and dominant genetic models of inheritance respectively indicating thereby that GG genotype and G allele of EDN1 rs2071942 and CC genotype of ADRB2 rs1042714 conferred acclimatization benefit.

Conclusion

Sea level acclimatized individuals shared similarity with the adapted natives in certain high altitude relevant genetically based trait variation suggesting advantageous consequence as well as commonality in gene regulatory pathways in which these gene products function both during process of acclimatization and adaptation in high altitude environment.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-015-0268-y) contains supplementary material, which is available to authorized users.

Asthma: Gln27Glu and Arg16Gly polymorphisms of the beta2-adrenergic receptor gene as risk factors

Background

Asthma is caused by both environmental and genetic factors. The ADRB2 gene, which encodes the beta 2-adrenergic receptor, is one of the most extensively studied genes with respect to asthma prevalence and severity. The Arg16Gly (+46A > G) and Gln27Glu (+79C > G) polymorphisms in the ADRB2 gene cause changes in the amino acids flanking the receptor ligand site, altering the response to bronchodilators and the risk of asthma through complex pathways. The ADRB2 polymorphisms affect beta-adrenergic bronchodilator action and are a tool to identify at-risk populations.

Objective

To determine the frequency of these two polymorphisms in allergic asthma patients and healthy subjects and to correlate these data with the occurrence and severity of asthma.

Methods

Eighty-eight allergic asthma patients and 141 healthy subjects were included in this study. The ADRB2 polymorphisms were analyzed using the amplification-refractory mutation system – polymerase chain reaction (ARMS-PCR) technique. The statistical analysis was performed with the SPSS 21.0 software using the Fisher’s Exact and χ² tests.

Results

The ADRB2 polymorphisms were associated with asthma occurrence. The Arg16Arg, Gln27Gln and Gln27Glu genotypes were risk factors; the odds ratios were 6.782 (CI = 3.07 to 16.03), 2.120 (CI = 1.22 to 3.71) and 8.096 (CI = 3.90 to 17.77), respectively. For the Gly16Gly and Glu27Glu genotypes, the odds ratios were 0.312 (CI = 0.17 to 0.56) and 0.084 (CI = 0.04 to 0.17), respectively. The haplotype analysis showed that there were associations between the following groups: Arg16Arg-Gln27Gln (OR = 5.108, CI = 1.82 to 16.37), Gly16Gly-Glu27Glu (OR = 2.816, CI = 1.25 to 6.54), Arg16Gly-Gln27Glu (OR = 0.048, CI = 0.01 to 0.14) and Gly16Gly-Gln27Glu (OR = 0.1036, CI = 0.02 to 0.39). The polymorphism Gln27Glu was associated with asthma severity, as the Gln27Gln genotype was a risk factor for severe asthma (OR = 2.798, CI = 1.099 to 6.674) and the Gln27Glu genotype was a protective factor for mild (OR = 3.063, CI = 1.037 to 9.041) and severe (OR = 0.182, CI = 0.048 to 0.691) asthma.

Conclusions

The Arg16Gly and Gln27Glu polymorphisms in the ADRB2 gene are associated with asthma presence and severity.

Alterations of cAMP-dependent signaling in dystrophic skeletal muscle

Autonomic regulation processes in striated muscles are largely mediated by cAMP/PKA-signaling. In order to achieve specificity of signaling its spatial-temporal compartmentation plays a critical role. We discuss here how specificity of cAMP/PKA-signaling can be achieved in skeletal muscle by spatio-temporal compartmentation. While a microdomain containing PKA type I in the region of the neuromuscular junction (NMJ) is important for postsynaptic, activity-dependent stabilization of the nicotinic acetylcholine receptor (AChR), PKA type I and II microdomains in the sarcomeric part of skeletal muscle are likely to play different roles, including the regulation of muscle homeostasis. These microdomains are due to specific A-kinase anchoring proteins, like rapsyn and myospryn. Importantly, recent evidence indicates that compartmentation of the cAMP/PKA-dependent signaling pathway and pharmacological activation of cAMP production are aberrant in different skeletal muscles disorders. Thus, we discuss here their potential as targets for palliative treatment of certain forms of dystrophy and myasthenia. Under physiological conditions, the neuropeptide, α-calcitonin-related peptide, as well as catecholamines are the most-mentioned natural triggers for activating cAMP/PKA signaling in skeletal muscle. While the precise domains and functions of these first messengers are still under investigation, agonists of β₂-adrenoceptors clearly exhibit anabolic activity under normal conditions and reduce protein degradation during atrophic periods. Past and recent studies suggest direct sympathetic innervation of skeletal muscle fibers. In summary, the organization and roles of cAMP-dependent signaling in skeletal muscle are increasingly understood, revealing crucial functions in processes like nerve-muscle interaction and muscle trophicity.

Rapid Desensitization of Lipolysis in the Visceral and Subcutaneous Adipocytes of Rats

In adipocytes, short and long term stimulation of beta adrenergic receptors (beta AR) induces the desensitization to catecholamines, leading to a decrease in the intracellular accumulation of cAMP, but the roles played by this in lipolysis is not clear. In this study, we assessed the catecholamine-induced desensitization of lipolysis and compared this in adipocytes isolated from visceral and subcutaneous fat tissues of rats. When adipocytes were pretreated with isoproterenol (ISO), the norepinephrine (NE)-induced lipolysis was significantly reduced dose- and time-dependently. A similar reduction of the lipolytic response was also found in NE-, dobutamine-, terbutaline- or BRL37344-induced lipolysis. The ISO- and each beta AR agonist-induced lipolysis in the visceral fat was not only higher than in the subcutaneous fat, but also markedly reduced by ISO- or NE-pretreatment. These results showed that short-term treatment of three subtypes of beta AR by each agonist induces a rapid reduction in the lipolytic response to beta AR stimulation. This suggests some common mechanism for the rapid desensitization of beta AR-agonist-induced lipolysis, in contrast with previous reports on the characteristics of beta AR subtypes. In addition, the regional difference of adipose tissue not only in inducing lipolysis but also in rapid desensitization was also apparent.

Effect of intermittent hypoxia on cardiovascular function, adrenoceptors and muscarinic receptors in Wistar rats

The usual model of intermittent hypoxia (sleep apnoea) corresponds to repeated episodes of hypoxia from a few seconds to a few hours interspersed with episodes of normoxia. The aim of this study was to evaluate in rats the effect of two periods of intermittent exposure for 2 months to hypoxia (IHX1, 24 h in hypoxia (428 Torr), 24 h in normoxia; IHX2, 48 h in hypoxia (428 Torr), 24 h in normoxia) as a new model of hypoxia simulating intermittent exposure to high altitude experienced by Andean miners. We assessed the haematological parameters, time course of resting heart rate and systolic blood pressure. We also evaluated the expression of adrenergic and muscarinic receptors. IHX1 and IHX2 produced an increase in haematocrit, haemoglobin concentration and mean corpuscular volume as previously seen in most hypoxic models. IHX1 and IHX2 induced a similar sustained elevation of systolic blood pressure (132 +/- 2 and 135 +/- 3 mmHg, respectively, vs. the control level of 121 +/- 16 mmHg) after 10 days of exposure without change in heart rate. Right ventricular (RV) hypertrophy (225 +/- 13 and 268 +/- 15 mg g(-1), vs. 178 +/- 7 mg g(-1) and downregulation of alpha1-adrenoceptor (RV: 127 +/- 21 and 94 +/- 16 fmol mg(-1) vs. 157 +/- 8 fmol mg(-1); left ventricle (LV): 141 +/- 5 and 126 +/- 9 fmol mg(-1) vs. 152 +/- 5 fmol mg(-1)) have been found in both groups, with right ventricular hypertrophy being greater and alpha1-adrenoceptor density being lower in IHX2 than in HX1 groups. These data indicate that both parameters are related to the time of exposure to hypoxia. IHX1 and IHX2 produced the same magnitude of upregulation of muscarinic receptors (LV, 60%; RV, 40%), and no change in beta-adrenoceptors. In conclusion, exposure to intermittent hypoxia led to polycythaemia and RV hypertrophy as observed in other types of hypoxia. A specific cardiovascular response was seen, that is an increase in blood pressure without change in heart rate, which was different from the one observed in episodic and chronic hypoxia. Furthermore, this model involved specific modifications of alpha1-adrenergic and muscarinic expression.

Pharmacogenetics of asthma

Recent advances in the extent of knowledge regarding interindividual genetic variation in drug treatment targets and drug metabolizing enzymes has resulted in studies designed to assess the contribution of genetic variability to treatment response in a range of diseases. This review describes the current state of knowledge of genetic variability in key airway targets important in the treatment of asthma. Whilst the genes coding for some key treatment targets contain little polymorphic variation (e.g. the muscarinic M2 and M3 receptors) other genes whose products are important targets in the treatment of asthma contain extensive genetic variation. The best examples of the latter are the beta2-adrenoceptor and the 5-lipoxygenase genes. Genetic variability in both of these genes may account in part for interindividual variability in treatment response. Finally, a number of key targets within the airways remain to be adequately screened for polymorphic variation.

Coupling of beta-adrenergic receptors to cardiac L-type Ca2+ channels: preferential coupling of the beta1 versus beta2 receptor subtype and evidence for PKA-independent activation of the channel

Beta1- and beta2-adrenergic receptors (beta-ARs) co-exist in mammalian heart, and it is generally accepted that both activate adenylyl cyclase (AC), resulting in increased levels of cAMP and subsequent activation of L-type Ca2+ channels (CaCh). To investigate the contribution of each beta-AR subtype in AC and CaCh coupling, we stably expressed cardiac CaCh alpha1 and beta2 subunits along with either beta1-AR or beta2-AR in CHW fibroblasts. Co-expression of either beta-AR with CaCh subunits conferred responsiveness of AC and CaCh to isoproterenol (ISO), which was not observed in non-transfected cells. ISO-promoted cAMP formation occurred at a lower EC50 through the beta2-AR than through the beta1-AR (0.13 +/- 0.01 vs. 0.6 +/- 0.14 nM). In contrast, activation of CaCh was more efficacious via the beta1-AR than the beta2-AR (EC50 for CaCh activation = 238 +/- 33 vs. 1057 +/- 113 nM). Pre-treatment with pertussis toxin (PTX) had no effect upon the responsiveness of either cAMP formation or CaCh activation through either receptor. We conclude (1) that beta1-ARs exhibit preferential coupling to CaCh activation, versus that observed for the beta2-AR; (2) that this preferential coupling cannot be explained solely by cAMP-dependent processes; and (3) that the relative attenuation of beta2-AR-promoted CaCh activation is not due to receptor coupling to PTX-sensitive G proteins. Thus, it is likely that other subtype-specific, cAMP-independent coupling of the beta-AR to CaCh is present.

Absence of early resetting of coronary baroreceptors in anaesthetized dogs

1. Both carotid and aortic arch baroreceptors have been shown to reset after as little as 20 min exposure to a different conditioning pressure; the mid-point of the stimulus-response curve is displaced towards the conditioning pressure. 2. Coronary baroreceptors operate over much lower pressures and induce slower reflex vasoconstriction than the other baroreceptors and this investigation was designed to determine whether their resetting characteristics are also different. 3. In chloralose anaesthetized dogs, a perfusion circuit allowed independent control of pressures distending carotid, aortic and coronary baroreceptors. Stimulus-response curves were obtained for carotid and coronary baroreceptors after maintaining the distending pressure at 60 or 180 mmHg for 20 min. 4. Neither the magnitude of the responses nor the baroreceptor pressure corresponding to 50 % of the response (BP50) of the coronary curves was changed by the conditioning regime. In contrast, conditioning carotid baroreceptors with the same regime produced significant shifts in the BP50 towards the conditioning pressure. 5. No changes were obtained after conditioning the coronary baroreceptors at 60 or 120 mmHg for 40 min. 6. These results confirm early resetting of carotid baroreceptors but show that coronary baroreceptors do not reset over a period of at least 40 min.

Influence of adrenergic agonists on the release of amino acids from rat skeletal muscle studied by microdialysis

The microdialysis technique was used to study the effects of adrenergic agonists on the release of amino acids from rat skeletal muscle. The release was monitored indirectly by measurements of interstitial concentrations. To distinguish metabolic from vasoactive effects, the adrenaline and isoprenaline results were compared with those of vasopressin, alpha-agonists and adenosine. As determined by the microdialysis ethanol technique, adrenaline, alpha-agonists and vasopressin induced vasoconstriction, whereas isoprenaline and adenosine induced vasodilatation. The lactate-to-pyruvate ratio increased fourfold with adrenaline (P < 0.001) and by 54% with isoprenaline (P < 0.05), whereas no change was observed with alpha-agonists and adenosine. Vasopressin induced a fivefold increase in the lactate-to-pyruvate ratio (P < 0.001), but with an unchanged pyruvate concentration, indicating that the effect may have been secondary to ischaemia. Adrenaline induced a twofold and vasopressin a 34% increase in the concentration of alanine (P < 0.001), whereas isoprenaline, adenosine and alpha-agonists had no significant effect. Adrenaline-perfusion induced an initial anabolic effect as evidenced by a reduced concentration of tyrosine. A significant decrease in the glutamate-to-glutamine ratio was observed with adrenaline and isoprenaline (22 and 27%, P < 0.01) whereas alpha-agonists, vasopressin and adenosine were without effect. In conclusion, the present study showed that adrenaline, via a beta-adrenergically mediated activation of glycogenolysis, possibly further stimulated by ischaemia, induced an increased release of alanine from skeletal muscle. The study indicates a beta-adrenergic stimulation on the glutamine synthetase step and a short lasting anabolic effect of adrenaline. Differences in the magnitude of the effects of adrenaline and isoprenaline could be related to their different vasoactive properties.

Beta2 adrenergic receptors mediate cAMP, tissue-type plasminogen activator and transferrin production in rat Sertoli cells

FSH is the main regulator of Sertoli cell function. Nevertheless, several other effectors such as catecholamines can also stimulate these cells through the adenylyl cyclase transduction pathway. However, the expression of beta adrenergic receptors in Sertoli cells is a subject of controversy. The aim of the present study was to determine if there are physiologically functional beta adrenergic receptors in Sertoli cells and to which subtype(s) they belong. In freshly isolated Sertoli cells, isoproterenol, a non selective beta-adrenergic agonist, was found to stimulate cAMP production and tissue-type plasminogen activator secretion. Specific transcripts for the beta1 and beta2, but not beta3, subtypes were detected by RT-PCR analysis. Beta2 transcripts were the form expressed predominantly in Sertoli cells. Binding experiments carried out on freshly isolated and on cytospined Sertoli cells indicated that in both conditions, [125I]iodocyanopindolol binding was inhibited by a non-selective and a 2 selective antagonist, whereas a beta1 selective antagonist had no effect. Scatchard analysis of beta2 specific inhibition revealed a dissociation constant of 0.3 nM and a receptor density of 14000 sites per cell. In freshly isolated Sertoli cells, we observed that cAMP and tissue-type plasminogen activator were stimulated by isoproterenol and a beta2 selective agonist, but not by beta1 or beta3 selective agonists. Accordingly, the isoproterenol-stimulated tissue-type plasminogen activator responses were abolished by the beta2 selective antagonist only. In cultured Sertoli cells, the trend was the same: tissue-type plasminogen activator and transferrin secretions were increased by isoproterenol and beta2 but not by beta1 or beta3 selective agonists. We conclude that freshly isolated Sertoli cells express beta2 adrenergic receptors which are functionally coupled to adenylyl cyclase and that these characteristics are preserved in cell culture. For the tested parameters, catecholamines and FSH effects were similar, but response magnitudes were systematically lower with beta agonists than with FSH. As norepinephrine is normally present in physiologically-relevant amounts in the interstitial fluid, it can be suspected to play a role in the regulation of Sertoli cell function.

Myocardial signaling defects and impaired cardiac function of a human beta 2-adrenergic receptor polymorphism expressed in transgenic mice

A threonine to isoleucine polymorphism at amino acid 164 in the fourth transmembrane spanning domain of the beta 2-adrenergic receptor (beta 2AR) is known to occur in the human population. The functional consequences of this polymorphism to catecholamine signaling in relevant cells or to end-organ responsiveness, however, are not known. To explore potential differences between the two receptors, site-directed mutagenesis was carried out to mimic the polymorphism. Transgenic FVB/N mice were then created overexpressing wild-type (wt) beta 2AR or the mutant Ile-164 receptor in a targeted manner in the heart using a murine alpha myosin heavy chain promoter. The functional properties of the two receptors were then assessed at the level of in vitro cardiac myocyte signaling and in vivo cardiac responses in intact animals. The expression levels of these receptors in the two lines chosen for study were approximately 1200 fmol/mg protein in cardiac membranes, which represents a approximately 45-fold increase in expression over endogenous beta AR. Myocyte membrane adenylyl cyclase activity in the basal state was significantly lower in the Ile-164 mice (19.5 +/- 2.7 pmol/min/mg) compared with wt beta 2AR mice (35.0 +/- 4.1 pmol/min/mg), as was the maximal isoproterenol-stimulated activity (49.8 +/- 7.8 versus 77.1 +/ 7.3 pmol/min/mg). In intact animals, resting heart rate (441 +/- 21 versus 534 +/- 17 bpm) and dP/dtmax (10,923 +/- 730 versus 15,308 +/- 471 mmHg/sec) were less in the Ile-164 mice as compared with wt beta 2AR mice. Similarly, the physiologic responses to infused isoproterenol were notably less in the mutant expressing mice. Indeed, these values, as well as other contractile parameters, were indistinguishable between Ile-164 mice and nontransgenic littermates. Taken together, these results demonstrate that the Ile-164 polymorphism is substantially dysfunctional in a relevant target tissue, as indicated by depressed receptor coupling to adenylyl cyclase in myocardial membranes and impaired receptor mediated cardiac function in vivo. Under normal homeostatic conditions or in circumstances where sympathetic responses are compromised due to diseased states, such as heart failure, this impairment may have important pathophysiologic consequences.

Differential regulation of circulating Mg2+ in the rat by beta 1- and beta 2-adrenergic receptor stimulation

Extracellular Mg2+ homeostasis was studied in vivo in the anesthetized rat. Animals were infused with isoproterenol (ISO) for 10 minutes, and serum Mg2+ was measured after the infusion and then 10 and 20 minutes later. A dose-dependent increase in circulating Mg2+ was observed in animals infused with ISO at a rate of 0.1 microgram.kg-1.min-1 or higher. The time course of the effect demonstrated that circulating Mg2+ continued to increase 20 minutes after the end of the ISO infusion. A predicted maximal increase in serum Mg2+ concentration of 19.3% was derived with a predicted EC50 of 0.08 microgram.kg-1.min-1. The maximal percent increase corresponded to a net increase of 6.7 mumol/300 g body wt. Because infusion of ISO resulted in changes in hemodynamic parameters, most notably a drop in blood pressure, a group of animals was infused with nitroprusside to mimic the hypotensive response via a nonadrenergic mechanism. Under these conditions, there was a transient increase in circulating Mg2+ that was largely inhibited by propranolol, indicating that hypotension per se was not responsible for the mobilization of Mg2+. Infusion of salbutamol, but not prenalterol, also induced an increase in circulating Mg2+. Pretreatment with butoxamine, ICI-118551, or propranolol prevented the ISO-induced increase in serum Mg2+. Pretreatment with atenolol minimally affected the ISO-induced changes in circulating Mg2+. Pretreatment with CGP-20271A actually enhanced the ISO-induced increase in circulating Mg2+. This evidence demonstrates the existence of a pool of Mg2+ that is mobilized into the circulation in response to selective beta 2-adrenergic stimulation.

Characterization of lymphocyte beta-adrenoceptor activity and Gs-protein in patients with rheumatic heart valvular disease

In order to test whether the beta-adrenoceptor activity in rheumatic heart valvular disease depends on the ventricular load conditions, we determined their density and binding affinity to [125I]-iodocyanopindolol in lymphocytes, as well as plasma catecholamine and cAMP levels in 69 patients with regurgitant and stenotic lesions of the aortic and mitral valves. The patients were classified as having left ventricular pressure overload (LVP), left ventricular volume overload (LVV), mixed lesions (MOL) or right ventricular pressure overload (RVP). The beta-adrenoceptor activity was determined by radioligand binding methods, catecholamines by high performance liquid chromatography using an electrochemical detector and cAMP by radioimmunoassay. The mean beta-adrenoceptor density (Bmax) of the control group was 60.1 +/- 9.5 fmol (n = 29) per 10(6) lymphocytes. In the study population, the density was decreased by 83% in LVP, 78% in LVV, 87% in MOL and 86% in RVP. Plasma norepinephrine was elevated by 89% in LVP and 60% in MOL, epinephrine by 43% in LVP, 50% in VOL, 115% in MOL and 20% in RVP, while dopamine was not significantly changed, and cAMP was slightly elevated in all four groups. Screening for activating mutational changes in the Gs alpha-protein gave negative results, possibly dissociating the elevation in plasma cAMP from stimulatory effects of such abnormalities in the Gs-protein signaling.(ABSTRACT TRUNCATED AT 250 WORDS)

alpha2-adrenergic agonist-induced inhibition of cyclic AMP formation in transfected cell lines using a microtiter-based scintillation proximity assay

Human embryonic kidney cells (HEK-7) were transfected with the gene for the human alpha2C receptor (alpha2C4). Cells were grown in 96-well microtiter plates and cyclic AMP levels were measured by scintillation proximity assay, a modified radioimmunoassay technique. Radioactivity was quantified using a TopCount Scintillation detector. Cyclic AMP was increased in a dose-dependent manner by the addition of exogenous forskolin. The forskolin-induced enhancement of cyclic AMP was inhibited dose-dependently by the addition of alpha2-adrenergic agonists, and this inhibition was blocked by the addition of adrenergic antagonists. The extent of the inhibitory response caused by alpha2-adrenoceptor agonists was related to the receptor density in clonal cell lines derived from the transfected parental HEK-7 cells. By using cells grown in microtiter format, and employing the technological advantages of scintillation proximity assay and TopCount detection, it was possible to simultaneously evaluate the effects of multiple experimental permutations on cellular production of cyclic AMP with minimal disturbance of the cells and minimal and/or automated manipulation of the cyclic AMP formed. This combination of techniques should allow rapid testing of the actions of adrenergic agonists and antagonists on cells transfected with receptors linked to cyclic AMP formation.

The palmitoylated cysteine of the cytoplasmic tail of alpha 2A-adrenergic receptors confers subtype-specific agonist-promoted downregulation

Most guanine nucleotide binding protein (G protein)-coupled receptors have a conserved cysteine in the C-terminal cytoplasmic tail near the seventh transmembrane spanning region. This cysteine is known to be palmitoylated in rhodopsin, the beta 2-adrenergic receptor (beta 2AR) and the alpha 2A-adrenergic receptor (alpha 2AAR). For the beta 2AR, this cysteine has been shown to be important for stimulatory G protein (Gs) coupling and agonist-promoted desensitization. For the alpha 2AAR (human alpha 2 C10) palmitoylation occurs at Cys-442, but it is not known what function such fatty acid acylation subserves. The closely related alpha 2CAR subtype denoted alpha 2C4 lacks a cysteine in this region and has different G-protein-coupling characteristics and agonist regulatory properties as compared to alpha 2C10. To assess the role of the palmitoylcysteine in alpha 2AR function, we constructed a mutated alpha 2C10 having a phenylalanine (the analogous amino acid in the alpha 2C4 in this position) substituted for Cys-442, denoted alpha 2C10(Phe-442), and expressed this along with wild-type alpha 2C10 and alpha 2C4 in CHO cells. Functional coupling to inhibitory G protein (Gi) and to Gs was identical between wild-type alpha 2C10 and alpha 2C10(Phe-442). Agonist-promoted desensitization of both the Gi and Gs-mediated pathways was also found to be unaffected by this mutation. Cellular trafficking induced by agonist exposure was evaluated by delineation of intracellular (sequestered) versus cell surface receptors and by determination of net receptor loss. Mutation of Cys-442 did not alter the extent or rate of agonist-promoted sequestration induced by agonists or the recovery from sequestration. However, the downregulation of receptor number after prolonged agonist exposure was completely abolished by this mutation and converted alpha 2C10 to an alpha 2C4 phenotype in regard to this adaptive response. Another mutated alpha 2C10, in which Cys-442 was replaced by alanine, also failed to downregulate. Thus, the function of this cytoplasmic palmitoylcysteine is distinctly different between the alpha 2AR and other G-protein-coupled receptors such as the beta 2AR and rhodopsin, and this suggests that this molecular attribute may subserve diverse roles among members of this family of receptors. For the alpha 2ARs, this may represent an evolved feature that provides for differing needs for regulation of the alpha 2C10 and alpha 2C4 subtypes by agonist.

Protein kinases and protein synthesis are involved in desensitization of the plasminogen activator response of rat Sertoli cells by follicle-stimulating hormone

Tissue-type plasminogen activator (tPA) secretion is a specific response of Sertoli cells to follicle-stimulating hormone (FSH), which is lower after preincubation of the cells with low FSH concentrations because of FSH receptor/Gs protein uncoupling. In this report, we present evidence that this desensitization induced by the lowest FSH concentrations is suppressed by specific peptidic inhibitors of endogenous PKA and PKC in permeabilized Sertoli cells. In contrast, desensitization promoted by slightly higher FSH concentrations is not mediated through PKA or PKC activation but is dependent on protein neosynthesis.